What flow rate does a shower need?

A good shower experience requires 8-12 litres per minute (0.13-0.20 l/s). Below 8 l/min feels weak; above 12 l/min is excellent. Power showers can use 15-20 l/min. Low-flow eco showerheads deliver 6-8 l/min while maintaining a good spray pattern. Mains-pressure (combi or unvented) systems typically deliver 10-15 l/min; gravity-fed systems from a loft tank may deliver only 5-10 l/min without a pump.

What flow rate does a combi boiler deliver?

A modern combi boiler delivers approximately 10-15 litres per minute of hot water (at a 35°C temperature rise). This is sufficient for one shower or bath fill at a time. Higher-output combi boilers (35-40 kW) can deliver up to 15-17 l/min. If the incoming mains flow rate is less than the boiler output, the mains flow rate becomes the limiting factor — always check mains flow rate before specifying a combi boiler.

How do I measure flow rate on site?

The simplest method is to time how long it takes to fill a known container. A standard bucket is approximately 10 litres. If it fills in 40 seconds, the flow rate is 10 ÷ 40 = 0.25 l/s or 15 l/min. For mains flow rate testing, open the kitchen cold tap fully (usually the nearest draw-off to the incoming main) and time the bucket fill. Test at different times of day as mains pressure varies.

Flow Rate Conversions — l/s, l/min, m³/h

Flow rate unit conversions for UK plumbing, heating and ventilation. Litres per second, litres per minute, cubic metres per hour and gallons per minute.

Flow Rate Conversion Factors

From	To	Multiply By
l/s	l/min	60
l/min	l/s	0.01667
l/s	m³/h	3.6
m³/h	l/s	0.2778
l/min	m³/h	0.06
l/s	UK gal/min	13.20
UK gal/min	l/s	0.07577
l/s	US gal/min	15.85
US gal/min	l/s	0.06309

Common UK Flow Rates

Application	l/s	l/min	m³/h
Kitchen tap	0.10 – 0.25	6 – 15	0.36 – 0.90
Basin tap	0.07 – 0.15	4 – 9	0.24 – 0.54
Standard shower	0.13 – 0.20	8 – 12	0.48 – 0.72
Power shower	0.25 – 0.33	15 – 20	0.90 – 1.20
Bath fill (taps)	0.25 – 0.50	15 – 30	0.90 – 1.80
Combi boiler DHW output	0.17 – 0.28	10 – 17	0.60 – 1.02
WC flush (6 litre)	—	~6 litres per flush	—
Washing machine fill	0.10 – 0.17	6 – 10	0.36 – 0.60
Garden hose	0.12 – 0.25	7 – 15	0.42 – 0.90