Gas Water Heater Size Calculator
Pick the outlets in use, the shower head and the winter inlet temperature to see the peak hot water demand in litres per minute, the heater size that covers it, and the gas meter it needs.
A gas water heater is sized by flow, not by family size. The number on the box is litres per minute at a 25 °C rise: draw two showers at once, and a 13 L unit delivers half of what each shower wants. Pick what runs at the same time and how cold the mains water gets in winter; the page returns the peak demand and the size to buy.
How you use hot water
Everything updates instantly.
Peak demand
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Heater size to buy
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Gas meter
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Winter multiplier
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Flow per outlet
| Outlet | Flow | Note |
|---|---|---|
| Handheld shower | 8 L/min | Standard head with an aerator |
| Rain shower head | 12 L/min | — |
| Kitchen tap | 4 L/min | Only counts if used at the same time |
| Second bathroom | 8 L/min | Two showers at once is the real test |
Common size classes
- 10–12 L — one handheld shower, small flat
- 13 L — one rain head, or shower plus kitchen
- 16 L — one rain head plus kitchen, or two simple showers
- 20–24 L — two bathrooms used at once
Before you buy
- The litre figure is measured at a 25 °C rise. In winter, cold mains water means the same unit delivers less hot water — always size for winter, not summer.
- A rain head is the hidden cost: it flows 50 percent more than a handheld, so the heater has to be one size up.
- Check the gas meter. A 20 L unit on a G1.6 meter will not reach full output; most installers require G2.5 from 16 L upwards.
- For two bathrooms, a recirculation line or a storage tank is often more practical than an oversized instantaneous unit.
- Keep the flue and the exhaust where the manual requires; a heater that cannot vent properly will derate or lock out.
Four common mistakes
- Sizing by the number of people instead of the number of outlets used at the same time.
- Choosing by the summer experience and then finding the shower lukewarm in January.
- Ignoring the gas meter and the pipe diameter, then discovering the unit cannot reach its rated output.
- Mounting the heater far from the bathroom, so every shower starts with 20 seconds of cold water.
Reference figures for ordinary flats. Actual flow depends on water pressure, pipe diameter and the shower head; confirm the gas supply with the installer.