How much rainwater from a roof in a 1-inch storm?

Homeowners usually ask a simpler question than engineers do: how many gallons come off my roof when it rains an inch? You do not need a full cistern design to get a useful ballpark. You need footprint area, the 0.623 rule, and an honest efficiency guess. AsphaltTons publishes that math in the rainfall roof runoff calculator so you can size barrels — and keep overflow off the asphalt driveway.

Try the roof rainwater calculator → Driveway runoff gallons →

The one-inch mental model

Every square foot of horizontal roof footprint collects about 0.623 gallons when one inch of rain falls. A 20×30 ft section (600 ft²) therefore has a theoretical 600 × 0.623 = 373.8 gallons before losses. A 1,200 ft² footprint sketches to about 747.6 gallons. Those are potential gallons at perfect capture — real systems miss some.

Multiply by your own footprint: measure the building outline on the ground (or use plans), not the sloping roof surface area. Runoff volume for this purpose tracks the horizontal catch, which is why length × width of the footprint is what the calculator asks for.

Efficiency and first flush, in plain language

Gutters leak at joints. Screens clog. Wind blows rain past the eaves. Many DIY systems intentionally dump the dirty “first flush” so barrels stay cleaner. Together those effects mean 70–90% of theoretical volume is a common homeowner planning band; our calculator defaults to 85% and lets you move the slider.

Example: 1,000 ft² × 1″ × 0.623 × 0.85 = 529.55 gallons after efficiency. That is still more than two typical 55-gallon barrels. If you only have one barrel, most of a solid one-inch storm becomes overflow — by design of the storage, not a bug in the math.

When a barrel overflows (and why asphalt cares)

Overflow starts the moment stored volume hits tank capacity. If after-efficiency storm volume is 530 gallons and the tank is 60 gallons, roughly 470 gallons still leave the outlet. Where that water lands matters for asphalt: a splash pad beside the drive keeps the shoulder wet and can feed soft spots — see soft spots. Aim overflow to a garden, dry well (where allowed), or a designed drain, not the pavement edge.

Use the optional tank field on the roof calculator to see captured vs overflow for your numbers. Pair with pavement volume on the driveway rainfall runoff calculator if both house and drive wet the same corner.

Quick checklist before you buy storage

• Measure footprint ft² (or L×W) for the roof section that actually feeds the downspout you care about.
• Assume a 1″ planning storm, then scale for heavier local events.
• Apply an efficiency below 100%; start near 85% if unsure.
• Compare after-efficiency gallons to barrel/cistern capacity — expect overflow on big storms.
• Plan the overflow path away from asphalt edges before the first fill.

Disclaimer

Planning estimate only. Uses area × rainfall inches × 0.623 × a runoff/collection factor. Not a licensed engineering, stormwater, plumbing, or potable-water design. Local codes, soil, slope, first-flush, and system losses vary — confirm with a qualified pro before installing drainage or rainwater systems. Rainwater for drinking requires proper treatment and may be restricted by local rules.

More tool detail: rainfall roof runoff calculator guide. Drainage angle for the drive: asphalt driveway rain runoff & drainage.

Roof gallons calculator → Driveway runoff calculator → Browse driveway shop →