Question 1

What flow rate does a shower need?

Accepted Answer

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.

Question 2

What flow rate does a combi boiler deliver?

Accepted Answer

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.

Question 3

How do I measure flow rate on site?

Accepted Answer

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.

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

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

l/s	m³/h	Typical Use
6	21.6	WC extract (Part F)
15	54	Bathroom extract (Part F)
30	108	Kitchen extract (adjacent hob)
60	216	Kitchen cooker hood (Part F)

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

Flow Rate Conversion Factors

Common UK Flow Rates

Ventilation Flow Rates (Air)

Related Pages

Frequently Asked Questions

Related Calculators

Related Calculators

Duct Sizing Tables

Part F Ventilation Rates

Part L Air Permeability

UK Airbrick Sizes

Was this calculator helpful?