Garden & Outdoor calculator
What Water Butt Size Do I Need?
Use contributing roof area, rainfall event depth, collection efficiency to find water butt capacity.
Enter your measurements
What Water Butt Size Do I Need uses contributing roof area, rainfall event depth, collection efficiency and the other configured measurements to return water butt capacity with a formula written specifically for this tool.
- Contributing roof area — Required
- Rainfall event depth — Required
- Collection efficiency — Required
- Fraction of event to capture — Required
Water butt capacity
Enter the measurements above to calculate water butt capacity.
How the calculation works
V9 calculation: collectableYield=roofArea×designRainfall×collectionEfficiency; tankCapacity=collectableYield×desiredCaptureFraction. The result is reported as water butt capacity.
Worked example
Example: Contributing roof area = 50 m²; Rainfall event depth = 60 cm; Collection efficiency = 85%; Fraction of event to capture = 20%. Result: Tank Capacity = 5100 L; Collectable Yield = 25.5.
Before you decide
Use measured dimensions rather than nominal labels. This page calculates the stated geometry or quantity only; product-specific construction or installation details can still affect the final choice. For what water butt size do i need, the limiting result on this page is water butt capacity.
Direct measurement/geometry calculation. No external reference is required for the arithmetic itself; the result is derived only from the user-entered measurements and the formula stored in calculationLogicJson.
Frequently asked questions
What does the Water Butt Size calculator tell me?
It returns water butt capacity from the measurements entered on this page. The calculation is specific to water butt size, so the result should be compared with the exact product or specification you are considering.
Which measurements should I take for water butt size?
Measure contributing roof area, rainfall event depth, collection efficiency, fraction of event to capture. Use clear, usable dimensions rather than nominal room or product labels.
Should I round the water butt capacity?
Keep the calculated value for comparison, then choose a real product that meets or stays within the returned water butt capacity while allowing for the clearances, tolerances or ratings shown by its manufacturer.