Rainfall Calculator — Volume and Collection
Convert rainfall depth over an area into total water volume in gallons, litres and cubic metres.
Formula and sources updated · Data sources · 2View algorithm, constants and default inputs (JSON)
About this calculator
Multiply rainfall depth by catchment area to estimate total water volume. Choose the input units, then apply your own collection percentage. Gallons are US liquid gallons; the 55-gallon output is equivalent capacity and may include a partly filled barrel.
Worked example
calculated with the default values below
Rainfall depth 1 in · Catchment area 1,000 sq ft · Depth unit Inches · Area unit Square feet · Collection percentage 85 % Water volume 83.33 cu ft · Water volume 623.4 gal · Water volume 2,359.7 l · Water volume 2.360 m³ · Harvestable water 529.9 gal · Equivalent 55-US-gallon capacity 9.6
Method and assumptions
Convert depth to metres and area to square metres, then multiply them for cubic metres. One millimetre over one square metre equals one litre. One US liquid gallon is 3.785411784 litres; one square foot is 0.09290304 square metres, and an acre here is 43560 international square feet. Collected volume is total volume times the entered percentage divided by 100. The default 1 inch over 1000 square feet gives about 623.4 US gallons, or 529.9 at 85% collection. Divide that collected amount by 55 for equivalent barrel capacity; 9.6 means nine full barrels plus part of another, not ten full barrels. Depth accepts 0–100 in the selected unit, area 0.1–10000000 in its selected unit, and collection 0–100%. Changing a unit reinterprets the number; it does not convert the entered value.
Data sources
The collection percentage is a user assumption, not a site-specific runoff forecast. Storage limits, first-flush diversion, evaporation, infiltration and overflow are not modelled. Use this volume estimate as one input to planning; it does not size a drainage system.
Disclaimer
These calculators give general estimates. Sizing, pricing, availability and standards vary by brand, retailer and country, so treat the output as a starting point and confirm it against the seller's or authority's own figures.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
V = dₘ × Aₘ²; L = 1000V; G = L/3.785411784; H = Gp/100; B = H/55
- dₘ
- Rainfall depth in metres
- Aₘ²
- Catchment area in square metres
- V,L,G
- Total cubic metres, litres and US liquid gallons
- p
- Entered collection percentage
- H
- Estimated collected US gallons
- B
- Equivalent 55-US-gallon capacity, including fractions
Convert inches with ×0.0254, millimetres with ×0.001 and centimetres with ×0.01 to metres. Convert square feet with ×0.09290304 and acres with ×4046.8564224 to square metres. Barrel capacity is a volume equivalent, not a count of full barrels.
Frequently asked questions
Are these US or imperial gallons?
All gallon outputs use US liquid gallons. One US gallon is 3.785411784 litres. The barrel row divides estimated collected US gallons by 55 and can include a partial barrel.
How does the collection percentage affect the result?
It multiplies total rainfall volume. At 0% collection the collected volume is zero; at 100% it equals the total. The default 85% is an editable assumption and does not account for tank capacity or local losses.
Does changing the unit convert my input?
No. Select the unit you intend for the displayed number, or replace that number with a converted measurement. For example, 1 millimetre over 1 square metre gives 1 litre; changing millimetres to inches makes that same entered 1 mean one inch.