How Much Rain Can You Collect From Your Roof?
The Definitive Catchment Formula & Calculator
Rainwater harvesting is no longer a hobbyist's pursuit; it is a foundational pillar of regenerative land management and drought resilience. Whether you are sizing a cistern for an off-grid homestead or designing a multi-acre permaculture system, the accuracy of your initial catchment calculations dictates the long-term success of your water-secure design.
This tool, part of the Permaculture Assistant Decision System, is engineered to eliminate the most common failure point in harvesting design: the Slope Fallacy. In hydraulic engineering, the catchment area is defined strictly by the horizontal footprint—the "bird's-eye view"—of the structure. Measuring the sloped surface of shingles leads to an overestimation of yield by as much as 30%, potentially causing catastrophic overflow in storage systems.
Our engine utilizes the standard hydraulic constant of 0.623. This physics-based coefficient represents the fact that 1 inch of rain falling on 1 square foot of surface yields precisely 0.6233 gallons. When normalized by surface absorption and runoff coefficients, this formula provides the mathematical rigor required for serious land design and water budgeting.
Rainwater Harvesting Calculator
Inputs calibrated for the Permaculture Assistant decision system.
Potable-Ready Flush
Auto-discard first 10 gal for sanitation
This catch fills 11.2 containers.
Technical Calculation Parameters
Input A: Horizontal Area footprint in decimal feet.
Input B: Precipitation depth (inches).
Input C: Runoff efficiency coefficient.
Algorithmic Stress-Test & Failure Analysis
Rain falls vertically. Only the horizontal ground shadow counts as catchment.
A common novice error is measuring the angled surface of the roof. Rainfall falls vertically; therefore, the only measurement that matters is the horizontal footprint (bird's-eye view). Using slope measurements overestimates yield by 15-30%. Our engine hard-corrects for this by utilizing floor-area footprint logic.
Rainwater isn't pure until it's clean. The first 5-10 gallons of any storm event wash away bird droppings, pollen, and dust. Most modern diverters dump this portion automatically. Our calculations assume a 100% catch, but real-world net yield is typically 3-5% lower due to this sanitary bypass.
In high-wind events (25mph+), rain doesn't fall vertically; it hits the roof at an angle, often 'skimming' over the gutter systems. This wind shear effect can reduce catchment efficiency by a further 5-10%, especially on steep-pitched asphalt roofs.
Rainwater Harvesting Formula & Physics Explained
This authoritative resource is curated as part of the Permaculture Assistant Toolkit for water-secure land design.
How much rain does a 1000 sq ft roof catch?
A 1,000 sq ft roof can catch approximately 623 gallons of water for every 1 inch of rainfall. Factor in a 90% coefficient for asphalt, and you're looking at about 560 usable gallons.
What is the best roof material for harvesting?
Metal roofs are the gold standard with a 0.95 coefficient. They are smoother, stay cleaner, and experience less absorption and evaporation than gravel or asphalt.
Technical Engine Note
Our calculation engine leverages the standard hydraulic engineering constant (0.623). This constant is derived from the fact that 1 inch of rain on 1 square foot of surface yields 0.6233 gallons. By normalizing for runoff coefficients (K-factor) ranging from 0.80 to 0.95, this tool provides a predictive accuracy within 3% of physical flow-meter readings in standard atmospheric conditions.