Water Tank Filling Time Calculator
How the formula works
The calculator finds the volume of water to add, then divides it by the net inflow, or finds the flow needed for a given time.
Formulas
Rectangular tank volume (L) = Length × Width × Depth × 1000 (in metres)
Cylindrical tank volume (L) = π / 4 × Diameter² × Depth × 1000 (in metres)
Volume to fill = Volume × (Fill-up % − Starting %) / 100
Net flow = Inflow − Outflow
Filling time = Volume to fill / Net flow
Required inflow = Volume to fill / Time + Outflow
Pipe velocity = Flow / Pipe area
Where:
- 1 m³ = 1,000 litres
- 1 m³/h = 16.67 L/min; 1 US GPM = 3.785 L/min
- Outflow is the water used while the tank is filling
Typical values
Pipe velocity of 1 to 2 m/s is common for water supply lines. The calculator shows a warning above 3 m/s. A normal house tap line gives about 10 to 30 L/min. A borewell or booster pump may give 50 to 300 L/min. These are typical values. Always measure the real flow, because pressure and pipe length change it. Use the Custom option where available for your own figures.
How to use the calculator: choose the calculation, enter the tank details, select the flow unit, enter the inflow or the time, then click Calculate.
Important notes: a float valve slows the flow as the tank fills. For a better estimate, measure the flow with a bucket test.
Worked example
Example 1: Rectangular tank
3 m × 2 m × 1.5 m = 9,000 L. Inflow 40 L/min, fill from 0 to 100 %.
Time = 9,000 / 40 = 225 minutes = 3 h 45 min (3.75 hours). Filling rate = 2,400 L/h.
Example 2: Cylindrical tank with outflow
Diameter 1.2 m, depth 1.8 m, volume 2,035.8 L. Inflow 1.2 m³/h (20 L/min), outflow 5 L/min, filling from 20 % to 90 %.
Volume to fill = 1,425 L. Net flow = 15 L/min. Time = 1 h 35 min.
Example 3: Volume known, flow in GPM
10,000 L tank, inflow 15 US GPM (56.78 L/min), pipe 25 mm.
Time = 10,000 / 56.78 = 176 minutes = 2 h 56 min. Pipe velocity = 1.93 m/s.
Example 4: Flow needed in a given time
5,000 L tank to be filled in 1 h 30 min, outflow 10 L/min, pipe 32 mm.
Net flow = 5,000 / 90 = 55.56 L/min. Required inflow = 65.56 L/min = 3,933 L/h = 3.93 m³/h = 17.32 US GPM. Pipe velocity = 1.36 m/s.
Common mistakes
- Using the full tank size when the tank is only partly empty.
- Forgetting that water is used while the tank is filling.
- Mixing up litres per minute and litres per hour, or GPM and L/min.
- Using the pump’s best flow instead of the actual flow at the delivery height.
- Measuring tank dimensions to the brim instead of to the working water level.
- Ignoring the float valve, which reduces the flow near the end.
- Using a small pipe, which causes high velocity, noise and pressure loss.
- Assuming the supply pressure stays the same all day.
- Using outside dimensions of a tank with thick walls.
Frequently asked questions
How do I calculate how long a tank takes to fill?
Divide the volume of water to add by the net flow rate. Net flow is the inflow minus any outflow.
How many litres is my tank?
For a rectangular tank, multiply length, width and depth in metres and then by 1,000. This calculator can do it for you.
How do I find my flow rate?
Fill a bucket of known size and time it. Divide the litres by the minutes to get L/min.
How do I convert L/min to L/hour?
Multiply L/min by 60. For example, 40 L/min is 2,400 L/h.
How do I convert GPM to L/min?
Multiply US GPM by 3.785. For example, 15 GPM is 56.8 L/min.
What flow do I need to fill a tank in one hour?
Divide the volume in litres by 60. For a 5,000 L tank this is about 83 L/min, plus any outflow.
Why does my tank take longer than calculated?
Pressure drops, a float valve, a long pipe or other users can reduce the real flow.
What is a safe pipe velocity?
About 1 to 2 m/s is common. Above 3 m/s the pipe may be noisy and lose a lot of pressure.
Can I fill part of the tank?
Yes. Enter the starting level and the fill-up level in percent.
How do I size the tank?
Use the Water Tank Size Calculator and the Daily Water Demand Calculator.
Which pump do I need for this flow?
Use the Borewell Pump HP Calculator with the flow from this page.
Is this an exact time?
No. It is an estimate. Measure the real flow for a better result.