Match the head size to the pressure you have, not to the space on the ceiling. A 30 cm head on a 1.5 bar supply feels like drizzle; a 20 cm head on the same supply feels like a shower.
The arithmetic
Coverage is flow divided by area, and area grows with the square of the diameter. A 30 cm head has more than twice the area of a 20 cm one, so the same litres per minute are spread more than twice as thinly.
What to read on the listing
The stated flow rate with its pressure, and the minimum operating pressure the maker asks for. Both are published by the makers who mean it. A large head with no minimum stated is the one to be careful with.
The practical sizes
20 cm suits almost every domestic supply and is the safe default. 25 cm is comfortable on a decent mains pressure. 30 cm and above wants pressure, and it is where most disappointment in this family lives.
The detail that decides year three
Whether the nozzles are flexible cones you can rub clean. In hard water that is the difference between a head that still works and one that sprays sideways.
Questions people actually ask
Does a ceiling-mounted head need more pressure than a wall arm?
A little, because the water has further to rise, but the difference is small compared with the head size itself. What a ceiling mount really changes is the plumbing — it needs a pipe in the ceiling, which is a renovation.
Can I fit a rain head to an existing shower hose outlet?
Only on an arm; a fixed head has no handle and needs to be held by something. Rail sets with a fixed head at the top and a hand shower below exist precisely to avoid opening the wall.
Last reviewed 5 September 2026