TryBuildCalcRectangular Water Tank Calculator(Liters, Gallons, Fill Level & Fill Time)
Estimate tank capacity, fill level, and fill time.
🕒 Last updated: July 29, 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.
5,625 L total tank capacity based on the entered dimensions.
Tank Summary
Entered Dimensions: 2.5 m × 1.5 m × 1.5 m
Tank Shape: Rectangular tank
Total Tank Capacity: 5,625 L
Water Weight (Full): 5,625 kg (12,401 lb)
Capacity Conversions
Liters: 5,625 L
Cubic Meters: 5.6 m³
US Gallons: 1,486 US gal
Imperial Gallons: 1,237.3 imp gal
Cubic Feet: 198.6 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.
- Above about 5,000 L, get a structural engineer to check the supporting stand or rooftop slab before installation.
- 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.
Looking for the verification checklist, reference tables, or tips and mistakes?See the complete Water Tank Capacity Calculator.
Rectangular tank capacity
This page is pre-filled for a larger rectangular sump-style tank — 2.5 m x 1.5 m x 1.5 m — giving 5,625 liters (5.625 m³) of total capacity.
Rectangular tanks are the most common shape for underground sumps and custom-built RCC tanks, since length, width, and height can each be sized independently to fit an available footprint.
- Length, width, height inputs.
- Liters output.
- Fill level optional.
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 | 2.5 x 1.5 x 1.5 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 = 2.5 x 1.5 x 1.5 (m) | 5.625 m³ |
| Total capacity | 5.625 m³ x 1,000 L/m³ | 5,625 L |
Step 2 — Full-Tank Water Weight
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Water weight | 5,625 L x 1 kg/L | 5,625 kg (12,401 lb) |
Therefore, this rectangular tank holds approximately 5,625 L (5.625 m³), weighing about 5,625 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 in the complete Water Tank Capacity Calculator.
This page keeps things focused on the calculation above. For the full construction guide — verification checklist, reference tables, usage steps, tips, common mistakes, and limitations.See the complete Water Tank Capacity Calculator.