Rain Barrel Size Calculator
Calculate how much rainwater a roof can collect and what barrel size makes sense.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
Estimate roof runoff using catchment area, rainfall and a user-selected loss factor. Annual potential assumes the entered rainfall and loss percentage; it does not account for roof material, local distribution, storage limits or first-flush design details.
Worked example
calculated with the default values below
Roof length draining to the downspout 40 ft · Roof width draining to the downspout 25 ft · Rainfall per storm 1 in · Annual rainfall 35 in · Loss to splashing and first flush 15 % · Barrel size 55 gal Roof catchment area 1,000 sq ft · Water per inch of rain 529.6 gal · Water from this storm 529.6 gal · Annual collection potential 18,534 gal · Barrels filled by this storm 9.6 · Note Most barrels overflow — plan an overflow route away from the house foundation
General background references
Disclaimer
These are general gardening estimates. Planting windows, growth, water and nutrient needs vary by place and plant. Check local extension guidance and product labels before acting on an estimate.
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
G = L×W×0.623×(1−q÷100)×r; B = G÷V
- L
- Roof length (ft)
- W
- Roof width (ft)
- q
- Loss factor (%)
- r
- Rainfall (in)
- G
- Gallons collected
- V
- Barrel size (gal)
- B
- Barrels filled
0.623 gal per ft² per inch of rain. B = barrels filled per storm; annual potential uses annual rainfall instead of r.
Frequently asked questions
Why does one inch of rain fill more than nine barrels here?
Catchment is roof area times 0.623 gallons per square foot per inch, then cut by the splashing loss, so 1,000 sq ft at 15% loss yields about 530 gallons per inch. A 1 inch storm therefore fills roughly 9.6 of the default 55 gallon barrels, and the 35 inch annual rainfall implies about 18,500 gallons a year if every drop were caught.