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DIY Cost Tools

DIY Cost Tools

Rain Barrel Fill Time Calculator

Estimate time to fill a rain barrel during an assumed steady rainfall rate.

Number from 1 to 100000 sq ft.
Number from 0.001 to 20 in/hour.
Number from 1 to 100 %.
Number from 0.1 to 100000 gal.

Enter values in the units shown. Empty fields, fractions, comma-formatted numbers, values outside the listed ranges, and non-finite values are rejected.

Capture rate
Fill time

How to use this calculator

  1. Measure the project or enter costs from a current quote or product label.
  2. Adjust the quantities, coverage, package size, and allowances to match your situation.
  3. Select Calculate, then check the method, worked example, and limitations below.

Formula and method

Captured gallons/hour = roof area × rainfall inches/hour ÷ 12 × 7.48052 × efficiency%. Fill time = remaining gallons ÷ capture rate.

Whole packages and pieces are rounded up. Results disappear when an input changes until the new values are calculated.

Worked example

Roof catchment area 600 sq ft; Rainfall rate 0.2 in/hour; Collection efficiency 80 %; Empty barrel capacity 55 gal. Results: Capture rate 59.84 gal/hour; Fill time 0.92 hours.

The example uses the values initially displayed in the fields.

Limitations

Rainfall rarely remains steady; first-flush losses, gutters, overflow, and barrel connections change actual fill time.

Review and reference

Reviewed October 5, 2026. The default example, input boundaries, arithmetic, rounding, and invalid input behavior were checked. The method is explained on How Our Calculators Work; replace example inputs with measurements or current product specifications. This is a planning estimate.

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