Capacity • Current level • Fill time

Know how much water your tank holds—and how long it will take to fill.

For homeowners, plumbers, installers and facility teams. Enter internal measurements for a transparent geometric estimate.

Step 1

Enter tank details

Tank shape

Select its unit independently from tank height.

Step 2

Your result

Enter your tank measurements to calculate capacity.

Flow rate is optional.
Don’t know your flow rate?

Time how long your inlet takes to fill a bucket. Keep the tap or pump setting unchanged.

Measured flow—
How the calculation works

Rectangular: length × width × water height.

Cylindrical: π × radius² × water height.

Fill time: remaining litres ÷ litres per minute.

All measurements are converted to metres internally; 1 m³ = 1,000 L.

Limits and safety
  • Measure inside the tank; external dimensions overstate capacity.
  • Irregular shapes, rounded corners, fittings, dead storage and freeboard reduce usable capacity.
  • Flow can change with pressure and pump head; measure under normal conditions.
  • Water is treated as approximately 1 kg/L. This tool does not verify structural support.
Formula sources

Tank-volume formulas follow published public-health guidance. Unit conversions use exact international definitions where available.

SA Health: rainwater supply guidanceWorkSafe Victoria: tank capacity

Sources reviewed 26 August 2026.

Use this calculator to estimate a rectangular or cylindrical water tank's total capacity, current water volume, remaining litres and fill time. Choose the tank shape and enter measurements in the units requested by the tool.

Measure before entering values

For a rectangular tank, measure its inside length, width and height. For a cylindrical tank, check whether the tool asks for diameter or radius and measure the internal height. Enter the current water depth separately. Wall thickness and unusable space can make the practical capacity lower than the outside dimensions suggest.

Estimate fill time

Use a measured inlet flow rate when possible. Flow can change with supply pressure, pipe restriction, pump condition and voltage, so the displayed fill time is an estimate. If the tank is already partly full, use the remaining volume rather than total capacity.

Check the result

Confirm that all units are correct before acting on the output. Do not use this screen alone to size structural supports, overflow pipes or a pump. For installation decisions, verify site measurements and safety requirements with a qualified professional.