Rainwater Tank Sizing Calculator
Multiplies roof footprint by rainfall depth for a storm event planning check — not a continuous water balance.
Step 1: Units
Step 2
Enter project measurements
Use decimals if needed (e.g. 3.5).
How to use this calculator
Enter roof catchment area and design rainfall depth in inches for one event.
What the math is doing
Capture ≈ area × rainfall depth (converted to volume).
Frequently asked questions
First flush?
Subtract diverted volume manually.
Runoff coeff?
This model assumes 100% of rain reaches tank opening—adjust mentally.
Codes?
Potable rainwater is heavily regulated.
Mosquitoes?
Screen all openings.