Rainwater volume from a roof calculator

How much water comes off your roof in a storm? Footprint × inches of rain × 0.6234 × runoff — the number for rain-barrel and discharge planning.

Measure your eave runs and confirm capacity, spacing and coverage against the exact product you buy. Allow extra for corners, waste and slope (~5–10%). Sizes, capacities, spacings and panel lengths vary by product and brand — read the label and the manufacturer’s data.

Calculator

ft²
Plan-view area — length × width of the roofed footprint.
in
Inches of rain in the event you are sizing for.
Roof runoff is ~0.90–0.95; little soaks in.
Result
Rainwater volume947.6 gal
Per inch of rain947.6 gal/in
Roof footprint1,600 ft²
Runoff coefficient0.95

A 1,600 ft² roof sheds about 948 gallons in 1.00 inch(es) of rain (948 gal per inch). 1 inch over 1 ft² = 0.6234 US gallons; roof runoff is ~0.90–0.95. For rain-barrel or discharge planning only — NOT a stormwater-detention or storm-sewer design (that’s a pro / local code).

Catchment planning starts with one honest number: the gallons your roof actually sheds. It rides on the horizontal footprint, not the sloped surface — a steeper roof over the same footprint catches the same rain.

The constant 0.6234 is exact: one inch of rain over one square foot is 144 cubic inches, which is 0.6234 US gallons. Roofs shed almost all of it, so the runoff coefficient sits near 0.90–0.95. Multiply footprint by inches by 0.6234 by runoff and you have the volume for a rain barrel, a cistern first-flush, or a discharge sanity check.

This is planning, not stormwater engineering. Detention, retention and storm-sewer connections are code-governed and belong to a professional.

Formula

gallons = footprint_sqft × rainfall_in × 0.6234 × runoff_coeff

0.6234 US gal = 1 inch of rain over 1 ft² (144 in³). Runoff ~0.90–0.95 for a roof.

Worked example

1,600 ft² footprint, 1 inch of rain, runoff 0.95:

1,600 × 1 × 0.6234 × 0.95 = 947.6 gallons per inch.

A 2-inch storm doubles it to about 1,895 gallons — which is why one 55-gallon barrel overflows in the first few minutes. Size the overflow and the discharge, not just the barrel.

Reading the number right

Footprint, not slope area. Rain falls vertically; a 6/12 and a flat roof over the same footprint shed the same volume. (Pitch matters for gutter sizing, not for total gallons.)

Runoff isn’t 1.00. A little wets the surface and evaporates; 0.90–0.95 is realistic for shingles and metal.

Barrels overflow fast. Use the per-inch figure to plan overflow routing to a safe discharge 4–6 ft from the wall, then hand off to grading if it goes farther.

Not a design tool. Cisterns for real supply, detention basins and storm-sewer sizing are engineered to local code — this is a homeowner planning estimate.

Reference table

Gallons a roof sheds per inch of rain = footprint × 0.6234 × runoff. 1 inch over 1 ft² is exactly 144 in³ = 0.6234 US gallons; roof runoff is ~0.90–0.95.

Roof footprintGallons per inch of rain
800 ft²473.8 gal/in
1,200 ft²710.7 gal/in
1,600 ft²947.6 gal/in
2,000 ft²1,184.5 gal/in
2,500 ft²1,480.6 gal/in
3,000 ft²1,776.7 gal/in

Runoff coefficient 0.95 applied. Rain-barrel and discharge planning only — NOT a stormwater-detention or storm-sewer design.

Frequently asked questions

How many gallons of water come off my roof per inch of rain?
About 0.6234 gallons per square foot of footprint per inch, times the runoff coefficient. A 1,600 ft² roof sheds roughly 948 gallons per inch at 0.95 runoff.
Do I use the sloped roof area or the footprint?
The footprint — the plan-view length × width. Rain falls vertically, so slope doesn’t add volume. Slope only changes how fast the gutter must drain, which the gutter size calculator handles.
What runoff coefficient should I use?
Around 0.90–0.95 for a typical roof — nearly everything runs off. Lower it slightly for very rough or absorptive surfaces.
Can I size a stormwater system with this?
No. This is rain-barrel and discharge planning only. Detention, retention and storm-sewer connections are engineered to local code by a professional.

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