Rainwater Harvesting Calculator
Enter your roof area, the rainfall, and a runoff factor to see how many litres you can capture — because one millimetre of rain on one square metre is about one litre.
Plan your rain barrel
Enter your roof's plan area, the rainfall in millimetres, and a runoff coefficient to get the harvestable volume in litres.
Use the plan area
Measure the flat footprint your roof covers, not the sloped surface — rain falls on the horizontal area below the roof.
What is rainwater harvesting?
Turning roof runoff into stored water
A rainwater harvesting calculator turns three simple measurements — your roof area, the rainfall, and a runoff coefficient — into the volume of water you can collect, in litres. The idea rests on a neat physical fact: one millimetre of rain falling on one square metre of surface delivers about one litre of water. Multiply the roof's plan area by the depth of rain and you have the water that lands on it; multiply by a runoff coefficient and you account for the share that actually reaches your barrel or tank after splashing, soaking, and first-flush losses. It is the number behind sizing a rain barrel, planning garden irrigation, and judging how much mains water a downpour could save.
Enter your roof area in square metres, the rainfall in millimetres, and a runoff factor between 0 and 1 to get the collectable volume in litres instantly.
The collectable volume is the roof area multiplied by the rainfall and then by the runoff coefficient.
Litres = area (m²) × rainfall (mm) × runoffBecause one millimetre of rain on one square metre is roughly one litre, the area in square metres times the rainfall in millimetres gives the litres that land on the roof. The runoff coefficient — typically 0.8 to 0.9 for a hard roof — scales that down to what you actually capture after losses. Keep the area in square metres and the rainfall in millimetres and the result comes back in litres.
Suppose you have a 50 m² roof, a rain event drops 20 mm, and your system captures 85% of it (a runoff coefficient of 0.85).
Multiply area by rainfall
50 × 20 = 1000 — the litres that fall on the roof footprint.
Apply the runoff coefficient
1000 × 0.85 = 850 — the share that reaches your barrel after losses.
Read the result
You can collect about 850 L from that single 20 mm shower.
The figure is the litres you can realistically capture from one rain event, and it scales directly with all three inputs. Double the roof area or the rainfall and you double the harvest; a runoff coefficient of 1 would mean every drop is captured, while 0.85 reflects a typical hard-roof system that loses about 15% to splashing, evaporation, and the first-flush diverter. To estimate an annual yield, use your local annual rainfall total in place of a single shower's depth: a 50 m² roof under 800 mm of rain a year at 0.85 runoff gathers roughly 34,000 L. That number tells you how big a tank to buy, how much garden watering a wet month can cover, and how much mains water you could offset. Remember it is a planning estimate — a tank that is already full overflows, so storage capacity, not just roof size, often sets the real limit on what you keep.
The formula is simple, but a few practical points keep the estimate honest.
Roof material, first-flush losses, and plan area
The runoff coefficient depends on the roof: smooth metal and tile shed water efficiently (around 0.9), while rough, porous, or gently sloped surfaces lose more. The first-flush of a storm carries dust and debris and is often diverted away, which lowers the real yield. Crucially, use the plan (footprint) area — the flat area the roof covers when seen from above — not the longer sloped surface, because rain is measured as a vertical depth. This calculator estimates collectable volume only; it does not size gutters, downpipes, or account for a tank that overflows once full.