# Rainwater harvesting calculator

> Rainwater harvesting calculator: yearly litres or gallons a roof collects from its area, rainfall and efficiency, and the share of household use it covers.

Interactive version: https://www.calcopenly.com/earth/rainwater-harvesting-calculator
Subject: Weather, earth and fun calculators

The yearly harvest is roof footprint area × rainfall × collection efficiency. One millimetre of rain on one square metre is one litre, and one inch on one square foot is 0.623 US gallons. The efficiency, or runoff coefficient, removes the water lost to first-flush diversion, splash-out from gutters, evaporation and overflow.

Homeowners use it to size a rain barrel or tank and to see how far the water goes. The default roof of 100 m² under 800 mm of rain at 0.85 efficiency collects 68,000 litres a year, 186 litres a day on average, which is 62% of a household using 300 litres a day.

The result is a yearly average. Rain arrives in bursts, so a full tank overflows in wet months and runs dry in long dry spells; tank size decides how much of the average you can actually use.

## Inputs

- **Roof footprint area**: The area the roof covers on the ground, not the sloped surface.
- **Rainfall per year**
- **Collection efficiency (runoff coefficient)**: Share of the rain that reaches the tank after first-flush, splash and evaporation losses; installers typically assume 0.75–0.90.
- **Household water use**

## Results

- Water collected per year (L) — main result
- In other units (m³)
- In other units (gal)
- Average per month (L)
- Average per day (L)
- Share of household use covered

## Formula

$$
V\ (\text{L}) = A\ (\text{m}^2) \times R\ (\text{mm}) \times C
$$

## Worked examples

### 100 m² roof, 600 mm of rain, 0.8 efficiency

- Roof footprint area: 100 m²
- Rainfall per year: 600 mm
- Collection efficiency (runoff coefficient): 0.8
- **Water collected per year: 48,000 L**
- **In other units: 48.00 m³**
- Checked against: 1 mm on 1 m² = 1 L, so 100 × 600 × 0.8 = 48 000 L by hand

### 1,000 ft² and 1 inch with nothing lost

- Roof footprint area: 1000 ft²
- Rainfall per year: 1 in
- Collection efficiency (runoff coefficient): 1
- **In other units: 623 gal**
- Checked against: TWDB manual: about 0.62 gal per ft² per inch; exact 1000 × 0.09290304 m² × 25.4 mm / 3.785411784 L = 623.377 gal (Python decimal)

### 2,000 ft² roof, 32 inches, 85%

- Roof footprint area: 2000 ft²
- Rainfall per year: 32 in
- Collection efficiency (runoff coefficient): 0.85
- **In other units: 33,912 gal**
- Checked against: Python decimal: 2000 ft² × 32 in × 0.85 = 33 911.7 US gal (the TWDB 0.62 rule of thumb gives 33 728)

### No rain collected when efficiency is 0

- Roof footprint area: 150 m²
- Rainfall per year: 900 mm
- Collection efficiency (runoff coefficient): 0
- **Water collected per year: 0 L**
- Checked against: Definition: V = A × R × 0

### Share of a 300 L/day household

- Roof footprint area: 100 m²
- Rainfall per year: 800 mm
- Collection efficiency (runoff coefficient): 0.85
- Household water use: 300 litres a day
- **Water collected per year: 68,000 L**
- **Share of household use covered: 62.1%**
- Checked against: 68 000 L ÷ (300 L × 365) = 62.10% by hand

## Questions

### How much rainwater can I collect from my roof?

Multiply the roof footprint by the rainfall and a collection efficiency. Each millimetre of rain on a square metre gives one litre, and each inch on a square foot gives 0.623 US gallons, which the Texas Water Development Board rounds to 0.62. A 2,000 ft² roof under 32 inches of rain at 85% efficiency collects about 33,900 US gallons a year.

### What collection efficiency should I use?

Between 0.75 and 0.90 for most systems: the Texas Water Development Board manual says most installers assume 75 to 90%. Use the lower end for composite or asphalt shingle roofs, which the manual puts at about a 10% loss, and where a first-flush diverter discards the first 1 to 2 US gallons per 100 ft² of roof in every storm.

### Should I measure the sloped roof area or the footprint?

The footprint: the horizontal area the roof covers, length × width from eave to eave, whatever the pitch (Texas Water Development Board). A steep roof whose eaves enclose 10 × 10 m has a 100 m² catchment, the same as a flat one. If only some gutters feed the tank, count only the area they drain.

### How much water does a household use per day?

The Texas Water Development Board puts a water-conserving household at 25 to 50 US gallons (95 to 189 litres) per person per day, and cites a 1999 American Water Works Association study in which water-conserving homes averaged 49.6 gallons. Enter your household total under More options: a 300 litre-a-day household gets 62% of its use from the default 100 m² roof.

### What size rainwater tank do I need?

Enough to carry the household through the longest expected spell without rain, which is the rule in the Texas Water Development Board manual. At 300 litres a day, a 30-day dry spell needs about 9,000 litres of storage. A tank much smaller than that spills in wet months and runs dry between storms, so it captures less than the yearly figure shown.

### How accurate is the rainwater harvesting calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 5 worked examples whose answers come from independent sources; for example, “100 m² roof, 600 mm of rain, 0.8 efficiency” is checked against 1 mm on 1 m² = 1 L, so 100 × 600 × 0.8 = 48 000 L by hand.

### Where does the method come from?

Texas Water Development Board, The Texas Manual on Rainwater Harvesting, 3rd ed. (2005), ch. 4 — 0.62 gal per ft² per inch; 75–90% efficiency; BS EN 16941-1:2018 — On-site non-potable water systems, part 1: systems for the use of rainwater.

## Sources

- [Texas Water Development Board, The Texas Manual on Rainwater Harvesting, 3rd ed. (2005), ch. 4 — 0.62 gal per ft² per inch; 75–90% efficiency](https://www.twdb.texas.gov/publications/brochures/conservation/doc/RainwaterHarvestingManual_3rdedition.pdf)
- BS EN 16941-1:2018 — On-site non-potable water systems, part 1: systems for the use of rainwater