Rainwater barrel calculator
Enter the roof area, the rain that fell or is forecast and how efficiently the roof sheds water. The calculator returns the volume that reaches the gutter and how many barrels of your size it would fill.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Pick your units and enter the roof footprint area.
- Enter the rainfall in mm or inches, the runoff coefficient and the size of one barrel.
- Read the volume collected and how many barrels it fills.
How it works
Litres = roof area (m²) × rainfall (mm) × runoff coefficient. In US units the area is converted from ft² and the rain from inches to the same metric basis; gallons = litres ÷ 3.785411784. Barrels = volume ÷ barrel size, rounded up.
Worked example
A 100 m² roof, 10 mm of rain and 90% runoff: 100 × 10 × 0.9 = 900 L, or 238 US gallons. With 200 L barrels that is 4.5, so 5 barrels.
Tips
- Check the local rainfall for the month, not the year, when sizing storage.
- Fit an overflow pipe to a drain or garden so the barrel cannot flood.
- A first-flush diverter keeps the dirtiest first water out of the barrel.
Frequently asked questions
How much water does a roof collect?
1 mm of rain on 1 square metre is 1 litre. In US units, 1 inch of rain on 1,000 square feet is about 623 gallons. Multiply by the runoff coefficient for the share that reaches the barrel.
What is the runoff coefficient?
The share of rain that actually leaves the roof through the gutter. The rest evaporates, soaks into the surface, splashes away or overflows. Smooth, sloped roofs are usually higher than rough or flat ones; the default is 90%, so adjust it for your roof.
Which roof area should I enter?
The footprint seen from above, which is what the rain falls on, not the longer sloped surface. For a gable roof, multiply length by width of the building including overhangs.
Is rainwater from a roof safe to drink?
No. This tool only estimates volume. Roof water collects dirt, droppings and roofing residues; barrels are normally used for watering plants, not drinking or cooking.