Calculate how many gallons of rainwater you can collect from your roof, plus the roof area or tank size you need to cover dry spells.
Rainwater Harvesting Formula
The harvesting modes estimate roof runoff before the separate first-flush deduction and any tank overflow:
V = A * R * 0.623 * C
In metric units the conversion factor drops out, because 1 mm of rain on 1 square meter is exactly 1 liter:
V = A * R * C
Variables:
- V is estimated roof runoff before first-flush diversion and tank overflow (US liquid gallons or liters)
- A is the roof footprint area (sq ft or m²), meaning the area the roof covers on the ground, not the sloped surface
- R is the rainfall depth over the period (inches or mm)
- 0.623 is the rounded number of US liquid gallons in one inch of rain over one square foot
- C is the assumed roof-runoff fraction after modeled roof losses, before the separately entered first-flush diversion and tank overflow. Use a value greater than 0 and no more than 1.
The nine modes estimate runoff from roof area or roof dimensions; solve roof area for a target runoff volume; size storage for a dry period or calculate days of supply; and solve total rainfall volume, area length, area width, or rain depth without runoff losses. Roof area uses A = V / (R * 0.623 * C), with V before first-flush diversion. Storage uses T = D * d or d = T / D, assuming a full usable tank, constant daily use, and no refill. The separate first-flush detail subtracts the entered diversion rate times the connected area and entered event count. Its default of 1 US gallon per 100 sq ft per event (approximately 0.40746 liters per square meter) is an illustrative allowance,not a water-quality design standard. The calculator uses the exact144/231 US-gallon factor; the article examples below use the rounded0.623 factor.
Runoff Coefficients and Yield by Rainfall
These coefficients are illustrative roof-runoff assumptions, not measured collection guarantees. Actual losses depend on the surface, rainfall, and collection system. Roof coatings and other components can affect water quality; a material or runoff coefficient alone does not establish that collected water is safe to drink.
| Roof material | Runoff coefficient | Notes for collection |
|---|---|---|
| Metal (standing seam, corrugated) | 0.90 – 0.95 | Smooth metal can be effective for collection; coatings and other components still require water-quality assessment |
| Asphalt shingle | 0.85 – 0.90 | May shed granules or leach chemicals; assess water quality for the intended nonpotable use |
| Clay or concrete tile | 0.85 – 0.90 | Porous tile can lose water; sealants and paints may also affect water quality |
| Flat built-up or gravel | 0.70 – 0.80 | Ponding and slow drainage lose more water between storms |
| Green (vegetated) roof | 0.30 – 0.50 | Designed to retain rain; assess runoff yield and water quality for the particular system |
Using the rounded0.623 factor,this table shows illustrative annual runoff from 1,000 sq ft at an assumed 0.9 roof-runoff coefficient, before first-flush diversion and overflow. The barrel column gives volume equivalents rounded up, not the number of storage barrels a site needs. Rainfall values are scenarios, not location forecasts or climate normals.
| Annual rainfall | Yield per 1,000 sq ft | 55-gallon barrels |
|---|---|---|
| 10 in | 5,607 gal | 102 |
| 20 in | 11,214 gal | 204 |
| 30 in | 16,821 gal | 306 |
| 40 in | 22,428 gal | 408 |
| 50 in | 28,035 gal | 510 |
Example Problems
Example 1: How much water can a house collect in a year?
Your roof footprint is 1,500 sq ft, annual rainfall is 32 inches, and you assume C = 0.9 for the asphalt-shingle roof:
V = 1,500 * 32 * 0.623 * 0.9 = approximately 26,914 US gallons per year before first-flush diversion and overflow. At an assumed 1 US gallon per 100 sq ft, the planned diversion is 15 gallons per event when enough runoff is available. Annual diversion depends on the actual events; the remaining water is not all guaranteed to be stored.
Example 2: What tank size covers a summer dry spell?
You irrigate with 60 gallons per day and want to ride out 21 dry days:
T = 60 * 21 = 1,260 gallons of usable stored water for the assumed dry period. A nominal 1,500-gallon tank meets this arithmetic only if it starts with enough usable water. Refilling, unusable capacity, reserves, and overflow need separate consideration; this calculation does not guarantee capture of large storms or a reliable year-round supply.
Frequently Asked Questions
Does roof pitch change how much rainwater I can collect?
Use the connected roof’s horizontal footprint, including overhangs, rather than its sloped surface area. Roofs with the same footprint have the same geometric rainfall potential for the same rainfall depth, but pitch, surface texture, and gutter performance can change actual runoff losses and the volume delivered to storage.
Why is my real yield lower than the formula predicts?
Here C models roof losses before the separate first-flush deduction; do not deduct first flush again if a supplied coefficient already represents total system efficiency. First-flush requirements vary with roof condition, dry intervals, and rainfall, so a fixed allowance is only an estimate. The rate is multiplied by the entered event count; the default assumes one diversion event in the rainfall period. Seasonal estimates require the corresponding event count. A full tank can overflow. Rainfall timing, demand, and storage capacity together determine supply and spill; dry-spell demand alone does not establish reliable system size.
Is it legal to harvest rainwater?
Rainwater rules depend on location and use. Colorado’s ordinary residential allowance permits up to two barrels with combined capacity of 110 US gallons for qualifying residential rooftops, for outdoor use on the same property; drinking and indoor household use are excluded. Separate permitted provisions apply to some qualifying well properties. Utah permits up to two covered containers, each no more than 100 US gallons, without registration; registered combined storage is limited to 2,500 US gallons, with capture and beneficial use on the same owned or leased parcel. See Colorado DWR guidance and Utah registration conditions. Confirm current state and local requirements before installation, especially for indoor or potable use.
