TryBuildCalcRainwater Harvesting Calculator(Calculate Yield, Tank Size & Water Savings)
Calculate rainwater yield, tank storage, overflow, and demand coverage.
🕒 Last updated: July 23, 2026
Inputs
Section 1: Catchment & Rainfall
ℹ️Use roof plan area, terrace area, or paved collection area.
ℹ️Typical annual rainfall can range from 250 to 3,000 mm. Dry regions may be lower; very wet regions can exceed this.
Section 2: Collection Efficiency
ℹ️Surface type sets a default runoff coefficient. Choose custom to enter your own value.
ℹ️Typical roof values range from 0.75 to 0.95. Lower values collect less water.
ℹ️Allows for filter, gutter, conveyance, and practical collection losses.
ℹ️Optional rainfall depth diverted before storage. Enter 0 if not used.
Section 3: Storage & Demand
ℹ️Optional. Used to estimate overflow and usable stored water.
ℹ️Optional. Used for demand coverage and days of supply.
ℹ️Used to recommend a practical tank size from daily demand.
💧 You can harvest up to 57,420 litres of rainwater per year.
Effective capture is 71.8% after runoff, first flush, and system efficiency losses.
This can cover 62.9% of your annual water demand.
Inputs Used
Catchment Surface: Concrete roof / terrace
Catchment Area: 100 m²
Annual Rainfall: 800 mm
Runoff Coefficient: 0.8
System Efficiency: 90%
First Flush Depth: 2 mm
Other Conversions
Cubic Meters: 57.42 m³
US Gallons: 15,168.8 US gal
Imperial Gallons: 12,630.6 imp gal
Cubic Feet: 2,027.8 ft3
Current Tank & Demand Planning
Entered Tank Capacity: 5,000 L
Max Stored at One Time: 5,000 L
Limited by the entered tank capacity.
Annual Overflow With This Tank: ⚠️ 52,420 L will overflow without additional storage
Compares total annual harvest with the entered tank size. Actual overflow depends on rainfall timing and water use.
Annual Demand: 91,250 L
Potential Demand Coverage: 62.9%
Based on total annual harvest, before detailed seasonal timing.
Backup Water Still Needed: 33,830 L
Supply From Full Entered Tank: 20 days
At 250 L/day demand.
Recommended Backup Tank: 2,000 L
Sized for 7 dry days plus 10% safety. Calculated need: 1,925 L. This is not sized to store all annual rainfall.
Loss Breakdown
Gross rainfall volume is 80,000 L. Runoff capture is 64,000 L, with 200 L reserved as first flush loss.
Harvestable water is calculated from catchment area, rainfall, runoff coefficient, first flush loss, and system efficiency.
Approximate results for planning only. Verify with a professional.
Purpose of a Rainwater Harvesting Calculator
A rainwater harvesting calculator estimates how much water can be collected from a roof, terrace, or paved catchment using local rainfall and practical collection losses.
It helps homeowners, contractors, plumbers, and site engineers size storage tanks, check expected overflow, and compare rainwater availability with non-potable daily water demand. Use the Water Tank Capacity Calculator alongside this estimate when you need to size the actual storage tank dimensions properly.
How the rainwater harvesting calculation works
The calculator converts the catchment area to square metres and rainfall to millimetres. Since 1 mm of rain over 1 m² equals 1 litre, the water quantity is simple to estimate.
Step 1 - Calculate gross rainfall volume
Litres = m² × mm
This is the theoretical water falling on the roof or terrace before any surface, gutter, filter, or first flush losses.
Step 2 - Apply runoff coefficient
The runoff coefficient adjusts for the catchment surface. Metal roofs collect more water, while rough or porous surfaces collect less.
Step 3 - Subtract first flush loss
Water After First Flush = Runoff Captured Water - First Flush Loss
First flush is the initial dirty roof water diverted away from the storage tank. If you enter 0, the calculator skips this loss.
Step 4 - Apply system efficiency
System efficiency accounts for practical losses in gutters, downpipes, filters, leakage, splash, and maintenance conditions.
Step 5 - Check storage, overflow, and demand
Period Demand = Daily Demand × Days in Period
Demand Coverage = Net Harvestable Water / Period Demand × 100
Backup Tank Need = Daily Demand × Dry Days × Safety Factor
Storage planning compares net harvest with the entered tank capacity and daily demand. The recommended backup tank is sized for dry days, not for storing all annual or monthly rainfall. If you already know the required litres, the Water Tank Capacity Calculator can convert that storage requirement into rectangular or cylindrical tank dimensions.
Real-World Rainwater Harvesting 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 |
|---|---|---|
| Rainfall Period | Annual | Determines whether results are shown per year or per month |
| Catchment Area | 100 m2 | Multiplied by rainfall to get gross rainfall volume |
| Annual Rainfall | 800 mm | Multiplied by catchment area to get gross rainfall volume |
| Catchment Surface | Concrete roof / terrace (coefficient 0.8) | Reduces gross rainfall to account for surface losses |
| System Efficiency | 90% | Reduces captured water for filter, gutter, and conveyance losses |
| First Flush Depth | 2 mm | Diverts the initial dirty runoff before it reaches storage |
| Existing Tank Capacity | 5000 L | Used to estimate overflow and maximum stored water |
| Daily Water Demand | 250 L/day | Used to estimate demand coverage and days of supply |
| Backup Dry Days | 7 days | Used to size a practical backup tank |
| Safety Factor | 10% | Adds a buffer to the recommended backup tank size |
| Primary Output Unit | liters | Sets the unit used for capacity figures in the result card |
Step 1 — Gross Rainfall Volume
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Gross rainfall | Catchment Area x Rainfall = 100 m² x 800 mm | 80,000 L |
Step 2 — Runoff Captured
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Runoff captured | Gross Rainfall x Runoff Coefficient = 80,000 L x 0.8 | 64,000 L |
Step 3 — First Flush Loss
| Calculation | Formula / Substitution | Result |
|---|---|---|
| First flush loss | Catchment Area x First Flush Depth = 100 m² x 2 mm | 200 L |
| Water after first flush | Runoff Captured − First Flush Loss = 64,000 − 200 | 63,800 L |
Step 4 — Net Harvestable Water
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Net harvest | Water After First Flush x System Efficiency = 63,800 x 90% | 57,420 L |
Step 5 — Stored Water and Overflow
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Max stored at one time | min(Net Harvest, Tank Capacity) = min(57,420, 5000) | 5,000 L |
| Overflow | Net Harvest − Tank Capacity = 57,420 − 5000 | 52,420 L |
Step 6 — Demand Coverage
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Annual demand | Daily Demand x Days in Period = 250 x 365 | 91,250 L |
| Demand coverage | (Net Harvest ÷ Period Demand) x 100 = (57,420 ÷ 91,250) x 100 | 62.9% |
| Backup water still needed | Period Demand − Net Harvest = 91,250 − 57,420 | 33,830 L |
Step 7 — Days of Supply From Stored Water
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Days of supply | Stored Water ÷ Daily Demand = 5,000 ÷ 250 | 20 days |
Step 8 — Recommended Backup Tank Size
| Calculation | Formula / Substitution | Result |
|---|---|---|
| Calculated backup tank | Daily Demand x Dry Days x (1 + Safety Factor) = 250 x 7 x (1 + 10%) | 1,925 L |
| Practical rounded tank size | Round up to the next practical tank size | 2,000 L |
Therefore, this catchment can harvest approximately 57,420 L per year (effective capture 71.8%), covering about 62.9% of the entered yearly demand, with a recommended backup tank of 2,000 L.
Actual yield can vary with roof slope, gutter layout, leaf guards, maintenance, and real rainfall timing. Cross-check the recommended tank against the Reference Tables section below.
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Catchment Area+-
- Catchment area was measured as the plan (horizontal) area of the roof — not the slope area of pitched roofs.
- All roof surfaces draining to the collection system were included — multiple slopes and sections were summed.
- Surfaces that do not drain to the collection point — walls, adjacent roofs, skylights draining internally — were excluded.
- Dimensions were entered in consistent units.
✓Rainfall Data+-
- Rainfall data was obtained from the IMD (India Meteorological Department) or local authority for the specific location — not a regional average.
- Annual rainfall was split into monthly data to identify dry months and size storage accordingly — annual average alone is insufficient for sizing.
- For recharge pit sizing, the 1-hour peak rainfall intensity (mm/hr) for the location was obtained — not the annual total.
✓Runoff and Collection Efficiency+-
- Runoff coefficient was set appropriately for the roof material — RCC terrace 0.85–0.95, clay tile 0.75–0.90, corrugated sheet 0.70–0.90.
- First-flush diverter volume was calculated and a first-flush device was specified — first 2.5mm of rainfall per 100 m² catchment is diverted.
- Collection efficiency factor of 0.70–0.85 was applied to account for evaporation, spillage, and first-flush losses.
- Gutters and downpipes were sized to carry peak rainfall intensity without overflow — undersized gutters lose runoff before it reaches the tank.
✓Storage Tank Sizing+-
- Storage tank volume was sized for the longest dry period in the local rainfall pattern — not the annual total.
- Daily demand for the intended end use (toilet flushing, irrigation, car washing) was calculated and used as the drawdown rate.
- Rainwater harvesting is not intended for drinking without treatment — end use was confirmed as non-potable.
- Overflow from the storage tank was directed to a soak pit or municipal drain — not to a neighbour's property.
- For groundwater recharge pits, the soil permeability (infiltration rate) was confirmed — clay soil cannot absorb at the required rate.
✓Regulatory and Maintenance+-
- Local municipal or state authority rainwater harvesting bye-law requirements were confirmed — several Indian cities mandate RWH for plots above a minimum size.
- Recharge pit location is at least 15m from any septic tank or soak pit to prevent groundwater contamination.
- Structural adequacy of the terrace for the additional water load during peak rainfall was confirmed with the structural engineer.
Full QC Checklist+−
Verify catchment area, rainfall data, runoff, first flush, storage demand, overflow, treatment, and maintenance.
✓Catchment Area+-
- Catchment area was measured as the plan (horizontal) area of the roof — not the slope area of pitched roofs.
- All roof surfaces draining to the collection system were included — multiple slopes and sections were summed.
- Surfaces that do not drain to the collection point — walls, adjacent roofs, skylights draining internally — were excluded.
- For terraced roofs, the net area was used after deducting parapet wall footprint, AC units, and overhead tank base.
- Dimensions were entered in consistent units.
✓Rainfall Data+-
- Rainfall data was obtained from the IMD (India Meteorological Department) or local authority for the specific location — not a regional average.
- Annual rainfall was split into monthly data to identify dry months and size storage accordingly — annual average alone is insufficient for sizing.
- A design rainfall event (e.g. 75th percentile annual rainfall) was used for conservative sizing — not the maximum or average.
- Rainfall data used is at least a 10-year average — shorter records do not capture year-to-year variability.
- For recharge pit sizing, the 1-hour peak rainfall intensity (mm/hr) for the location was obtained — not the annual total.
✓Runoff and Collection Efficiency+-
- Runoff coefficient was set appropriately for the roof material — RCC terrace 0.85–0.95, clay tile 0.75–0.90, corrugated sheet 0.70–0.90.
- First-flush diverter volume was calculated and a first-flush device was specified — first 2.5mm of rainfall per 100 m² catchment is diverted.
- Collection efficiency factor of 0.70–0.85 was applied to account for evaporation, spillage, and first-flush losses.
- Gutters and downpipes were sized to carry peak rainfall intensity without overflow — undersized gutters lose runoff before it reaches the tank.
- Leaf guards and mesh filters were specified at all gutter inlets to prevent blockage and debris entry.
✓Storage Tank Sizing+-
- Storage tank volume was sized for the longest dry period in the local rainfall pattern — not the annual total.
- Daily demand for the intended end use (toilet flushing, irrigation, car washing) was calculated and used as the drawdown rate.
- Rainwater harvesting is not intended for drinking without treatment — end use was confirmed as non-potable.
- Overflow from the storage tank was directed to a soak pit or municipal drain — not to a neighbour's property.
- For groundwater recharge pits, the soil permeability (infiltration rate) was confirmed — clay soil cannot absorb at the required rate.
- Storage tank is covered, shaded, and constructed of food-grade material if water is used for any human contact purpose.
✓Regulatory and Maintenance+-
- Local municipal or state authority rainwater harvesting bye-law requirements were confirmed — several Indian cities mandate RWH for plots above a minimum size.
- Recharge pit location is at least 15m from any septic tank or soak pit to prevent groundwater contamination.
- First-flush diverter was designed to be manually or automatically reset after each rainfall event.
- Filter media in the recharge pit — gravel, coarse sand, fine sand layers — was specified and maintenance access was provided.
- Annual cleaning and inspection schedule for gutters, filters, first-flush device, and storage tank was confirmed.
- Structural adequacy of the terrace for the additional water load during peak rainfall was confirmed with the structural engineer.
Reference Tables
Runoff Coefficient by Catchment Surface
Use this as a sanity check against the auto-filled coefficient above — actual values vary with slope, surface condition, and maintenance.
| Catchment Surface | Typical Coefficient |
|---|---|
| Metal / GI roof | 0.85 - 0.95 |
| Tile roof | 0.75 - 0.90 |
| Concrete roof / terrace | 0.80 - 0.90 |
| Paved area (concrete/asphalt) | 0.60 - 0.80 |
| Gravel / landscape area | 0.20 - 0.40 |
Typical Non-Potable Water Demand
Use these as a starting point for the Daily Water Demand field above, then adjust to your actual intended use.
| Use | Typical Demand |
|---|---|
| Toilet flushing (per person, per day) | 20 - 30 L |
| Garden / landscape irrigation (per m², per week) | 10 - 20 L |
| Car washing (per wash) | 40 - 100 L |
| Floor cleaning / general washdown (per day) | 20 - 50 L |
| Cooling tower / HVAC makeup (small residential) | 100 - 300 L/day |
How to Use the Rainwater Harvesting Calculator
- Choose annual or monthly rainfall and enter the rainfall amount for your location.
- Enter the roof, terrace, or paved catchment area.
- Select the catchment surface or enter a custom runoff coefficient.
- Add first flush, tank capacity, and daily demand if you want storage planning.
- Review net harvest, tank overflow, days of supply, and recommended tank size.
Rainwater Harvesting Tips & Best Practices
- Split annual rainfall into monthly figures where possible — a location with the same annual total but concentrated monsoon rainfall needs a differently sized tank than one with evenly spread rainfall.
- Always divert first flush, even a small 1-2mm depth — it removes most of the dust, bird droppings, and leaf debris that accumulate on a roof between rain events.
- Size the backup tank for dry-day coverage, not for storing a full year of rainfall — a tank sized for annual volume is usually far larger and costlier than needed.
- Check local rainwater harvesting bye-laws before finalizing tank size — some jurisdictions mandate minimum harvesting capacity or recharge structures for plots above a certain size.
- Combine this estimate with the Water Tank Capacity Calculator to convert the recommended litres into real tank dimensions.
Common Mistakes to Avoid
- Using roof slope area instead of plan (horizontal) area. Rainfall is measured as depth falling vertically, so the catchment area that matters is the horizontal footprint of the roof, not its sloped surface area — using slope area overstates harvest by 10-40% depending on pitch.
- Sizing the tank from annual rainfall instead of dry-day demand.A tank that stores the full annual harvest is usually far bigger and more expensive than needed — the backup tank should cover the longest realistic dry spell, not the whole year's rainfall.
- Skipping first flush diversion. Without it, the first few millimeters of dust, bird droppings, and roof debris go straight into storage, degrading water quality for the entire tank, not just the first flush volume.
- Ignoring overflow routing. A tank that overflows onto a neighboring property, into a foundation, or against a boundary wall is a common source of disputes and water damage — overflow needs a planned discharge point (soak pit or storm drain).
- Treating demand-coverage percentage as guaranteed year-round. A 60% annual demand-coverage figure does not mean 60% of every month is covered — concentrated rainfall means some months may have surplus overflow while dry months have zero harvest.
Rainwater Harvesting Calculator Limitations
- Rainfall is assumed as a total depth for the selected period, not a day-by-day rainfall pattern.
- Overflow is a simplified estimate and does not model storm timing or tank drawdown between storms.
- Water quality, filtration, disinfection, and local code requirements must be checked separately.
- Actual yield can vary with roof slope, gutter layout, leaf guards, maintenance, and leakage.
Related Calculators
Use the Water Tank Capacity Calculator to calculate the physical volume of a rectangular or cylindrical storage tank.
The Roofing Sheet Calculator can help estimate roof area when planning roof-based collection.
Use the Pipe Volume Calculator for plumbing line capacity after the tank.