Size the required design flow for a rainwater harvesting treatment or transfer system. Enter daily volume, the hours available to process it, a peak factor, and planned downtime. The calculator converts average daily demand into an adjusted hourly and per-minute flow so pumps and treatment components can be screened against the operating window.
Required hourly flow = (daily volume ÷ operating hours) × peak factor ÷ (1 − downtime fraction)
Required gallons per minute equals adjusted hourly flow divided by 60.
The main result is the treatment or transfer rate needed during available operating hours after applying peak and downtime allowances.
Hydraulic equipment should also be checked against instantaneous fixture discharge, pump curves, head loss, minimum turndown, equalization storage, and manufacturer limits.
For 1,200 gal/day processed over 12 hours, a 1.5 peak factor and 10% downtime allowance require (1,200 ÷ 12) × 1.5 ÷ 0.90 = 166.67 gal/hour, or 2.78 gpm.
Equipment must process the daily volume within the hours it is actually available to run.
It adds capacity for variation above the base hourly average; choose it from measured patterns or design criteria.
The adjusted flow is divided by the available fraction, one minus the downtime percentage.
No. Pump head, efficiency, controls, solids handling, and operating range also matter.
No. This calculator treats the input as hours available within one design day.
| 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. |