Water
Rain Barrel and Water Harvest Calculator
Work out how many gallons or litres your roof can collect per storm and per year, and how many barrels you need.
Water harvest estimate
Size storage to a typical storm, then add overflow routing or extra barrels for larger events.
How roof runoff turns into usable water
Rain falling on a roof doesn't disappear — it sheets off the surface, funnels into gutters, and drops down a downspout, where a barrel or cistern can catch it before it soaks into the ground. The volume you can capture depends on three things: the flat footprint of your roof (not the sloped surface area, since rain falls vertically), how much rain falls in a given storm, and how efficiently your gutter and downspout system channels that water into storage rather than losing it to overshoot, evaporation, or absorption.
The core conversion is simple: one inch of rain on 1,000 square feet of flat roof footprint produces roughly 623 gallons of runoff. Multiply your roof's length by width to get square footage, multiply by the rainfall depth in inches, multiply by 0.623, and then apply an efficiency factor for your roofing material and gutter condition to get a realistic collection estimate rather than a theoretical maximum.
Why roofing material changes your yield
| Roof type | Typical collection efficiency | Why |
|---|---|---|
| Metal | 85-95% | Smooth, low friction, sheds water fast with minimal absorption |
| Asphalt shingle | 80-90% | Slightly textured surface causes minor splash and evaporation loss |
| Tile | 75-85% | Overlapping tiles create small gaps and channels that redirect some flow |
| Wood shake | 65-75% | Porous wood absorbs water before it can run off |
Sizing your storage to real storms
A common mistake is sizing rain barrels to annual rainfall totals, which vastly overstates what a single barrel needs to hold. In practice, a barrel fills and overflows within one storm, and any capacity beyond that storm's yield is only useful if you're actively using water between rain events. Instead, size your storage — or your number of linked barrels — to a typical storm for your area, and plan for overflow routing (a French drain, rain garden, or simply directing overflow away from the foundation) for the larger storms an annual total implies.
What harvested rainwater is good for
- Watering ornamental beds, containers, and lawns without drawing on municipal supply.
- Topping up ponds or water features, since rainwater lacks the chlorine of tap water.
- Washing tools, pots, and outdoor furniture.
- Supplementing drip irrigation systems on a gravity-fed or pumped basis.
- Reducing stormwater runoff and erosion around your foundation during heavy rain.
Connecting barrels and managing overflow
Multiple barrels can be linked at the base with a bulkhead fitting and short length of hose, allowing water to flow between them so a single downspout fills several barrels in sequence rather than overflowing the first one. Always include an overflow outlet near the top of the highest barrel, directed at least several feet from your foundation, so storms bigger than your storage capacity don't pool water against the house.
How much water can I really collect from my roof?
As a rule of thumb, one inch of rain falling on 1,000 square feet of roof yields about 623 gallons of runoff before losses. Multiply your roof's footprint (not the sloped surface area) by rainfall and by a collection efficiency factor, typically 75-90%, to account for splash, evaporation, and first-flush diversion.
Why isn't collection 100% efficient?
Some water is lost to overshoot at gutters during heavy storms, evaporation off hot roofing material, absorption by porous roofing like wood shakes, and any first-flush diverter you use to route the dirtiest initial runoff away from the barrel.
How many barrels do I actually need?
Size storage to your typical storm, not your annual total, since a single 55-gallon barrel fills fast and any rain beyond its capacity overflows unless you have a second barrel or overflow diversion. Compare the water from an average storm event to your total storage capacity.
Is harvested rainwater safe to use on vegetables?
Rainwater from an asphalt shingle roof is commonly used for ornamental beds and irrigation of established plants, but many gardeners avoid direct contact with edible leafy greens close to harvest, and check local regulations since some regions restrict rainwater harvesting or require first-flush diverters and mosquito screening.
Last reviewed February 2026