Estimate practical greywater reuse storage by balancing daily non-potable demand, expected supply, and the number of dry days the system should bridge. The result shows a demand-based tank volume, usable volume after reserve allowance, and whether the entered supply can support the intended demand. Use this planning estimate to compare tank sizes before detailed hydraulic design.
Nominal storage = daily demand × autonomy days ÷ (1 − reserve fraction)
Usable storage is nominal storage after subtracting the reserve. Supply coverage compares average supply with demand; it does not model rainfall timing or hourly greywater peaks.
The main result is the nominal tank capacity needed to deliver the entered daily demand through the selected no-supply period while preserving the reserve.
Final tank selection should also account for local plumbing rules, overflow, treatment equipment, seasonal supply patterns, and available standard tank sizes.
With 180 gal/day of demand, 5 dry days, and a 10% reserve, nominal capacity is 180 × 5 ÷ 0.90 = 1,000 gal. Usable storage is 900 gal.
The calculator divides usable demand storage by one minus the reserve fraction, so a larger reserve increases nominal tank size.
Yes. Storage can still be sized from demand and autonomy, but the coverage result will show that stored water is not replenished on average.
No. Add any dedicated fire-storage requirement separately and follow applicable codes.
Use demand and supply from the design season when shortages are most likely; annual averages can hide seasonal gaps.
No. Round up to a suitable standard size after allowing for site, overflow, access, and treatment constraints.
| Input concept | Planning use |
|---|---|
| Volume or flow | Use the same gallon basis throughout the calculation. |
| Efficiency or allowance | Enter percentages as displayed, from 0 to 100. |
| Operating condition | Base it on the intended design period, not an unrelated annual peak. |