Formula
Gallons = roof area (sq ft) × rainfall (in) × 0.623. Example: a 1,000 sq ft roof in 1 inch of rain collects about 623 gallons — far more than one barrel holds.
How to use it, step by step
Measure the roof area that drains to your barrels — the footprint of the roof section, not the sloped surface area. Enter rainfall for a single storm or for a month, depending on what you want to know: a storm figure tells you barrel capacity needed, a monthly figure tells you total harvest. Divide the result by your barrel size to see how many barrels one storm fills.
Worked example in detail
A 1,000 sq ft roof in a 0.5-inch storm collects about 312 gallons, or about 6 × 55-gallon barrels. In a month with 3 inches of rain, the same roof collects about 1,869 gallons. Most gardeners cannot store a month's rain, so the practical question is usually how many barrels to place under each downspout, not total monthly harvest.
What affects real collection
Roof material, gutter condition, first-flush diverters and overflow all reduce collected water below the theoretical figure. A dirty gutter or a blocked downspout collects nothing, however large the roof. Water from roofs is fine for garden use, but do not use it for drinking without proper treatment, and keep barrels covered to prevent mosquitoes and algae.
Frequently asked questions
Is rainwater better than tap water for plants?
Usually yes — it is soft, chlorine-free and at ambient temperature. Plants watered with rainwater often look better than those watered with hard tap water, though the difference is modest for most vegetables.
How many barrels do I need?
Start with one per downspout you use, then add more if barrels overflow regularly in storms. Linked barrels share overflow and are more useful than separate barrels that fill unevenly.
Can I use rainwater in winter?
Yes, but barrels can freeze and crack. Drain or insulate barrels before hard frosts, and divert downspouts away from the house when barrels are disconnected.