Enter your catchment and rainfall details. The layout adapts to screen size for faster entry.
| Catchment (m²) | Rainfall (mm) | Surface | Efficiency | Losses | First flush (L) | Net captured (L) | Storage (L) | Coverage at 40 L/day |
|---|---|---|---|---|---|---|---|---|
| 50 | 25 | Metal roof (0.90) | 90% | 5% | 10 | 951.88 | 500 | 12.50 days |
| 80 | 15 | Asphalt (0.85) | 85% | 8% | 15 | 782.64 | 750 | 18.75 days |
| 30 | 40 | Tile (0.80) | 88% | 6% | 8 | 786.11 | 300 | 7.50 days |
Runoff coefficient represents how much rain becomes usable runoff. Efficiency and losses reflect real-world collection and storage behavior in home gardens.
- Measure your roof or collection area and select the unit.
- Enter rainfall depth for your chosen period (event, month, or year).
- Select surface type, then adjust the coefficient if needed.
- Set efficiency based on gutter condition and filtration.
- Estimate losses and first-flush diversion for cleaner collection.
- Add storage capacity and daily demand to see coverage days.
- Press Calculate, then download CSV or PDF.
Catchment Surfaces and Coefficients
Smooth roofs shed most rainfall as runoff, so coefficients commonly range from 0.80–0.95. Metal often performs near 0.90, while tile sits closer to 0.75–0.85. Hardscape pavers may land around 0.60–0.75, and lawns can drop to 0.20–0.35 because infiltration absorbs water. Use the override when your site experience differs.
Rainfall Inputs and Seasonal Variation
This calculator treats rainfall depth as a total over your selected period. For storm planning, enter the event total from a local gauge. For monthly or annual budgeting, use long‑term averages or the sum of recorded totals. Because rainfall is rarely uniform, consider testing a wet month and a dry month to see best‑ and worst‑case capture. If you have roof shading, adjust efficiency slightly downward for moss buildup.
Losses, First-Flush, and Water Quality
Real systems lose volume through overflow, splash, leaks, and evaporation. A conservative planning loss of 5–15% is typical for small garden setups. First‑flush diversion improves water quality by discarding the initial roof wash that carries dust and bird droppings. Increase the first‑flush value when roofs are dusty, trees overhang, or gutters collect debris.
Storage Sizing for Practical Use
Captured water is only useful when storage matches your harvesting pattern. Small barrels (200 L) are easy to place, but fill quickly during intense storms. Larger tanks (1000 L or more) reduce overflow and stabilize supply between rains. Include capacity to see “storable” liters, then compare the quick sizing counts to decide whether multiple barrels or one tank fits your space.
Irrigation Coverage and Cost Planning
Daily demand converts stored liters into coverage days, helping you plan watering schedules. Add up drip lines, hose use, and container watering to estimate liters per day. When coverage is low, you can increase area, reduce losses, or expand storage. Over time, captured water can offset municipal use, reduce runoff, and keep sensitive plants hydrated during heat.
1) What catchment area should I use for a pitched roof?
Use the roof’s plan-view footprint, not the sloped surface. The horizontal area best matches how rainfall depth is measured and keeps your capture estimate consistent with local gauge totals.
2) Why is net captured lower than gross volume?
Net capture applies the runoff coefficient, collection efficiency, and losses, then subtracts first-flush diversion. Each factor represents real-world limits like wetting, splash, leaks, and overflow.
3) How do I choose the runoff coefficient?
Start with the surface default, then adjust for texture and debris. Smooth, clean roofs trend higher, while porous or vegetated surfaces trend lower. If you have measured performance, use the override.
4) Does the calculator account for rainfall intensity?
It uses rainfall depth, not storm intensity. High intensity can increase overflow and gutter bypass, so reflect that by increasing the losses percentage when your system can’t convey peak flow.
5) How much first flush should I set?
Begin with a small diversion (for example, 5–20 L) and adjust after observing water clarity. Increase it for dusty seasons, heavy leaf drop, or bird activity on the collection surface.
6) Can I use captured rainwater for vegetables and herbs?
Often yes, especially with clean roofing and basic filtration. Avoid collecting from contaminated surfaces, keep tanks covered, and follow local guidance if you plan to spray edible leaves directly.