Rainwater Collection Calculator
Harvest gallons from roof area, rainfall, and efficiency.
What it does
Rain harvest multiplies roof area by rainfall depth: each inch on each square foot holds 0.623 gallons before gutters take their cut. Efficiency below 100% accounts for splash, leaks, and first flush diverters.
The tool above takes roof area with rainfall and capture efficiency in imperial or metric. It returns gallons with liters and 55 gallon barrels beside them.
The default 1000 ft² with 1 inch at 95% collects 592 gallons (2,240 L, 10.8 barrels). The metric mirror of 92.9 m² with 25.4 mm reads the same 592 gallons with 2,242 liters.
The 0.623 factor follows extension rainwater guides such as Kentucky with DOE federal harvesting references behind them. Both apply an efficiency below unity for real gutters.
How to use
- Pick imperial or metric for roof and rain.
- Enter the roof footprint area draining to gutters.
- Enter the rainfall depth for the event or month.
- Set efficiency, 95% for tight gutters.
- Read gallons first, then liters and barrels.
Formula
- 0.623
- Gallons per ft² per inch of rain.
- 1:1
- One mm over one m² is exactly one liter.
- eff
- Share surviving splash and diverters.
Footprint area, not sloped shingle area, since rain falls vertically. Monthly rainfall totals project monthly harvest the same way as single storms.
Worked example
Example: 1000 ft², 1 inch, 95%
Imperial defaults: 1000 ft², 1 in, 95%.
- Raw: 1000 x 1 x 0.623 = 623 gal.
- Captured: 623 x 0.95 = 591.85, shown 592 gal.
- Liters: 591.85 x 3.78541 = 2,240 L.
- Barrels: 591.85 / 55 = 10.8 barrels.
- Tool shows 592 gal, 2,240 L, 10.8 barrels.
Reference table
| Rain | Gallons | Barrels |
|---|---|---|
| 0.5 in | 296 gal | 5.4 barrels |
| 1 in | 592 gal | 10.8 barrels |
| 2 in | 1,184 gal | 21.5 barrels |
| 4 in | 2,367 gal | 43 barrels |
Limits
- Volume only; potability needs treatment.
- Footprint area assumed, not slope surface.
- Storm intensity can overwhelm gutters.
- Storage sizing needs monthly demand beside supply.
Common mistakes
- Measuring sloped roof surface instead of footprint.
- Forgetting efficiency and planning 100% capture.
- Mixing inches with feet without converting.
- Sizing barrels to one storm instead of dry months.
- Drinking untreated roof runoff.
Try it yourself
Quick checks. Answers sit tucked under each question.
1500 ft², 2 in, 95%?
1500 x 2 x 0.623 x 0.95 = 1,776 gal.
Barrels for 1776 gal?
1776 / 55 = 32.3 barrels.
Metric: 92.9 m², 25.4 mm, 95%?
92.9 x 25.4 x 0.95 = 2,242 L.
Questions
How much rain from my roof?
ft² times inches times 0.623 times efficiency: 1000 ft² at 1 in is 592 gal.
Why 0.623?
One inch over one square foot is 1/12 ft³, times 7.48 gal/ft³.
Metric shortcut?
m² times mm equals liters directly, times efficiency.
What efficiency?
95% for tight gutters; lower with leaks and big first flush.
Is it drinkable?
Not raw. Filter and disinfect to potable standards first.
How many barrels?
Divide gallons by 55 for standard drum count.