BuildMeter guide · Water & Mechanical
Rainwater Harvesting Planning Guide
Estimate roof catchment yield while separating rainfall, collection efficiency, storage, demand, first-flush treatment, overflow and local-use restrictions.
Direct answer
What to know first
Potential collection is roof plan area × rainfall depth × an editable collection factor. Storage size must also consider rainfall timing, water demand, tank starting level, overflow route, water quality and permitted uses. Annual yield alone does not determine a reliable tank size.
Open Rainwater Harvesting CalculatorPotential collection comes from roof plan area and rainfall depth
For a simple planning estimate, multiply horizontal roof catchment area by rainfall depth and an editable collection factor. Use plan area rather than sloped roof surface area because rainfall depth is measured on a horizontal plane.
Potential collected volume = roof plan area × rainfall depth × collection factor
| Rain on 1,000 ft² roof | Geometric volume before losses |
|---|---|
| 0.25 in. | About 156 US gal |
| 0.50 in. | About 312 US gal |
| 1.00 in. | About 623 US gal |
| 2.00 in. | About 1,247 US gal |
Actual captured volume is lower when water is lost to initial wetting, splash, gutter overflow, first-flush diversion, screens, leaks or a full tank.
Annual yield does not determine tank size by itself
A site can receive substantial annual rainfall but still experience long dry periods. Tank sizing requires a time-based water balance: rainfall timing, catchment yield, daily demand, tank starting level and overflow. A tank that is large relative to the roof may rarely fill; one that is small relative to intense storms may overflow often.
| Input | What to verify |
|---|---|
| Catchment area | Only roof sections actually connected to the tank. |
| Rainfall series | Local daily or event data rather than annual total alone. |
| Collection factor | Roof material, conveyance and first-flush losses. |
| Demand | Permitted uses and realistic daily or seasonal consumption. |
| Tank usable volume | Difference between inlet, outlet, dead storage and overflow elevations. |
| Overflow destination | A safe, erosion-resistant route when the tank is full. |
Worked event example
A connected roof plan area is 1,500 ft². A storm produces 0.8 in. of rain. Geometric runoff is about 748 gallons. With an editable 85% collection factor, potential captured volume is about 636 gallons.
If the tank has only 300 gallons of free capacity before the storm, at least 336 gallons must leave through overflow or other controlled discharge during or after the event. The overflow system must be checked for peak flow, not only total volume.
Water quality and permitted use matter
Roof runoff can carry debris, animal waste, metals, coatings and atmospheric pollutants. Screens, first-flush devices and treatment may improve quality, but requirements depend on the intended use and local rules. Do not assume untreated roof water is potable.
Common mistakes
- Using sloped roof area instead of horizontal catchment area.
- Applying 100% collection efficiency.
- Sizing only from annual rainfall.
- Ignoring overflow when the tank is already full.
- Combining potable and non-potable systems without required protection.
- Forgetting pump energy, maintenance and sediment management.
Use the calculators
Use the Rainwater Harvesting Calculator for event or annual yield, the Rainwater Tank Overflow Calculator for a simplified inflow-capacity comparison and the Rainwater Harvesting Payback Calculator for editable cost assumptions.
Sources and professional limits
EPA resources describe rain barrels and cisterns as systems that collect roof runoff for reuse or controlled release. Local plumbing, health, stormwater and water-right rules may apply, especially for indoor or potable use.