How many GPM do I need?

Add up fixtures running at once: shower 2–2.5, faucet 1–1.5, dishwasher 1–1.5.

What temperature rise should I use?

Winter inlet to about 105–120 °F — often 60–70 °F in the north.

Whole-house units can need 100–150 A at 240 V.

What size tankless for two showers?

About 5 gpm at your winter rise — often 180–199k BTU gas.

How many kW for a tankless electric?

Whole-house units are 18–36 kW.

Tanks store a reserve; tankless gives endless hot water within its flow limit.

Water heater sizing calculator

Hot water flow and inlet and outlet temperatures: you get the heat output needed, the equivalent heater rating and gas use.

Required output

20.93 kW

Required output20.93 kW
Equivalent heater rating (l/min at 25 °C rise)12 l/min
Natural gas use2.17 m³/h

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How it's calculated (visible formula)

P (kW) = flow (l/s) × 4.186 × ΔT; 1 kW = 3,412 BTU/h

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Decide how much hot water you need at once (gpm).
Take winter inlet water temperature.
Set the desired outlet temperature.
Output = flow × 4.186 × temperature rise (kW).
Pick a heater rated for that flow at your rise.

Worked example: 2.6 gpm (10 l/min) shower, 50 to 104 °F

10 ÷ 60 × 4.186 × 30 = 20.9 kW
≈ 71,400 BTU/h output

→ A unit giving ~3 gpm at a 54 °F rise (about 85,000 BTU input).

Common mistakes

Comparing GPM ratings at different temperature rises.
Ignoring two fixtures running at once.
Forgetting colder winter inlet water.

Pro tips

Add fixtures used simultaneously, not every fixture in the house.
For gas units, check venting and combustion air.
Hard water? A softener extends heat exchanger life.
For tank heaters, size by first-hour rating.

From calculation to quote

Add labour, overhead and your margin, and generate the professional PDF.

Create quote Calculate margin & GST

FAQs

How many GPM do I need?: Add up fixtures running at once: shower 2–2.5, faucet 1–1.5, dishwasher 1–1.5.
What temperature rise should I use?: Winter inlet to about 105–120 °F — often 60–70 °F in the north.
Electric tankless?: Whole-house units can need 100–150 A at 240 V.
What size tankless for two showers?: About 5 gpm at your winter rise — often 180–199k BTU gas.
How many kW for a tankless electric?: Whole-house units are 18–36 kW.
Tank or tankless?: Tanks store a reserve; tankless gives endless hot water within its flow limit.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.