Rainwater Tank Size Calculator(Roof Catchment, Rainfall & Tank Storage)
Estimate rainwater collection and tank storage.
🕒 Last updated: April 25, 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 60,996 litres of rainwater per year.
Effective capture is 67.8% after runoff, first flush, and system efficiency losses.
This can cover 55.7% of your annual water demand.
Inputs Used
Catchment Surface
Concrete roof / terrace
Catchment Area
120 m2
Annual Rainfall
750 mm
Runoff Coefficient
0.8
System Efficiency
85%
First Flush Depth
2 mm
Other Conversions
Cubic Meters
60.996 m3
US Gallons
16,113.4 US gal
Imperial Gallons
13,417.2 imp gal
Cubic Feet
2,154.1 ft3
Current Tank & Demand Planning
Entered Tank Capacity
6,000 L
Max Stored at One Time
6,000 L
Limited by the entered tank capacity.
Annual Overflow With This Tank
⚠️ 54,996 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
1,09,500 L
Potential Demand Coverage
55.7%
Based on total annual harvest, before detailed seasonal timing.
Backup Water Still Needed
48,504 L
Supply From Full Entered Tank
20 days
At 300 L/day demand.
Recommended Backup Tank
3,500 L
Sized for 10 dry days plus 10% safety. Calculated need: 3,300 L. This is not sized to store all annual rainfall.
Loss Breakdown
Gross rainfall volume is 90,000 L. Runoff capture is 72,000 L, with 240 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.
Popular rainwater harvesting calculator examples
Rainwater tank sizing
This page emphasizes storage sizing from demand and dry-day reserve.
This page is pre-filled for this rainwater harvesting use case. Change any input and the calculator results update from the active values.
- Daily demand: 300 liters.
- Dry days: 10.
- Tank capacity: 6000 liters.
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 meters and rainfall to millimeters. Since 1 mm of rain over 1 m2 equals 1 liter, the water quantity is simple to estimate.
Step 1 - Calculate gross rainfall volume
Liters = m2 x 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 x Days in Period
Demand Coverage = Net Harvestable Water / Period Demand x 100
Backup Tank Need = Daily Demand x Dry Days x 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.
Calculation example for Rainwater Tank Size Calculator
This example uses the active catchment area, rainfall, runoff coefficient, first flush loss, system efficiency, tank capacity, and demand inputs from this programmatic calculator page.
- Catchment Area = 120 m2
- Rainfall = 750 mm
- Surface Type = Concrete roof / terrace
- System Efficiency = 85%
- Tank Capacity = 6000 liters
Step 1 - Calculate gross rainfall volume
Gross Rainfall = Area x Rainfall
Gross Rainfall Volume = 90,000 liters
Step 2 - Apply runoff coefficient and first flush
Runoff Coefficient = 0.8
Runoff Captured = 72,000 liters
First Flush Loss = 240 liters
Step 3 - Apply system efficiency
System Efficiency = 85%
Net Harvest = 60,996 liters
Effective Capture = 67.77%
Step 4 - Compare tank storage and demand
Stored by Tank = 6,000 liters
Overflow = 54,996 liters
Days of Supply = 20 days
Recommended Tank = 3,500 liters
For this page, the active inputs estimate 60,996 liters of net harvest and 6,000 liters stored by the tank.
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 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.