Tank Fill Time Calculator
Work out how long a tank, cistern or pool takes to fill from its volume or its dimensions and the flow rate, or solve for the flow rate you need or the water added in a set time, with a partial fill, water drawn off while filling, finish time and a fill-progress table.
How it works
fill time = water to add ÷ (inflow − outflow) · water to add = capacity × (target % − current %) · flow rate needed = water to add ÷ time (+ outflow) · water added = net flow × time · rectangular V = L × W × H · vertical or horizontal cylinder V = π × (D ÷ 2)² × height or length · sphere V = π × D³ ÷ 6 · 1 ft³ = 7.4805 gal, 1 gal = 3.7854 L
Filling a tank is volume divided by flow rate, as long as the inflow stays steady. The calculator can start from the capacity or from the inside dimensions of a rectangular, cylindrical or spherical tank, and it counts only the water you still need to add when the tank is partly full or you stop short of the brim. If water is drawn off while the tank fills, from a house supply, a leak or an overflow, only the difference between inflow and outflow fills the tank. Flip the question with Solve for: the flow rate needed to finish within a set time, or the water that goes in during a set time. Real supplies slow as pressure falls or a float valve closes, so measure your flow with a bucket and a stopwatch and treat the answer as the quickest case.
Worked example
A 50 gallon tank fed at 5 GPM takes 50 ÷ 5 = 10 minutes to fill. Halve the flow to 2.5 GPM and it doubles to 20 minutes, because flow rate and time are inversely proportional. A 4 ft diameter, 4 ft tall round tank holds π × 2² × 4 = 50.3 ft³, or 376 gallons. If it is already a quarter full and fills at 5 GPM while the house draws 1 GPM, the 282 gallons still to add take 282 ÷ 4 = 70.5 minutes, about 1 h 11 min.
Sources: NIST Handbook 44 — US gallon (231 in³) and cubic-foot conversions
Frequently asked questions
What if my flow rate drops as the tank fills?
This calculator assumes a constant rate. If pressure falls as the tank fills against a float or a head of water, the real fill takes longer. Use the average flow over the whole fill, or measure the flow near the end and run the numbers again.
How do I measure my fill rate?
Time how long a known container takes to fill. A 5 gallon bucket in 60 seconds is 5 GPM, in 30 seconds 10 GPM. In metric, a 20 L bucket in 60 seconds is 20 L/min. Measure at the same tap and pressure you will use for the real fill.
How long does it take to fill a pool with a garden hose?
A garden hose gives roughly 5–12 GPM depending on pressure and hose size. A 10,000 gallon pool at 8 GPM takes 10,000 ÷ 8 = 1,250 minutes, about 21 hours. Choose a shape under Tank size from and enter the pool’s length, width and average depth to get the volume.
Can I work out the flow rate I need to fill in a given time?
Yes. Choose Flow rate under Solve for and enter the time you have. The calculator divides the water to add by that time, and adds any water drawn off while filling.
Can I use this for draining too?
For a pumped drain at a steady rate, yes: enter the pump’s flow rate. Gravity drains slow down as the water level drops, so real gravity drain times run longer than this simple division suggests.
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This calculator is for educational purposes. Double-check important results before acting on them.