At the pump and tank
Will there be enough water through the day?
It is easy to look at a large tank and feel covered. Yet the useful question is how much water the pump can replace during the day—and how much is actually in the tank when the day begins.
This calculator checks those two amounts against a daily water demand. It gives one answer for today and a separate check for following days. That distinction matters when a tank can bridge a shortage once but the pump cannot refill it fast enough to keep doing so.
How to use the calculator
Measure the pump's useful flow
Choose litres or US gallons, then enter the pump flow while it runs and the hours it can operate in a day. Use a field measurement at the intended pressure when available. If flow is measured before a known transfer loss, open the optional section and enter that loss.
Match the tank and daily demand
Enter usable tank capacity and the gross water volume your farm must receive in one day. The tank starts empty in the default check. If it already holds usable water, enter that amount under the optional section; a tank's labeled capacity is not the same as water available now.
Read both supply checks
The large result compares today's pump output plus current tank water with demand. Read the repeat-day line next: it compares demand with pump output alone. Use tank filling time to understand the storage scale, then check the actual hours when water will be drawn.
The daily balance, in plain numbers
A pump measured at 100 L/min and running for eight hours produces 48,000 L before any transfer loss. If the farm needs 40,000 L that day and the tank starts empty, the volume check shows an 8,000 L surplus. A usable 10,000 L tank takes 100 minutes to fill from empty at that flow, provided nothing is drawn while it fills.
Usable pump flow = measured flow × (1 − transfer loss ÷ 100)
Pump water per day = usable flow × 60 × operating hours
Available today = pump water per day + water in tank now
Today's balance = available today − daily demand
Repeat-day balance = pump water per day − daily demand
The tank's capacity sets the size of the buffer, but capacity alone does not add water to today's supply. Enter the water actually stored. The calculations keep full precision; displayed volumes are rounded for reading.
When the tank gets you through today, but not tomorrow
A useful warning
One good day can hide a continuing gap
Suppose that same 100 L/min pump has 10% transfer loss. At eight hours it supplies 43,200 L/day. If the tank holds 5,000 L now and demand is 45,000 L, there are 48,200 L available today: 3,200 L above today's demand.
Tomorrow's pump contribution is still only 43,200 L. Against 45,000 L/day, that is a 1,800 L/day repeat deficit. Unless another source refills the tank, the buffer shrinks. The empty-to-full fill time is about 111.11 minutes at 90 L/min useful flow.
A tank buys time; it does not create daily water.
What a short day looks like
Now consider a pump delivering 20 US gal/min for six hours. It produces 7,200 US gal. A tank with 1,000 US gal of usable capacity contains only 500 US gal at the start, so today's available water is 7,700 US gal.
If demand is 8,000 US gal, the result is 300 US gal short today. The repeat-day pump deficit is 800 US gal/day. Entering the full 1,000-gallon tank capacity as though it were stored water would hide the immediate shortage.
What tank filling time and equivalents mean
Empty-to-full time divides usable tank capacity by effective pump flow. For a 1,000 US gal tank and 20 US gal/min useful flow, that is 50 minutes. The figure assumes the pump runs continuously and nobody draws water during filling. It is a fill-time check, not a promise about automatic cycling.
The page also divides daily demand by usable tank capacity. If demand is 8,000 US gal and usable capacity is 1,000 US gal, the result is eight full-tank equivalents. This helps you picture the volume, but it does not mean the pump will start exactly eight times. Control settings and the timing of withdrawals determine real cycles.
Keep the two kinds of water loss separate
Daily demand should be the water the farm needs supplied to its irrigation or use point. If a crop plan gives water needed at the roots, its application efficiency belongs in the step that converts that net need into gross demand. The Crop Water Requirement Calculator can make that estimate.
The optional transfer-loss field here concerns water lost between the measured pump flow and usable supply. Do not enter irrigation application loss in both places. If you measured flow at the tank inlet or final supply point, zero additional transfer loss is usually the relevant arithmetic setting.
Before relying on the answer
- Check the pump under load. Actual flow can change with lift, pressure, filters and source level.
- Check the tank's usable range. Dead storage and control levels may make nominal capacity larger than drawdown available to you.
- Check the clock, not just the total. A pump that makes enough water by evening may still fail an early peak draw.
- Check the source. A well or intake must be able to sustain the entered flow for the entered hours.
This calculator does not size pipes or pumps, model well recharge, predict the hour a tank empties, or decide legal withdrawal limits. For one irrigation zone's operating time after its water target is known, use the Irrigation Runtime Calculator.