Rainwater Roof Yield Calculator

The formula

Yield = roof footprint × rainfall × collection coefficient − (first-flush volume per event × number of events). In metric, 1 mm of rain on 1 m² is 1 litre, so there is no constant. In US units one inch on one square foot is 144 in³ and a US gallon is 231 in³, giving 0.6234 gallons per sq ft per inch. The Texas manual rounds this to 0.62 and says that in theory about 0.62 gal/sq ft/inch can be collected, but in practice some is lost to first flush, evaporation, splash-out, overshoot from gutters and leaks. Use the footprint (eave to eave and front to rear), not the sloped surface.

Worked example

A 150 m² footprint, 800 mm of rain in the period, coefficient 0.90 and a 10 gal per 1,000 sq ft first flush over 60 events. Rain on the roof: 150 × 800 = 120,000 L. After the coefficient: 108,000 L. First flush per event: 10 × 3.7854 L × (150 m² ÷ 92.903 m² per 1,000 sq ft) = 61.1 L, or 3,667 L over 60 events. Yield: about 104,333 L (104.3 m³, 27,562 US gal), 86.9% of the rain that fell. The 800 mm, 60 events and 0.90 are example inputs, not recommendations for your site.

Where the numbers come from

The Texas Water Development Board's Texas Manual on Rainwater Harvesting, 3rd edition (2005), states that most installers assume a collection efficiency of 75 to 90 percent, and uses 85 percent in its water-balance examples. It gives a first-flush rule of thumb of at least 10 gallons per 1,000 sq ft of collection surface, a recommended range of 1 to 2 gallons per 100 sq ft, and cites one vendor study recommending 13 to 49 gallons per 1,000 sq ft, noting there is no exact calculation. Clay tile is described as possibly adding up to a 10 percent loss. The manual folds first flush into its overall 75 to 90 percent range, so entering a separate first-flush loss on top of a low coefficient counts it twice. That is why the default coefficient here is 0.90; lower it for rough roofs. Collection figures are as published in 2005 and are US practice, so they are a starting point for Canadian roofs, not a code requirement.

For rainfall, open the Environment Canada climate normals, pick a station and copy its precipitation total for the period you want. Rain that falls as snow is not modelled. Rainwater is not drinking water; treatment, plumbing and local rules are your responsibility.

Source: Texas Water Development Board, Texas Manual on Rainwater Harvesting, 3rd ed., 2005 (chapters on first-flush diverters, the collection surface and water balance; checked 2026-09-30).

To size the tank itself, use the Rainwater Tank Sizing Calculator. Related: Rain Garden Size Calculator, Irrigation Runtime & Water Use, Pond Volume, Liner & Pump.

FAQ

Why is the footprint used and not the roof slope?

Rain falls on the horizontal area, so a steeper roof does not collect more. The manual specifies the footprint for this reason.

How is this different from the tank sizing calculator?

That tool estimates annual harvest from one lumped efficiency and sizes storage for a dry spell. This one separates the coefficient from first-flush loss per event and shows each step.

What if the first flush is bigger than a light rain?

The loss is subtracted per event as a fixed volume, so very light events overstate the loss. If the total exceeds what the roof collects, the result is shown as zero.