Rainfall Calculator - Volume, Weight and Runoff
Enter your catchment area and rainfall depth to find how much water lands on that surface. Pick a surface type to apply a realistic runoff coefficient, and get the collectable volume in litres or gallons along with the water weight. You can also reverse-solve: enter a target volume and rainfall depth to find the area you need. Switch between metric and imperial units at any time.
Formula
Worked example
A tile roof (C = 0.90) measuring 10 m x 8 m = 80 m2 receives 25 mm of rain: gross volume = 80 x 0.025 = 2.0 m3 = 2000 L. Collectable volume = 0.90 x 2000 = 1800 L, weighing about 1796 kg.
How the rainfall volume formula works
The basic formula is straightforward: Volume = Area x Depth. If your roof covers 80 square metres and receives 25 mm of rain, 0.080 cubic metres of rain falls on each square metre, giving a gross total of 2.0 m3 (2000 litres). The same idea applies in imperial units: multiply the catchment area in square feet by the rainfall depth in feet (divide inches by 12 first), then multiply by 7.481 to convert cubic feet to US gallons. The result is the theoretical maximum - the gross volume before any losses.
What the runoff coefficient means
Not all rain that hits a surface reaches your tank. Some is lost to splash, evaporation, initial wetting of the surface, and for soft surfaces such as lawns, absorption into the ground. The runoff coefficient (C) represents the fraction that actually runs off. A metal or glazed tile roof has C around 0.90 to 0.95 because almost all water runs to the gutter. A flat lawn has C around 0.20 to 0.25 because most soaks in. The Rational Method formula Q = C x I x A is the standard engineering approach for estimating peak runoff, and the same coefficient applies here for volume. Choosing the correct surface type in this calculator applies the right coefficient automatically.
Rainwater harvesting and tank sizing
The collectable volume figure is the starting point for sizing a rainwater harvesting system. Compare it against your tank capacity: if a single storm event produces 3000 litres but your tank holds only 1000 litres, the excess will overflow. Many designers size tanks to handle the 90th percentile storm event for their region, or use the mean monthly rainfall multiplied by the catchment area to estimate annual yield. Once you have your target volume, use the reverse-solve mode in this calculator (Solve for: Area) to find how large a roof you need to collect it from a given rainfall depth.
Converting between volume units
One cubic metre of water equals exactly 1000 litres or 264.2 US gallons. One US gallon is 3.785 litres. One inch of rain on one square foot equals about 0.6234 gallons. These conversions are applied automatically when you toggle the unit switch. The weight of water is approximately 1 kg per litre (more precisely 0.998 kg/L at 20 degrees Celsius), or about 8.34 lb per US gallon. Knowing the weight is important for structural loading: a 1000-litre tank full of water adds roughly 1000 kg (one tonne) to whatever supports it.
Typical runoff coefficients by surface type
| Surface type | Runoff coefficient (C) | Typical use |
|---|---|---|
| Metal roof | 0.95 | Best for rainwater harvesting |
| Tile / concrete roof | 0.90 | Common residential rooftops |
| Asphalt / pavement | 0.90 | Driveways, parking lots |
| Concrete | 0.90 | Paths, hardstand areas |
| Shingle roof | 0.85 | Standard roofing |
| Compacted gravel | 0.75 | Gravel roads, car parks |
| Steep lawn / slope | 0.35 | Slopes > 2% gradient |
| Agricultural land | 0.30 | Crops, ploughed fields |
| Flat lawn / grass | 0.25 | Parks, flat gardens |
| Woodland / forest | 0.18 | Dense canopy, deep litter |
Coefficients from standard engineering hydrology guidance (Rational Method). Higher values mean more water runs off the surface.
Frequently asked questions
How do I calculate the volume of rainfall from my roof?
Multiply your roof area in square metres by the rainfall depth in millimetres, then divide by 1000 to convert to cubic metres (or leave in litres: area in m2 x depth in mm = litres). Apply your surface runoff coefficient to get the realistic collectable volume. For example, a 50 m2 roof and 20 mm of rain gives 1000 L gross; with a tile roof coefficient of 0.90, you can expect to collect about 900 L.
What is a runoff coefficient and why does it matter?
The runoff coefficient (C) is a dimensionless number between 0 and 1 that represents the fraction of rainfall that actually runs off a surface rather than soaking in or evaporating. A concrete driveway has a coefficient close to 0.90 because almost all water runs off, while a grassy lawn might have 0.20 to 0.35 because most water soaks in. Using the right coefficient gives you a realistic estimate of how much water you can actually collect.
How do I convert millimetres of rain to litres?
Multiply the area of your catchment in square metres by the rainfall depth in millimetres. The result is directly in litres. For example, 10 mm of rain on a 100 m2 surface: 100 x 10 = 1000 litres. This works because 1 mm of depth over 1 m2 equals exactly 1 litre.
Can I use this calculator for storm drain or flood sizing?
This calculator gives you the total volume of water from a rainfall event. For drain sizing you also need rainfall intensity (mm/hour or in/hour) and the time of concentration, which determines peak flow rate rather than total volume. The Rational Method formula Q = C x I x A uses the same runoff coefficient but works with flow rate units. Use this calculator for storage sizing, and consult a hydraulic engineer or a rational method calculator for pipe and drain design.
How do I work out what catchment area I need?
Switch the "Solve for" input to "Area", enter your target volume (how much water you want to collect) and the expected rainfall depth, then choose your surface type. The calculator back-calculates the minimum catchment area required. For instance, to collect 2000 litres from a 25 mm rain event with a tile roof (C = 0.90), you need about 88.9 m2 of roof.
How much does collected rainwater weigh?
Water weighs approximately 1 kg per litre (or 8.34 lb per US gallon) at typical temperatures. The calculator shows the weight of the collectable volume, which you should check against the load rating of any tank, stand, or gutter system. A 1000-litre tank full of water adds roughly 1 tonne of load, which requires adequate structural support.