WaterCalcLabRainwater Tank Size Calculator
Size usable storage for a chosen rainless interval and compare annual supply with constant annual demand. Rainfall here must be annual.
Your result
Bars compare only results in the same unit group. They are normalized, not a time-series forecast.
Planning result based on your inputs.
How this tool works
Size usable storage for a chosen rainless interval and compare annual supply with constant annual demand. Rainfall here must be annual.
Vtank = demand/day × dry days ÷ usable fractionExample calculation
The prefilled form is a worked example. Its base-unit inputs and expected outputs are shown below; use the reset button to restore it.
| Input | Example |
|---|---|
| Daily demand | 100 |
| Days | 21 |
| Usable storage fraction | 0.9 |
| Area | 100 |
| Rainfall for the selected period | 600 |
| Roof runoff coefficient | 0.85 |
| System efficiency (separate losses) | 0.9 |
Frequently asked questions
How should I choose the inputs?
Use measurements and local records for the stated period. All preset values are illustrative examples, not recommendations.
What does this result leave out?
A dry-spell target is not an optimized tank size or reliability guarantee. A daily rainfall-demand simulation and structural design are needed before purchase.
Continue your plan
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A collection system needs somewhere for incoming water to go after the tank reaches its operating limit. Plan that route from the start rather than treating every overflow as a malfunction. Distinguish a full-tank overflow from water escaping at a blocked gutter or inlet; the latter can occur even while substantial storage space remains available.
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A rainwater system is easiest to understand as a route with checkpoints. Identify the connected roof, follow each gutter to its downpipe, locate the inlet and then trace the outlet to the watering point. A large roof disconnected from the tank contributes nothing. Draw this route before comparing storage products; it often exposes the missing connection that changes a project.
Read guide ↗Sources & method references
Method version 1.0 · Last reviewed: 2026-10-10