How much rainwater from 25 mm of rain on a 100 m² roof?
25 mm of rain on a 100 m² roof is 2,500 litres of water. A roof that sheds 90% of it, less a 40 litre first flush, yields about 2,210 litres, enough to fill 12 barrels of 200 L.
How to work it out
- 100 m² × 25 mm = 2,500 L (1 mm on 1 m² = 1 L)
- × 0.90 (asphalt shingles) = 2,250 L − 40 L first flush (40 L per 100 m²) = 2,210 L
- 2,210 L ÷ 200 L = 12 barrels
Metric makes this easy: 1 mm of rain on 1 m² is exactly one litre, so 100 m² and 25 mm is 2,500 litres before losses. Twenty-five millimetres is a properly wet day in most temperate climates; lighter rain is far more common.
Adjust for your project
More options: First flush · Storms · Use
- Likely collected
- 2,210 L
- Barrels to hold it all
- 12 barrels
- Rain needed to fill one barrel
- 2.7 mm
- A single barrel fills in a light rain — plan an overflow that carries water away from the foundation.
- Rules on collecting rainwater differ by state and country, and some places regulate volume or use. Check yours before installing a large system.
Rainwater from 25 mm of rain, by roof area
| Roof area | Rain falling on that roof | Likely collected | Barrels to hold it all |
|---|---|---|---|
| 25 m² | 625 L | 553 L | 3 barrels |
| 50 m² | 1,250 L | 1,105 L | 6 barrels |
| 75 m² | 1,875 L | 1,658 L | 9 barrels |
| 100 m² | 2,500 L | 2,210 L | 12 barrels |
| 150 m² | 3,750 L | 3,315 L | 17 barrels |
What to know about 25 mm on a 100 m² roof
- Water butt sizes. Garden water butts are commonly sold from about 100 to 300 litres. With the whole 100 m² roof feeding one 200 L butt, it is full after about 2.7 mm of rain, counting the first flush.
- Per downpipe. Most houses split the roof between two to four downpipes. A butt on a downpipe draining 25 m² fills with about 9 mm of rain; on 50 m², about 5 mm.
- Roof type. Concrete and clay tiles, the usual UK roof, are rougher than metal and shed a little less; pick your roof in the calculator. Planted green roofs on sheds and extensions hold back half or more of the rain.
- Weight. A litre of water weighs a kilogram, so a full 200 L butt is around 200 kg on its stand. Set it on paving slabs or compacted gravel, not bare soil.
- Linking butts. Two or three butts joined at the base (or overflow to overflow) store 400–600 L, enough to catch a good share of a moderate day’s rain from one downpipe.
In US units, the same idea for a year of rain is How much rainwater a 2,000 sq ft roof collects in a year, and per storm, How much water from 1 inch of rain on a 1,000 sq ft roof.
Different size? Use the full Rain Barrel Calculator.
What you’ll need
- Rain barrel (50–65 gal)Mid-range price
A lidded barrel with a screened inlet, spigot and overflow. Linkable models make it easy to add a second one later.
See options on Amazon (opens in a new tab) - Rain barrel linking kitBudget price
Hoses and fittings to join barrels side by side, so overflow from the first fills the next instead of running off.
See options on Amazon (opens in a new tab) - Downspout diverterBudget price
Fits into the downspout and sends water to the barrel until it is full, then back down the spout.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How much rainwater from 10 mm of rain on a 100 m² roof?
1,000 litres falls on the roof; about 860 litres can be collected after roof losses and the first flush.
How much rainwater does a 50 m² roof collect from 25 mm of rain?
1,250 litres in total, about 1,105 litres after losses: 6 barrels of 200 L.
How much rain fills a 200 litre water butt from a 100 m² roof?
Only about 2.7 mm of rain, including the 40 litres a first-flush diverter takes.
Other common sizes
Sources
- Clemson Cooperative Extension (HGIC) — Designing for Rain Barrel Success (opens in a new tab) — Roof area × 0.623 = gallons from a 1 in rain; screen the inlet; emergency overflow away from the foundation; barrels raised 12–36 in.
- Clemson Cooperative Extension (HGIC) — Best Practices for Application of Harvested Rainwater on Edibles (opens in a new tab) — Water the soil, not the leaves; not on harvest day; wash produce in potable water; first-flush diversion.
- Texas Water Development Board — The Texas Manual on Rainwater Harvesting (3rd ed.) (opens in a new tab) — First flush: divert a minimum of 10 gallons per 1,000 sq ft of collection surface (1–2 gal per 100 sq ft); about 0.62 gal per sq ft per inch in theory; example uses 85% collection efficiency.
- New Mexico Office of the State Engineer — Roof-Reliant Landscaping, Ch. 7: Water Collection (opens in a new tab) — Runoff coefficients: metal 0.95, asphalt 0.90, concrete 0.90, tar and gravel 0.80–0.85.
- Texas A&M AgriLife Extension — Rainwater Harvesting: System Planning (B-6240) (opens in a new tab) — Roof runoff coefficients 0.75–0.95 (smooth surfaces high, rough low); monthly supply vs demand planning.
- Minnesota Stormwater Manual — Overview for green roofs (opens in a new tab) — Green roofs with 3–6 in of media typically cut annual runoff by 50–70%.