Enter the roof catchment area in square feet, measured as the footprint that drains to the gutters, the rainfall in inches for the period you are interested in and the collection efficiency as a percent. Use horizontal catchment area, not the sloping roof surface area. Collection efficiency is an entered assumption accounting for first-flush diversion, leakage, overflow and other losses.
Use measured yield where available; roof area alone does not establish efficiency. Use local rainfall data from the weather service.
Rainfall varies by season and year, so the average can overstate what is available in dry periods. Roof material affects water quality, and collected water is not safe to drink without treatment. Local rules on rainwater collection differ, and some areas require permits.
Storage tanks must be covered to prevent mosquitoes.
Rainwater Harvesting: worked example
| Roof catchment area (sq ft) | 1,200 |
|---|---|
| Rainfall (inches) | 1 |
| Collection efficiency | 85.0% |
| Gallons collected | 635.5 |
| Liters collected | 2,405.5 |
Common error: Using the whole roof area when only one slope feeds the downspout. The barrel fills from the part of the roof that drains into it.
Rainwater Harvesting: formula
CDC: collecting rainwater and health · NIST SP 811: Conversion factors by quantity
Rainwater Harvesting: changing Roof catchment area (sq ft)
| Roof catchment area (sq ft) | Gallons collected | Liters collected |
|---|---|---|
| 960 | 508.4 | 1,924.4 |
| 1,080 | 571.9 | 2,164.9 |
| 1,200 (example) | 635.5 | 2,405.5 |
| 1,320 | 699 | 2,646 |
| 1,440 | 762.6 | 2,886.6 |
Rainwater Harvesting: compare input ranges
| Input | Tested from | To | Gallons collected: low input | Gallons collected: high input | Swing |
|---|---|---|---|---|---|
| Roof catchment area (sq ft) | 1,080 | 1,320 | 571.9 | 699 | 127.1 (20%) |
| Rainfall (inches) | 0.9 | 1.1 | 571.9 | 699 | 127.1 (20%) |
| Collection efficiency | 83.0% | 87.0% | 620.5 | 650.4 | 29.9 (4.7%) |
Rainwater Harvesting: individual input comparisons
| Input | Value | Gallons collected | Liters collected |
|---|---|---|---|
| Roof catchment area (sq ft) | 960 | 508.4 | 1,924.4 |
| 1,080 | 571.9 | 2,164.9 | |
| 1,200 | 635.5 | 2,405.5 | |
| 1,320 | 699 | 2,646 | |
| 1,440 | 762.6 | 2,886.6 | |
| Rainfall (inches) | 0.8 | 508.4 | 1,924.4 |
| 0.9 | 571.9 | 2,164.9 | |
| 1 | 635.5 | 2,405.5 | |
| 1.1 | 699 | 2,646 | |
| 1.2 | 762.6 | 2,886.6 | |
| Collection efficiency | 81.0% | 605.6 | 2,292.3 |
| 83.0% | 620.5 | 2,348.9 | |
| 85.0% | 635.5 | 2,405.5 | |
| 87.0% | 650.4 | 2,462.1 | |
| 89.0% | 665.4 | 2,518.7 |
Rainwater Harvesting: combined input changes
| Roof catchment area (sq ft) / Rainfall (inches) | 0.8 | 1 | 1.2 |
|---|---|---|---|
| 960 | 406.7 | 508.4 | 610 |
| 1,080 | 457.5 | 571.9 | 686.3 |
| 1,200 | 508.4 | 635.5 | 762.6 |
| 1,320 | 559.2 | 699 | 838.8 |
| 1,440 | 610 | 762.6 | 915.1 |
Rainwater Harvesting: which assumptions move the result?
Rainwater Harvesting: values and definitions
| Quantity and definition | Value used |
|---|---|
| Roof catchment area (sq ft) — The footprint of the roof that drains to the gutters, not the sloped surface. | 1,200 |
| Rainfall (inches) | 1 |
| Collection efficiency — Losses to splash, first flush and overflow. 75 to 90 percent is common. | 85.0% |
| Gallons collected | 635.5 |
| Liters collected | 2,405.5 |
Rainwater Harvesting: half and double Roof catchment area (sq ft)
| Roof catchment area (sq ft) | Gallons collected |
|---|---|
| 600 | 317.7 |
| 1,200 | 635.5 |
| 2,400 | 1,270.9 |
Home & Construction calculators: Cubic Feet, Yards and Meters Calculator, Flooring Calculator, Material Volume and Bag Calculator, Tile Flooring Calculator, Roofing Calculator, Square Footage Calculator.
One inch of rain on one square foot of roof is 0.623 gallons. That single constant turns a roof size and a rainfall into a barrel count. A 1,200 square foot catchment in a 1 inch storm can yield about 635 gallons at 85 percent efficiency, enough to fill more than ten 55 gallon barrels.
Small roofs and small storms fill a single barrel quickly, which is the reason overflow needs a plan.
Use the footprint of the roof that drains to your gutters, not the sloped surface, and check local rules on collecting and using the water.
Get the roof area on the roofing and square footage calculators, volumes on the cylinder and cubic feet pages and conversions on the volume converter. The pool volume and lawn calculators relate to water use, and the sinking fund page helps plan for a cistern.
Does clear-looking roof water establish drinking safety?
No. CDC identifies possible germs and chemical contamination in collected runoff. Testing and treatment must address the actual contaminants; a first-flush device alone does not establish drinking safety.
What does a first-flush diverter do?
It diverts an initial portion of roof runoff away from storage. The design and diverted volume need the actual roof and collection conditions.
How much water does one inch of rain give?
About 0.623 gallons per square foot of surface, so a 1,000 square foot roof gives about 623 gallons per inch, before losses.
Is rainwater safe to drink?
Not without treatment and filtration. Collected roof water is best used for irrigation and other non-potable purposes such as washing.
What is a first flush diverter?
A device that discards the first rainfall from a roof, which carries the most dirt and debris, before collecting cleaner water.