Rainwater Harvesting Calculator(Roof Catchment, Rainfall & Tank Storage)

Estimate rainwater harvesting potential from roof area, rainfall, runoff, and storage tank size. Adjust roof catchment area, rainfall, runoff coefficient, efficiency, tank capacity, and demand.

Estimate rainwater collection and tank storage.

🕒 Last updated: July 29, 2026

Inputs

Section 1: Catchment & Rainfall

Rainfall Period

▾

Catchment Area

ℹ️Use roof plan area, terrace area, or paved collection area.

Annual Rainfall

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

Catchment Surface

▾

ℹ️Surface type sets a default runoff coefficient. Choose custom to enter your own value.

Runoff Coefficient

ℹ️Typical roof values range from 0.75 to 0.95. Lower values collect less water.

System Efficiency

ℹ️Allows for filter, gutter, conveyance, and practical collection losses.

First Flush Depth

ℹ️Optional rainfall depth diverted before storage. Enter 0 if not used.

Section 3: Storage & Demand

Existing Tank Capacity

ℹ️Optional. Used to estimate overflow and usable stored water.

Daily Water Demand

ℹ️Optional. Used for demand coverage and days of supply.

Backup Dry Days

ℹ️Used to recommend a practical tank size from daily demand.

Safety Factor

Primary Output Unit

▾

Large roof / site collection

💧 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.

Water Tank Capacity CalculatorConvert this recommended storage size into real tank dimensions.Roofing Sheet CalculatorEstimate roof area if you don't already know your catchment size.Pipe Volume CalculatorSize the conveyance pipework between the roof and storage tank.

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Looking for the verification checklist, reference tables, or tips and mistakes?See the complete Rainwater Harvesting Calculator.

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Rainwater harvesting estimate

This page is pre-filled with a general baseline scenario — a 100 m² concrete roof, 800mm of annual rainfall, 90% system efficiency, and a 5,000-liter tank — which nets about 57,420 liters of harvestable water a year, far more than the 5,000 L tank can store at once.

This is a broad starting point covering the most common inputs. Change the catchment area, rainfall, surface type, or tank size below and every downstream figure — net harvest, overflow, demand coverage, and backup tank size — recalculates immediately.

Catchment area: 100 m².
Annual rainfall: 800 mm.
Tank capacity: 5000 liters.

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

Gross Rainfall Volume = Catchment Area × Rainfall
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

Runoff Captured Water = Gross Rainfall Volume × 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

First Flush Loss = Catchment Area × First Flush Depth
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

Net Harvestable Water = Water After First Flush × System Efficiency

System efficiency accounts for practical losses in gutters, downpipes, filters, leakage, splash, and maintenance conditions.

Step 5 - Check storage, overflow, and demand

Overflow = Net Harvestable Water - Entered Tank Capacity
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 in the complete Rainwater Harvesting 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 Rainwater Harvesting Calculator.

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FAQ

Gross rainfall (100 m² x 800mm = 80,000 L) is first reduced by the concrete roof's 0.8 runoff coefficient (64,000 L), then by a 200 L first-flush loss (63,800 L), then by 90% system efficiency for gutter, filter, and leakage losses, giving 57,420 liters of net harvestable water for the year.

This figure is annual harvest potential, not what fits in the tank at once — with only a 5,000 L tank, roughly 52,420 liters would overflow across the year unless it's drawn down between rain events. A bigger tank, or higher daily usage that keeps the tank from staying full, both reduce this overflow.

At 250 liters/day demand (91,250 L/year), 57,420 liters of harvest covers about 62.9% of annual demand — meaning rainwater alone isn't quite enough to fully replace mains or borewell supply here, though it meaningfully reduces reliance on it.

About 1,925 liters, from Daily Demand x Dry Days x (1 + Safety Factor) = 250 x 7 x 1.10. This is a separate figure from total annual harvest — it's sized specifically to cover a realistic dry spell, not to store the whole year's rainwater.

A concrete/terrace roof uses a 0.8 runoff coefficient by default — less efficient than a metal roof (0.9) but more efficient than a paved area (0.7) or gravel surface (0.3). Check the Runoff Coefficient by Catchment Surface table below and adjust if your actual roof differs.

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