How much rain can I collect from my roof?

The catchment formula, a worked example, and a reference table by roof size — so you size your storage to your roof instead of guessing.

Before you buy a single barrel, you should know roughly how much water your roof actually produces. It's almost always more than people expect, which is why a 50-gallon barrel overflows after one decent storm. The math is one multiplication, and it answers two questions at once: how much you could capture, and how much storage is worth buying.

Some links below are affiliate links — if you buy through them, Moon Dog may earn a small commission at no extra cost to you. We only point to gear we'd actually use.

The formula

One inch of rain falling on one square foot of surface produces about 0.623 gallons of water. That constant is the whole calculation — it converts inches of depth over an area into gallons. Most people round it to 0.62:

Gallons collected	=
Catchment area (sq ft) × Rainfall (in) × 0.62 × Efficiency	gallons

Two things to note. First, catchment area is the roof's footprint — the flat area it covers as seen from above, not the sloped surface area. A steeper roof doesn't catch more rain; it catches the rain that falls on the ground it shadows. Second, efficiency accounts for what you lose to splash, evaporation, gutter spill, and the first-flush diverter — a realistic figure is about 0.85 (85%). For a quick upper-bound estimate, leave efficiency out.

Worked example: a 1,000 sq ft roof, 1 inch of rain

Say your roof footprint is 1,000 square feet and you get 1 inch of rain:

Upper bound: 1,000 × 1 × 0.62 = 620 gallons
Realistic (85% efficient): 620 × 0.85 = ~527 gallons

So a single ordinary storm produces enough to fill ten 50-gallon barrels — and most of that just ran into the yard before you started. Now scale to a year: if your area gets 36 inches of rain annually, that same roof yields 1,000 × 36 × 0.62 × 0.85 ≈ 18,972 gallons a year. The constraint is never how much falls; it's how much you can store and use between storms.

Reference table (per 1 inch of rain, before efficiency)

Find your roof footprint, then multiply the row by your storm's rainfall in inches:

Roof footprint	Gallons per inch of rain
200 sq ft (a shed)	124
500 sq ft	310
800 sq ft	496
1,000 sq ft	620
1,500 sq ft	930
2,000 sq ft	1,240
2,500 sq ft	1,550

Apply your own efficiency (multiply by ~0.85) for a realistic figure. Half-inch storms produce half these numbers; a heavy two-inch storm, double.

Size storage to a useful window, not to the year.

You don't need to store a year's worth — you need enough to bridge dry spells between storms. A common rule of thumb is enough capacity to hold the rain from one or two typical storms, sized against how fast you'll draw it down. If the table says one storm gives you 600 gallons and you use 50 a week, two 275-gallon IBC totes is a sane target; ten barrels is not.

Gear this guide uses

Once you know your number, the setup follows from it — a barrel for small roofs, totes for big ones.

Small roof / starter: a 50-gallon rain barrel with a downspout diverter kit for automatic overflow.
Bigger roof / serious storage: a food-grade 275-gallon IBC tote, expandable with an IBC tote linking kit.
Measure rainfall yourself with a simple rain gauge so your numbers match your yard, not a regional average.

Sources

Innovative Water Solutions — rainwater harvesting formula and the 0.623 constant
Surfrider Foundation — catchment potential calculations
BlueBarrel Rainwater — sizing a rain catchment system

Estimates only. Actual yield depends on your real rainfall, gutter condition, roof material, and losses to first-flush and overflow. Use a rain gauge and a conservative efficiency factor before you commit to storage size.