WaterCalcLabRain Barrel Calculator
Estimate how many empty barrels could contain one specified rainfall event.
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
Estimate how many empty barrels could contain one specified rainfall event.
N = ceil(event yield ÷ barrel capacity)Example 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 |
|---|---|
| Area | 100 |
| Rainfall for the selected period | 25 |
| Roof runoff coefficient | 0.85 |
| System efficiency (separate losses) | 0.9 |
| One barrel capacity | 200 |
Planning notes for this calculator
Compare the storage needed to contain runoff from one rainfall event using identical, initially empty barrels. This helps explain event capture and the need for overflow routing.
Choose meaningful inputs
- Area
- Measure the horizontal area that actually drains to the tank. Exclude disconnected roof sections.
- Rainfall for the selected period
- Enter the depth of one event, not an annual rainfall total.
- Roof runoff coefficient
- This fraction describes roof runoff before downstream collection losses. Use a documented assumption or a measured value.
- System efficiency (separate losses)
- This fraction applies to separate downstream losses. Do not count the same filter or first-flush loss in both coefficients.
- One barrel capacity
- Use the capacity available in one empty barrel. Equal capacity does not guarantee equal filling.
Read the result correctly
The calculator divides estimated event collection by one barrel capacity and rounds upward to a whole count. That count is a storage comparison, not a guarantee that the connections distribute water fast enough. Existing contents reduce free space, while a larger storm can still overflow the planned set.
Common mistakes to avoid
- Keep annual-yield calculations separate from single-event storage.
- Do not assume connected barrels were empty before the storm.
A useful next step
Check free space, stable supports, secure covers and an overflow route before selecting a barrel arrangement.
Follow a worked scenario
A separate illustrative scenario; these values do not change when you edit the form above.
- 100 m² × 25 mm × 0.85 × 0.90 = 1,912.5 L collected.
- 1,912.5 ÷ 200 = 9.5625, rounded up to 10 empty barrels.
Frequently asked questions
What if the barrels are already partly full?
Compare the event yield with total free space rather than total nameplate capacity. This calculator does not model different starting levels in individual barrels.
Will adding more barrels eliminate every overflow?
No. Both the incoming event volume and the transfer rate through connections matter. Plan a suitable overflow even when the nominal combined capacity exceeds your example storm.
Continue your plan
Rainwater Harvesting Calculator
Estimate catchment yield from horizontal roof area and period rainfall. One millimetre on one square metre is one litre.
Open calculator ↗Rainwater harvestingRoof Rainwater Collection Calculator
Measure the horizontal roof footprint draining to the connected downpipes, not the sloping roof surface.
Open calculator ↗Related reading
Sources & method references
- Texas Water Development Board: Rainwater Harvesting Manual
Collection, storage and system-planning context. Geometry and unit conversions on this page are stated explicitly.
Method version 1.1 · Explanations and examples reviewed: 2026-10-11