Flow Rate Calculator
Volumetric flow Q = A × v from a pipe's inside diameter and the water speed, speed from flow, flow from volume and time, pool pump turnover and irrigation totals.
Result…
How it's calculated
Q = A × v, with A = π d² ÷ 4 v = Q ÷ A Q = volume ÷ time Turnover time = pool volume ÷ pump flow Water used = flow × time (drip: emitters × rate per emitter × time)
Volumetric flow rate is the volume passing a point each second. In a full pipe it is the cross-section area times the average speed of the water, so halving the bore at the same flow makes the water four times faster. Units use exact factors: 1 US gallon = 3.785411784 L, 1 UK (imperial) gallon = 4.54609 L, 1 in = 25.4 mm, 1 ft = 0.3048 m.
Your numbers, step by step
About the Flow Rate calculator
This flow rate calculator turns pipe size and water speed into volumetric flow with Q = A × v, where A is the area of the pipe's bore. It also runs the other way, giving the speed of the water from a known flow and pipe size, and finds a flow from a volume and the time it took, such as timing how long a bucket or tank takes to fill. Two practical modes help around the home: how long a pool pump takes to move the whole pool through once, and how much water an irrigation run uses from a known flow or from a count of drip emitters. Results come in L/s, L/min, m³/h, US gal/min and UK gal/min, with exact factors for both kinds of gallon. Each step of the working is shown.
How to use it
- Choose what “I want”: flow from pipe size, speed from flow, flow from volume and time, pool turnover or irrigation total.
- For a pipe, enter the “Pipe inside diameter” and its unit, then the “Flow speed” or the “Flow rate”.
- For a pool, enter its “Volume”, the pump's “Flow rate” and the “Pump hours per day”.
- For irrigation, enter a known “Flow rate” or the “Number of emitters” and “Rate per emitter”, then the “Watering time”.
- Pick the “Flow unit” and “Volume unit” you want the answers in.
Worked example
Water moving at 1.5 m/s through a pipe with a 25 mm bore: the area is π × 0.025² ÷ 4 = 4.91 cm², so the flow is 0.000491 m² × 1.5 m/s = 44.2 L/min, or 11.7 US gal/min. A 45,000 L pool on a 150 L/min pump turns over once in 45,000 ÷ 150 = 300 min, which is 5 h, so 8 hours of pumping a day gives 1.6 turnovers. Sixty 2 L/h drip emitters running for 40 minutes use 60 × 2 × 40 ÷ 60 = 80 L.
Updated
Questions people ask
How do I calculate flow rate from pipe diameter and velocity?
Work out the bore area A = π × d² ÷ 4 in square metres, then multiply by the average speed in m/s to get m³/s. Multiply by 60,000 for litres per minute.
What is the formula for flow rate from volume and time?
Flow = volume ÷ time. If a 10 L bucket fills in 30 seconds, the flow is 10 ÷ 0.5 min = 20 L/min, a quick way to measure what a tap or hose delivers.
Should I use the inside or outside diameter of the pipe?
Use the inside diameter, the bore the water actually flows through. Nominal pipe sizes and outside diameters are larger, and using them overstates the flow, which is a common mistake.
How many pool turnovers a day do I need?
It depends on the pool, how much it is used and local health rules, so check the guidance that applies to you. The calculator shows the turnover time and the turnovers your pump hours give; for the pool's volume, the tank volume calculator can help.
Why are there US and UK gallons per minute?
The two gallons are different sizes: a US gallon is 3.785411784 L and a UK gallon is 4.54609 L, about 20% more. Check which one a pump or meter is rated in.
Does the flow rate calculator account for pressure?
No. Q = A × v only links flow, area and speed. The flow a real pipe delivers depends on the supply pressure, the pipe's length and fittings, and the pump, so for system design ask a plumber or irrigation specialist.