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
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
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.
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.