Rainwater Tank Overflow Calculator
Compare estimated roof runoff into a rainwater tank with an editable overflow capacity, freeboard volume, storm duration and safety allowance.
Default project preview
49.37 gpm
Estimated roof inflow
41.14 gpm
Entered outlet capacity
35 gpm
Storm inflow volume
2,469 gal
Net accumulation rate
6.14 gpm
Calculator
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Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Contributing roof area
Enter this project assumption in ft². The current imperial default is 2200 ft².
Planning rainfall intensity
Enter this project assumption in in/hr. The current imperial default is 2 in/hr.
Roof runoff reaching tank
Enter this project assumption in %. The current imperial default is 90 %. Minimum: 0. Maximum: 100.
Available tank capacity before overflow
Enter this project assumption in gal. The current imperial default is 250 gal. Minimum: 0.
Storm duration
Enter this project assumption in minutes. The current imperial default is 60 minutes.
Entered overflow outlet capacity
Enter this project assumption in gpm. The current imperial default is 35 gpm. Minimum: 0.
Capacity planning allowance
Enter this project assumption in %. The current imperial default is 20 %. Minimum: 0. Maximum: 50.
How to read the outputs
Estimated roof inflow
90% runoff reaching tank. Default preview: 41.14 gpm.
Entered outlet capacity
user-supplied planning value. Default preview: 35 gpm.
Storm inflow volume
60 minute duration. Default preview: 2,469 gal.
Net accumulation rate
inflow above entered outlet capacity. Default preview: 6.14 gpm.
Freeboard fill time
250 gal available capacity. Default preview: 40.7 min.
Unmanaged storm volume
after entered overflow and free capacity. Default preview: 119 gal.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Roof inflow equals roof area × rainfall intensity × runoff percentage. The safety allowance sets a planning overflow target. Entered outlet flow is compared with inflow, while free tank capacity temporarily absorbs any net accumulation.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Estimate peak overflow from roof catchment, rainfall intensity and tank inflow assumptions so overflow conveyance can be discussed.
Calculation steps
- 1Define connected roof area.
- 2Enter design rainfall intensity and runoff coefficient.
- 3Subtract only defensible storage or controlled outflow.
- 4Compare peak overflow with outlet and downstream capacity.
Worked numerical example
A 2,000 ft² roof under 3 in/hr rainfall with a 0.95 runoff coefficient produces about 59.2 gpm before any tank storage or inlet losses.
Purchasing and rounding
Use the result to discuss overflow pipe, screened outlet and safe discharge routing with the system designer.
Interpret the result
This simplified estimate does not replace local stormwater design or hydraulic verification.
Page-specific assumptions
- Uniform rainfall.
- All connected area contributes.
- Tank is full for conservative overflow.
Common mistakes
- Using annual rainfall.
- Assuming tank volume eliminates peak overflow.
- Discharging near foundations.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Rainwater Tank Overflow Calculator questions
Does tank storage replace an overflow outlet?+
No. Available storage may delay overflow, but a full tank still needs a safe route for incoming water.
Why is rainfall intensity editable?+
Design rainfall varies by location, storm duration and project criteria. Use a value appropriate to the site rather than a universal default.
Does this calculate pipe diameter?+
No. The entered outlet capacity must come from a separate hydraulic check that includes pipe geometry, slope, fittings, head and blockage risk.