TryBuildCalcWater Tank Capacity Calculator(Rectangular, Cylindrical, Spherical, Fill Time & Recommended Size)
Calculate tank capacity, stored water, fill time, and recommended size.
🕒 Last updated: July 15, 2026
Inputs
ℹ️Use fill level for current stored water, fill time for inlet planning, and recommend tank size to find the right size before buying.
Units
ℹ️Controls how total capacity, stored water, and remaining capacity are displayed in the result card. Daily usage and inlet flow rate are always entered in liters, independent of this setting.
ℹ️Choose external if you measured the outside of the tank walls — wall thickness below will be deducted automatically.
Rectangular Tank
Usable Capacity (Optional)
ℹ️Reserved space at the top for overflow — not usable storage.
ℹ️Height below the outlet where sediment settles — not usable storage.
Installation & Material
ℹ️Used only for practical installation notes below the result — does not affect the capacity calculation.
Usage Planning
ℹ️Use your local planning norm or average household water use per person. Always in liters per day, regardless of the Primary Output Unit selected above.
3,600 L total tank capacity based on the entered dimensions.
Tank Summary
Entered Dimensions: 2 m × 1.5 m × 1.2 m
Tank Shape: Rectangular tank
Total Tank Capacity: 3,600 L
Water Weight (Full): 3,600 kg (7,937 lb)
Capacity Conversions
Liters: 3,600 L
Cubic Meters: 3.6 m³
US Gallons: 951 US gal
Imperial Gallons: 791.9 imp gal
Cubic Feet: 127.1 ft³
Installation Notes
- Overhead tanks rely entirely on the supporting stand or slab — confirm it is designed for the full, filled weight, not just the empty tank.
- Use a UV-stabilized, food-grade tank for drinking water — unprotected plastic degrades and can discolor stored water with prolonged sun exposure.
Approximate results for planning only. Verify with a professional.
Water Tank Capacity Calculator: Liters, Gallons, Fill Time, and What Size to Buy
This calculator estimates how much water a rectangular, cylindrical, spherical, oval, or capsule tank holds, plus how much is currently stored, how much space remains, how long it takes to fill at a given inlet flow rate, and how much the full tank weighs. If you have not bought a tank yet, switch to recommend tank size mode to work backward from your household's daily usage and desired backup days to a real, purchasable tank size instead of guessing a round number.
It is useful for homeowners, plumbers, site engineers, contractors, and maintenance teams working with overhead tanks, underground sumps, RCC water tanks, plastic storage tanks, and other site water storage tanks — whether the question is "how many liters is my tank", "what size tank do I need", or "how long will it take to fill".
Horizontal cylindrical tanks use real circular-segment geometry, and spherical tanks use real spherical-cap geometry, for partial fill levels rather than a flat height-ratio approximation. Enter freeboard and dead storage height to see usable capacity alongside total capacity, and set installation type and tank material for practical structural, corrosion, and UV-exposure notes. Every result includes unit conversions (liters, cubic meters, US and imperial gallons, cubic feet) alongside the water's weight in kilograms and pounds — useful for checking that a tank stand or rooftop structure is designed for the full, filled weight.
How the water tank calculation works
The calculator first determines the correct tank geometry, then calculates total capacity, current stored water, remaining volume, unit conversions, and fill time where applicable.
Step 1 - Select the tank shape or mode
Use rectangular, cylindrical, spherical, oval, or capsule mode for total capacity by shape; fill level mode for current stored water (rectangular, cylindrical, or spherical); fill time mode to estimate how long it takes to fill the remaining space; and recommend tank size mode to work backward from household demand to a tank size you can actually buy. If you measured the outside of the tank walls rather than the inside, switch Dimension Input Type to external and enter wall thickness — every formula below then runs on the adjusted internal dimensions.
Step 2 - Calculate total tank capacity
Cylindrical Tank Volume = π x Radius² x Height / Length
Spherical Tank Volume = (4/3) x π x Radius³
Oval Tank Volume = π x Semi-Major Axis x Semi-Minor Axis x Length
Capsule Tank Volume = (π x Radius² x Cylindrical Length) + (4/3) x π x Radius³
All inputs are converted to meters first, then the calculator returns total tank volume in cubic meters and other common water storage units, plus the weight of that volume of water in kilograms and pounds (1 litre ≈ 1 kg). A capsule's cylindrical length is its overall length minus twice the radius, since the two rounded ends are part of the overall length you enter.
Step 3 - Calculate usable capacity (optional)
When freeboard or dead storage height is entered, the calculator works out the liquid volume between the dead-storage line near the outlet and the freeboard line near the overflow — using the same height-to-volume relationship as fill-level tracking below. This is offered for rectangular, cylindrical, and spherical tanks; oval and capsule shapes do not have a simple closed-form partial-volume formula, so usable capacity is not calculated for them.
Step 4 - Calculate current stored water
Vertical Cylinder Stored Water = π x Radius² x Current Water Level
Horizontal Cylinder Stored Water = Circular Segment Area x Tank Length
Spherical Stored Water = (π x Height² / 3) x (3 x Radius − Height)
For vertical tanks, stored volume is based on the current liquid height. For horizontal cylindrical tanks, the calculator uses circular segment geometry, and for spherical tanks it uses the standard spherical-cap formula, to estimate the water actually stored at the entered fill level.
Step 5 - Calculate remaining volume and fill percentage
Fill Percentage = (Current Stored Water / Total Capacity) x 100
These results help you understand how full the tank is, how much more water it can receive, and whether the current storage is enough for household or site use.
Step 6 - Estimate fill time
In fill time mode, the calculator converts the remaining capacity to liters and divides it by the entered inlet flow rate to estimate how long it will take to fill the tank.
Step 7 - Recommend a tank size from demand
Recommended Size = Next common tank size ≥ Required Volume
Recommend tank size mode multiplies daily per-person usage by the number of people and the number of backup days you want covered, then rounds the result up to the nearest common off-the-shelf tank size so you get a size you can actually order.
Step 8 - Installation notes (advisory only)
Based on the installation type (overhead, underground, or ground level) and tank material selected, the result card shows practical, non-geometric notes — structural sign-off thresholds, corrosion or buoyancy risks, and UV or food-grade material guidance. These do not affect the volume calculation.
Real-World Water Tank Calculation Example
This example uses the active inputs above and follows the same steps from the formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Mode | Rectangular tank | Determines which geometry formula and which optional fields apply |
| Shape | Rectangular | Sets the total-capacity formula used in Step 1 below |
| Dimensions | 2 x 1.5 x 1.2 m | Sets the tank's total geometric volume |
| Primary Output Unit | liters | Sets the unit used for capacity figures in the result card |
Step 1 — Total Tank Capacity (Rectangular Shape)
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Volume | Length x Width x Height = 2 x 1.5 x 1.2 (m) | 3.6 m³ |
| Total capacity | 3.6 m³ x 1,000 L/m³ | 3,600 L |
Step 2 — Full-Tank Water Weight
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Water weight | 3,600 L x 1 kg/L | 3,600 kg (7,937 lb) |
Therefore, this rectangular tank holds approximately 3,600 L (3.6 m³), weighing about 3,600 kg when full.
Actual usable capacity may vary depending on wall thickness, domed covers, baffles, and internal fittings not entered above. For a purchase-ready size, cross-check the result against the Reference Tables section below.
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Dimensions and Shape+-
- Tank dimensions were physically measured — not taken from the supplier's catalogue or a purchase invoice.
- Tank shape was correctly identified — rectangular, cylindrical, or L-shaped — before entering dimensions.
- For cylindrical tanks, the internal diameter was measured — not the external diameter. Wall thickness reduces the internal diameter by 20–60mm depending on material.
- For rectangular tanks, internal dimensions (length, width, depth) were measured after deducting wall thickness on all sides.
- Effective water depth was measured to the inlet pipe invert or overflow level — not to the top of the tank wall.
- Dimensions were entered in consistent units — all metres, all centimetres, or all millimetres.
- For underground or sump tanks, the usable depth was measured to the suction pipe invert — water below suction level is not accessible.
✓Demand Calculation+-
- Daily water demand was calculated from the number of occupants — IS 1172:1993 recommends 135 litres per person per day for residential use.
- Storage capacity covers the supply gap duration — number of days between municipal supply cycles was confirmed with the local authority.
- Overhead tank capacity is at least one day's demand; underground sump capacity is at least two to three days' demand.
✓Tank Material and Location+-
- Tank material is food-grade and BIS-certified — IS 12701 for plastic tanks, IS 2644 for ferrocement, IS 805 for steel.
- Overhead tank is structurally supported — the supporting structure was designed for the full tank weight when filled (1 litre = 1 kg).
- Tank location ensures adequate pressure head — minimum 2m height above the highest fitting for gravity-fed systems.
- Underground sump is located away from septic tank and soak pit — minimum 15m separation per IS 1172.
- Tank has a tight-fitting, vermin-proof lid — open tanks collect debris and mosquito breeding in still water.
✓Inlet, Outlet, and Overflow+-
- Inlet, outlet, and overflow pipe sizes were confirmed — undersized pipes restrict fill rate and create pressure loss.
- Overflow pipe discharges to a visible point — not directly into the drainage system where blockage would go unnoticed.
- A float valve or ball cock is fitted at the inlet to prevent overflow — confirmed as the correct size for the supply pipe.
✓Maintenance and Compliance+-
- Local municipal bye-law requirements for tank size, material, and location were confirmed before construction.
- For overhead tanks above 5,000 litres, structural engineer's certification for the supporting structure was obtained.
Full QC Checklist+−
Verify demand, tank geometry, usable capacity, freeboard, structure, overflow, and maintenance access.
✓Dimensions and Shape+-
- Tank dimensions were physically measured — not taken from the supplier's catalogue or a purchase invoice.
- Tank shape was correctly identified — rectangular, cylindrical, or L-shaped — before entering dimensions.
- For cylindrical tanks, the internal diameter was measured — not the external diameter. Wall thickness reduces the internal diameter by 20–60mm depending on material.
- For rectangular tanks, internal dimensions (length, width, depth) were measured after deducting wall thickness on all sides.
- Effective water depth was measured to the inlet pipe invert or overflow level — not to the top of the tank wall.
- Dimensions were entered in consistent units — all metres, all centimetres, or all millimetres.
- For underground or sump tanks, the usable depth was measured to the suction pipe invert — water below suction level is not accessible.
✓Demand Calculation+-
- Daily water demand was calculated from the number of occupants — IS 1172:1993 recommends 135 litres per person per day for residential use.
- Storage capacity covers the supply gap duration — number of days between municipal supply cycles was confirmed with the local authority.
- Overhead tank capacity is at least one day's demand; underground sump capacity is at least two to three days' demand.
- Water demand for non-domestic uses — garden irrigation, car washing, construction water — was estimated and added separately.
- Fire-fighting reserve was confirmed — IS 1172 and local bye-laws may require a dedicated fire reserve volume separate from domestic storage.
- Future occupancy increase was factored into the demand calculation for new constructions.
✓Tank Material and Location+-
- Tank material is food-grade and BIS-certified — IS 12701 for plastic tanks, IS 2644 for ferrocement, IS 805 for steel.
- Overhead tank is structurally supported — the supporting structure was designed for the full tank weight when filled (1 litre = 1 kg).
- Tank location ensures adequate pressure head — minimum 2m height above the highest fitting for gravity-fed systems.
- Underground sump is located away from septic tank and soak pit — minimum 15m separation per IS 1172.
- Tank has a tight-fitting, vermin-proof lid — open tanks collect debris and mosquito breeding in still water.
- A manhole or access opening of minimum 450mm diameter is provided for inspection and cleaning.
- Tank is painted or made of UV-resistant material if exposed to sunlight — direct UV degrades plain plastic tanks and promotes algae growth.
✓Inlet, Outlet, and Overflow+-
- Inlet, outlet, and overflow pipe sizes were confirmed — undersized pipes restrict fill rate and create pressure loss.
- Overflow pipe discharges to a visible point — not directly into the drainage system where blockage would go unnoticed.
- A float valve or ball cock is fitted at the inlet to prevent overflow — confirmed as the correct size for the supply pipe.
- The outlet pipe is 50–75mm above the tank floor to prevent sediment from entering the supply.
- A drain/washout valve is fitted at the lowest point of the tank for periodic cleaning.
- Inlet and outlet are on opposite sides of the tank to prevent short-circuiting of fresh and stale water.
✓Maintenance and Compliance+-
- Tank cleaning schedule — minimum once every six months — was confirmed with the building owner or maintenance team.
- Water quality testing was planned for new tanks before commissioning and annually thereafter.
- Local municipal bye-law requirements for tank size, material, and location were confirmed before construction.
- For overhead tanks above 5,000 litres, structural engineer's certification for the supporting structure was obtained.
- Tank capacity in litres was cross-checked against the result using: length (m) × width (m) × depth (m) × 1,000.
Reference Tables
Common Off-the-Shelf Tank Sizes
Use this as a quick sanity check against the recommended tank size mode above — these are the sizes most suppliers actually stock.
| Tank Size | Typical Use Case |
|---|---|
| 200 L | Single-person apartment, small overhead booster tank |
| 500 L | Small household (2-3 people) with regular municipal supply |
| 1,000 L | Typical family home overhead tank (4-5 people, one day's demand) |
| 1,500 L | Larger household or one day's demand for 6-8 people |
| 2,000 L | Family home with 1-2 day backup, or small commercial unit |
| 3,000 L | Multi-unit residential or a home with irregular supply |
| 5,000 L | Underground sump for a house, 2-3 day backup for a family |
| 10,000 L | Small apartment building or site-storage during construction |
| 20,000 L+ | Apartment complex, commercial building, or community storage |
Typical Daily Water Usage
Daily usage varies a lot by region, fixtures, and lifestyle — use these as a starting point for the demand fields above, then adjust to your local planning norm.
| Context | Usage per Day |
|---|---|
| Basic domestic use (drinking, cooking, minimal washing) | 40-70 L |
| Standard urban household with normal plumbing fixtures | 100-150 L |
| Household with a washing machine, dishwasher, and garden | 150-200 L |
| Hotel or serviced apartment (per guest, per day) | 150-300 L |
| Hospital (per bed, per day) | 300-450 L |
| Office or commercial building (per employee, per day) | 35-50 L |
How to Use the Water Tank Capacity Calculator
- Select the required mode: rectangular, cylindrical, spherical, oval, capsule, fill level, fill time, or recommend tank size.
- For shape-based modes, choose the dimension unit; for every mode, choose the preferred output unit for displaying results.
- If you measured the outside of the tank, set Dimension Input Type to external and enter wall thickness so the calculator uses internal dimensions.
- Enter the required tank dimensions for the selected shape.
- For fill-level mode, also choose whether the tank is rectangular, cylindrical, or spherical.
- For fill-level or fill-time mode, enter the current water level.
- Enter inlet flow rate in liters per minute when using fill time mode.
- Optionally enter freeboard and dead storage height to see usable capacity alongside total capacity.
- Select installation type and tank material to see practical installation notes with your result.
- For recommend tank size mode, enter daily usage per person, number of people, and desired backup days.
- Review total capacity, usable capacity, stored water, remaining capacity, fill percentage, water weight, and unit conversions in the result card.
Water Tank Tips & Best Practices
- Measure internal tank dimensions where possible; if you only have outside measurements, use external dimension input type with wall thickness instead of guessing an internal size.
- Use fill-level mode when you want a more practical estimate of currently available water — it now supports rectangular and spherical tanks, not only cylindrical.
- Enter freeboard and dead storage height for a realistic usable capacity figure, especially when sizing storage against actual household demand.
- Check both liters and gallons if you compare manufacturer data from different markets.
- For daily operations, combine capacity with household or site consumption to estimate refill intervals.
- Before buying a tank, run recommend tank size mode with your actual household headcount rather than guessing a round number.
- Check the water weight figure against your tank stand or roof structure's load rating before installing an overhead tank.
- Set installation type and tank material to see relevant structural, corrosion, and UV-exposure notes for your specific setup.
Common Mistakes to Avoid
- Using external tank dimensions instead of internal ones. Wall thickness on a plastic or RCC tank can reduce internal diameter by 20-60mm per side, which compounds into a noticeably smaller usable volume on larger tanks — set Dimension Input Type to external and enter wall thickness instead of guessing an internal size.
- Ignoring water weight when sizing an overhead tank stand.A 2,000 L tank weighs about 2,000 kg full — plus the tank's own weight — and an undersized stand or rooftop slab not designed for that load is a structural risk, not just a capacity question.
- Rounding required storage down instead of up. When sizing for backup days, a 1,080 L requirement rounded down to a 1,000 L tank leaves no margin — always round up to the next common size, which is exactly what recommend tank size mode does automatically.
- Treating horizontal cylindrical fill-level like a vertical tank. A horizontal cylinder does not fill linearly with height — the middle third of the tank height holds roughly half the total volume due to circular segment geometry, so a naive height-ratio estimate overstates how full a horizontal tank actually is at low and high fill levels.
- Forgetting the freeboard and inlet/outlet dead zones. A few centimeters near the top (freeboard for overflow) and a few centimeters near the bottom (sediment above the outlet) are not usable storage, so the calculated geometric capacity is always slightly higher than the practical usable capacity — enter both under Usable Capacity to see the real, practical figure instead of the geometric maximum.
- Assuming a capsule or oval tank fills the same way as a cylinder. Their curved cross-sections mean liquid volume does not scale linearly with height the way a rectangular tank does — this calculator intentionally limits capsule and oval modes to total capacity rather than showing an inaccurate partial-fill estimate.
Water Tank Calculator Limitations
- This calculator is intended for planning and estimation, not certified storage approval.
- It assumes ideal tank geometry and does not account for domed tops, internal baffles, or irregular shapes — wall thickness is handled separately via Dimension Input Type.
- Horizontal cylindrical fill-level calculations are based on standard circular segment geometry and assume uniform tank length.
- Fill time calculations do not account for fluctuating pressure, outlet usage during filling, or pump performance loss.
- Oval and capsule modes calculate total capacity only — partial fill-level tracking and usable-capacity trimming are not offered for these shapes, since their curved cross-sections do not have a simple closed-form partial-volume formula.
- Basic spherical mode (not fill-level or fill-time) shows total capacity only; use fill-level or fill-time mode with spherical shape selected for partial-fill tracking.
- Recommend tank size mode suggests the nearest common off-the-shelf size but does not check local availability, delivery constraints, or installation space.
- Installation notes are general, practical guidance based on installation type and material — not a substitute for a structural engineer's or local code review.
Related Calculators
If you are sizing a tank for roof water collection, use the Rainwater Harvesting Calculator to estimate roof yield, overflow, dry-day backup storage, and demand coverage before finalizing tank capacity.
For wastewater storage and preliminary septic sizing, use the Septic Tank Size Calculator to estimate septic tank capacity, sludge storage, and suggested dimensions.
If you need quick measurement conversion before entering tank dimensions, use the Unit Converter to switch between meters, centimeters, feet, inches, and common volume units.
For land and site size planning around water storage installations, the Plot Area Calculator can help estimate footprint and layout space.
If you are building an underground sump or water storage pit, the Excavation Calculator can help estimate digging volume before installation.
After tank placement, use the Backfill Calculator to estimate refill material around the tank and the Land Leveling Calculator if you need to plan final site grading around the storage area.