Tank Fill/Drain Calculator

To calculate tank filling time, divide the volume by the flow rate: filling time = volume ÷ flow rate. A 1,800-gallon tank filling at 15 GPM takes 1,800 ÷ 15 = 120 minutes, or 2 hours. Draining at a steady pumped rate works the same way. Enter the tank's shape and size, or its volume if you know it, and the flow rate.

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Tank calculation result

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How It Works

Fill time is the tank's volume divided by the flow rate going in, and drain time is the volume divided by the flow rate coming out. The calculator works out the volume from the tank's shape and dimensions, or takes the volume you enter, converts the flow rate to the same units and divides. It assumes the flow rate stays constant, which holds for a pump or a regulated supply. Use it to time a pumped transfer, size a pump for a fill window or see how long a 1,000-litre IBC takes to fill from a hose.

If you don't know the flow rate, time how long a bucket of known size takes to fill: litres ÷ seconds × 60 gives L/min. For a hose, the garden hose flow calculator estimates it from the hose size, length and supply pressure.

Volume Calculations

Tank volumes are calculated using standard geometric formulas:

  • • Rectangular: V = L × W × H
  • • Cylindrical: V = π × (D/2)² × H
  • • Spherical: V = (4/3) × π × (D/2)³
  • • Known volume: enter it in US gallons, litres, m³, ft³ or imperial gallons

Time Calculations

Time to fill or empty is calculated using:

Time = Volume / Flow Rate

Turn the formula round to find the flow rate you need: Flow Rate = Volume / Time. Keep the units matched: gallons with GPM and litres with L/min both give minutes.

Worked Examples

  • Fill time from dimensions. A round tank 1.5 m across and 2 m tall holds π × 0.75² × 2 = 3.534 m³, which is 3,534 litres. At 50 L/min it fills in 3,534 ÷ 50 = 70.7 minutes.
  • Drain time. A 600-gallon tank pumped out at 7.5 GPM empties in 600 ÷ 7.5 = 80 minutes.
  • Fill rate needed. To fill a 1,500-gallon tank in 2 hours you need 1,500 ÷ 120 = 12.5 GPM.

To cost a pumped fill, multiply the fill time in hours by the pump's input power in kW (the pump power calculator gives it from flow and head), then price the kWh at your own tariff with an electricity running cost calculator.

Tank Fill Time by Size and Flow Rate (US Gallons)

Tank 5 GPM 10 GPM 15 GPM 20 GPM
50 gal 10 min 5 min 3.3 min 2.5 min
100 gal 20 min 10 min 6.7 min 5 min
250 gal 50 min 25 min 16.7 min 12.5 min
500 gal 1 h 40 min 50 min 33.3 min 25 min
1,000 gal 3 h 20 min 1 h 40 min 1 h 7 min 50 min
1,500 gal 5 h 2 h 30 min 1 h 40 min 1 h 15 min
2,500 gal 8 h 20 min 4 h 10 min 2 h 47 min 2 h 5 min
5,000 gal 16 h 40 min 8 h 20 min 5 h 33 min 4 h 10 min
10,000 gal 33 h 20 min 16 h 40 min 11 h 7 min 8 h 20 min

Tank Fill Time by Size and Flow Rate (Litres)

Tank 10 L/min 20 L/min 50 L/min 100 L/min
100 L 10 min 5 min 2 min 1 min
250 L 25 min 12.5 min 5 min 2.5 min
500 L 50 min 25 min 10 min 5 min
1,000 L 1 h 40 min 50 min 20 min 10 min
2,500 L 4 h 10 min 2 h 5 min 50 min 25 min
5,000 L 8 h 20 min 4 h 10 min 1 h 40 min 50 min
10,000 L 16 h 40 min 8 h 20 min 3 h 20 min 1 h 40 min
25,000 L 41 h 40 min 20 h 50 min 8 h 20 min 4 h 10 min

Draining by Gravity

When a tank empties through an outlet at the bottom with no pump, the flow falls as the level drops, because the outlet flow depends on the depth of water above it. For a tank with vertical sides, the time to empty is twice what the starting flow rate suggests: t = 2V ÷ Q₀, where Q₀ is the flow when the tank is full.

A full 500-gallon tank that drains at 20 GPM at first takes 2 × 500 ÷ 20 = 50 minutes to empty. This calculator assumes a constant flow, so use it as it stands for a pumped drain. For a gravity drain from a tank with vertical sides, enter the flow when the tank is full and double the drain time it gives.

For the water held in the pipework itself, use the pipe volume calculator.

FAQ

Common questions about tank fill time, fill rate and drain time.

Frequently Asked Questions

How do I calculate tank filling time?

Divide the volume still to fill by the flow rate, in matching units: gallons divided by GPM gives minutes, and so do litres divided by L/min. A 1,800-gallon tank filling at 15 GPM takes 1,800 ÷ 15 = 120 minutes, or 2 hours.

How do I work out the fill rate needed to fill a tank in a set time?

Divide the volume by the time you have: flow rate = volume ÷ time. To fill a 1,000-litre tank in 20 minutes you need 1,000 ÷ 20 = 50 L/min, and to fill a 500-gallon tank in an hour you need 500 ÷ 60 = 8.3 GPM.

How long does it take to drain a tank?

At a steady pumped rate, divide the volume by the pump-out rate, as for filling. Draining by gravity through an outlet at the bottom takes longer, because the flow slows as the level falls: for a tank with vertical sides it takes twice as long as the starting flow rate suggests.

How long will a 500-gallon water tank last?

Divide the volume by the rate you draw from it. At a steady 2 GPM, a full 500-gallon tank lasts 500 ÷ 2 = 250 minutes, which is 4 h 10 min of continuous use.

How do I account for a tank that is already partly full?

The calculator assumes the tank starts empty. For a partly full tank, subtract the volume already in it from the full volume and divide what is left by the flow rate. A 500-gallon tank holding 200 gallons and filling at 10 GPM needs (500 − 200) ÷ 10 = 30 minutes.

What if my tank has an irregular shape?

Use the closest standard shape as an approximation, or split the tank into standard shapes and add their volumes. If the manufacturer states the capacity, choose Known volume and enter it directly.

How does tank orientation affect the calculation?

For a horizontal cylindrical tank, enter the tank's length as the height, because the full volume is π × (D/2)² × L either way. The time is for the whole tank: in a horizontal tank, depth does not rise in step with volume, so a part-fill cannot be read off as a fraction of the height.

What units can I use?

Dimensions in metres, centimetres, feet or inches, with a separate unit for each dimension; a known volume in US gallons, litres, m³, ft³ or imperial gallons; and flow in L/min, US GPM, m³/h or m³/s. The result gives the time and the volume in m³, litres and US gallons.

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