Rainfall is measured as a depth – the height of water that would stand on flat ground if none soaked in, ran off or evaporated. Turning that depth into a volume is simple: multiply by the area. One millimetre of rain on one square metre is exactly one litre, and one inch of rain on an acre is about 27,154 US gallons, which surprises most people the first time they work it out.
The calculation matters for rainwater harvesting (how big a tank does my roof fill?), for drainage and soakaways, for irrigation planning and for understanding flood volumes. Not all the water can be captured: a pitched metal roof sheds about 90–95% of the rain into the gutters, a tiled roof around 80–90%, while a lawn absorbs most of it. The runoff coefficient accounts for this.
Enter the area directly, as length × width, or as the radius of a round catchment, and the rainfall in millimetres or inches. The reverse mode tells you how much area you need to catch a given volume from a given rainfall.
How to use the Rainfall Volume Calculator
- Calculate
- Area given as
- Catchment area
- Length
- Width
- Radius
- Water volume wanted
- Rainfall depth
- Surface (runoff coefficient)
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
The calculator multiplies area by depth in base units (m² × m = m³), applies the runoff coefficient for the collectable share, and converts to litres, gallons and tonnes of water (1 m³ of water weighs about 1 tonne).
Worked example
A 50 m² backyard receives 5 cm (50 mm) of rain: V = 50 m² × 0.05 m = 2.5 m³ = 2,500 litres (660 US gallons), weighing 2.5 tonnes – enough for about 16.7 bathtubs. To collect 1,000 L from a 50 mm rain with a metal roof (C = 0.95) you need 1 ÷ (0.05 × 0.95) = 21.1 m² of roof.
Water from 1 inch or 25 mm of rain
| Area | Volume |
|---|---|
| 1 m² | 25.4 L |
| 100 ft² (9.29 m²) | 236 L (62.3 US gal) |
| 1,000 ft² roof | 2,360 L (623 US gal) |
| 1 acre | 102.8 m³ (27,154 US gal) |
| 1 hectare | 254 m³ |
Tips
- Use decimal degrees (negative for south and west) or the degrees–minutes–seconds fields.
- Great-circle distances assume a spherical Earth (error under 0.5%).
- Every field has its own unit menu.
- Everything is calculated in your browser.
Common mistakes to avoid
- Swapping latitude and longitude.
- Forgetting the minus sign for southern and western coordinates.
- Mixing magnetic and true north for bearings.
Inputs explained
| Field | What to enter |
|---|---|
| Calculate | Choose one: Rainfall volume from area and depth, Catchment area needed for a volume |
| Area given as | Choose one: Area, Length × width, Circle radius |
| Catchment area | Units: m², ft², yd², acre, ha, km² |
| Length | Units: m, ft, yd, cm, km |
| Width | Units: m, ft, yd, cm, km |
| Radius | Units: m, ft, yd, cm, km |
| Water volume wanted | Units: L, m³, US gal, UK gal, ft³, acre-ft |
| Rainfall depth | Units: mm, in, cm, m |
| Surface (runoff coefficient) | Choose one: All the rain (C = 1.00), Metal roof (C = 0.95), Tile or slate roof (C = 0.85), Concrete or asphalt (C = 0.85) … |
What each result means
- Rainfall volume = [A]×[Rainfall depth]
- Catchment area needed = [A]
- Water you can collect = [A]×[Rainfall depth]×[Surface (runoff coefficient)]
- Mass of the rain = [A]×[Rainfall depth]×1000
- Water per square metre = [Rainfall depth]×1000
- Bathtubs filled (150 L each) = [A]×[Rainfall depth]×[Surface (runoff coefficient)]÷0.15
- Area used = [A]
Formula
Volume V = rainfall depth h × area A · Collectable water = V × runoff coefficient C · Area needed A = V ÷ (h × C) · 1 mm of rain on 1 m² = 1 litre
Worked example
Inputs — Calculate: Rainfall volume from area and depth; Area given as: Area; Catchment area: 50 m²; Rainfall depth: 50 mm; Surface (runoff coefficient): All the rain (C = 1.00).
Results — Rainfall volume = 2,500 L; Water you can collect = 2,500 L; Mass of the rain = 2,500 kg; Water per square metre = 50 L/m²; Bathtubs filled (150 L each) = 16.7; Area used = 50 m².
Key terms
- Rainfall depth
- Height of water that rain would leave on flat, impermeable ground.
- Catchment area
- The horizontal area that collects the rain.
- Runoff coefficient
- Fraction of rainfall that flows off a surface.
Frequently asked questions
How much water is 1 mm of rain?
One litre on every square metre, or 1,000 m³ (one million litres) on a square kilometre.
How much water falls on a roof?
Multiply the roof's footprint by the rainfall: 50 m² × 50 mm = 2,500 L, of which a metal roof sheds about 2,375 L into the gutters.
How many gallons is an inch of rain on an acre?
About 27,154 US gallons (102.8 m³).
Do I use the roof's sloped area or its footprint?
The footprint (the area seen from above) – rain falls vertically, so the slope does not add catchment.
How big should my rainwater tank be?
Size it for your biggest regular rainfall event or for a few weeks of use; this calculator tells you how much a single rain event delivers.
What is a runoff coefficient?
The share of rain that runs off a surface rather than soaking in or evaporating: about 0.9 for roofs and 0.1–0.3 for lawns.