Calculator Inputs
Formula Used
Cylindrical capacity uses π multiplied by radius squared and height. Rectangular capacity multiplies length, width, and height. Tapered barrels use the standard frustum volume equation.
| Purpose | Formula |
|---|---|
| Vertical cylinder | V = πr²h |
| Horizontal cylinder | V = πr²L; partial fill uses a circular segment |
| Rectangular tank | V = L × W × H |
| Oval tank | V = πabH |
| Frustum | V = πh(R² + Rr + r²) ÷ 3 |
| Rain collection | Area × rainfall × runoff × efficiency − first flush |
| Barrel quantity | Ceiling(required storage ÷ barrel capacity) |
How to Use
Select the calculation mode matching your current planning task. Enter measurements using consistent units and realistic efficiency values. Review warnings before purchasing barrels or modifying drainage systems.
- Choose geometry, collection, sizing, or barrel quantity mode.
- Enter barrel, roof, rainfall, demand, and loss details.
- Select preferred output units and decimal precision.
- Press Calculate and review every capacity warning.
- Copy, print, or export the completed result.
Example Data
| Scenario | Key inputs | Expected use |
|---|---|---|
| Vertical barrel | 60 cm diameter, 90 cm height | Find geometric and safe capacity |
| Roof event | 100 m² roof, 20 mm rainfall | Estimate collection and overflow |
| Dry-period storage | 40 L daily, 14 dry days | Recommend total barrel capacity |
| Multi-barrel system | 800 L target, 200 L barrels | Calculate barrel quantity and cost |
Frequently Asked Questions
How accurate is the calculated barrel capacity?
The geometric result is accurate for regular tank shapes. Real barrels may have walls, fittings, and curved bases. Always compare results with the manufacturer’s published capacity first.
What runoff coefficient should I use?
Smooth metal roofs commonly use the highest runoff coefficient. Rougher roofs retain more water and collect more debris. Choose conservative values when roof performance remains uncertain outdoors.
Why include a first-flush loss?
A first-flush diverter removes early roof dirt and contaminants. That diverted water never reaches the main storage barrel. Include its volume for realistic collection estimates every time.
What is safe-fill capacity?
Safe-fill capacity excludes freeboard and unusable bottom storage. It may also include a practical sediment allowance. This value better represents water available for planned use.
Can connected barrels be calculated?
Yes, enter the number of barrels connected together. The calculator multiplies capacity, current volume, and total cost. Confirm connecting pipes can balance every barrel safely together.
How is horizontal partial volume calculated?
The calculator uses a circular-segment area at water depth. That area is multiplied by the cylinder’s measured length. Accurate depth readings improve partially filled tank estimates considerably.
How much roof rainfall fills one barrel?
The calculator reverses the collection formula for required rainfall. Roof area, losses, efficiency, and existing water affect results. Greater catchment area reduces rainfall needed for complete filling.
Can this estimate irrigation coverage?
Yes, enter the planned irrigation depth in millimetres. One litre covers one square metre by one millimetre. Actual coverage changes with soil, plants, wind, and application.
Does the cost estimate include installation?
The estimate multiplies barrel quantity by cost per barrel. It excludes stands, filters, gutters, pipes, and labour. Add those project costs before approving any purchase budget.
Should collected rainwater be used for drinking?
This calculator only estimates storage and collection performance. Potable use requires suitable treatment, testing, and local compliance. Follow local health guidance before consuming harvested rainwater.