Water Heater Size Calculator
First-hour rating beats tank size.
Work out Water Heater Size. First-hour rating beats tank size. Counts the thing everyone forgets to count.
Washing up, basins, appliances
202 L first-hour rating against 154 L of peak demand
Adequate
52 L/h recovery from a 3 kW element
First-hour rating — stored volume plus what the element reheats within the hour — is what actually matters, rather than tank size alone. A small tank with fast recovery frequently outperforms a larger slow one, which is why comparing tanks on litres is misleading. Storage below about 60 °C raises legionella risk, and anything above needs a mixing valve at the outlets to prevent scalding.
How the Water Heater Size Calculator works
Whether a storage water heater meets peak demand, comparing first-hour rating — stored volume plus what the element reheats within the hour — against the demand a morning actually creates.
Also known as: what size hot water cylinder do i need · first hour rating calculator · hot water tank sizing · why does my hot water run out
First-hour rating, not tank size
The figure that describes what a water heater actually delivers is first-hour rating: the stored volume plus what the element can reheat within the hour. Tank volume alone describes only the starting point.
Which means a small tank with fast recovery frequently outperforms a larger slow one. Comparing water heaters on litres is the standard mistake, and it is why two tanks of the same size can behave completely differently in a busy household.
The other half is that a shower does not draw its full flow from the tank. Mixed down to about 40 °C from 60 °C stored with 10 °C cold, a shower takes only around 60% of its flow as hot water — which makes the effective capacity larger than it first appears.
Sixty degrees, and why you cannot just turn it down
Legionella multiplies between roughly 20 and 45 °C and is killed above about 60 °C. Storing below that to save energy or prevent scalding creates a genuine health risk, particularly in a tank that sits partly used.
The correct answer to scalding is a thermostatic mixing valve at the outlets rather than a lower storage temperature. It blends the stored water down to a safe 43 °C or below at the tap while the tank itself stays hot enough to be safe.
Water at 60 °C causes a serious burn in seconds, and far faster on a child. Which is why the two requirements — hot enough to be biologically safe, cool enough to be physically safe — are met at different points in the system rather than by a compromise setting.
Standby loss, and what a heat pump changes
A storage tank loses heat continuously whether or not water is drawn. An older uninsulated cylinder can lose several kilowatt-hours a day, which makes an insulating jacket one of the cheapest energy measures available anywhere in a house.
That standing loss is the argument for a tankless heater, which has none and never runs out. Against it is a flow rate limit and a very large instantaneous power or gas demand — and whether the supply can carry that demand is usually what decides it.
A heat pump water heater changes the sizing arithmetic. Recovery is much slower than a resistance element, so a larger tank is generally needed to reach the same first-hour rating. Running cost is a fraction of resistance heating, which is what makes the larger tank worth it.
Where to go next
The Water Heater Size question rarely arrives on its own. These are the ones that usually come with it:
- Hot Tub Chemical Calculator — Small volume means the margin for error is tiny.
- Pipe Flow Rate Calculator — Flow goes with the square of the bore.
- Septic Tank Size Calculator — Sized on the greater of flow and bedrooms.
- Ring Size Calculator — Finger measurement to US, UK, EU and Japanese sizes.
Frequently asked questions
What size water heater do I need?
Size it on first-hour rating against peak demand rather than on tank volume. A small tank with fast recovery frequently outperforms a larger slow one.
What is first-hour rating?
Stored volume plus the volume the heating element can reheat within an hour. It is the figure that describes what the heater actually delivers during a busy morning.
How much hot water does a shower use?
Less than the flow rate suggests, because a shower mixes hot and cold. At 60 °C stored and 10 °C cold, a 40 °C shower draws only about 60% of its flow from the tank.
What temperature should I store hot water at?
Storing below about 60 °C raises legionella risk. Storing at or above it means a mixing valve is needed at the outlets, because water at 60 °C scalds in seconds.
How much does a hot water tank lose standing?
Roughly proportional to volume and insulation quality — an older uninsulated cylinder can lose several kilowatt-hours a day. An insulating jacket is one of the cheapest energy measures available.
Is a tankless heater better?
It removes standby loss and never runs out, at the cost of a flow rate limit and a much larger instantaneous power or gas demand. Whether the supply can carry that demand is usually the deciding constraint.
What is legionella and why does storage temperature matter?
A bacterium that multiplies between roughly 20 and 45 °C and is killed above about 60 °C. It is why storage below 60 °C is discouraged and why a mixing valve rather than a lower setting is the safe way to prevent scalding.
What is a thermostatic mixing valve?
A valve that blends stored hot water down to a safe delivery temperature at the outlet. It lets the tank stay above 60 °C for safety while the tap runs at 43 °C or below.
How much does an insulating jacket save?
On an older uninsulated cylinder, a great deal — standby loss can run to several kilowatt-hours a day, and a jacket typically pays for itself within months.
Should I turn the water heater off when away?
For more than a few days, yes. For a weekend, reheating from cold usually costs about as much as the standby loss avoided.
Why does my hot water run out mid-shower?
Peak demand exceeding the first-hour rating. Either the tank is too small, recovery is too slow, or the thermostat has been turned down enough to shrink the effective volume.
Does a heat pump water heater change the sizing?
Yes — recovery is slower than a resistance element, so a larger tank is usually needed for the same first-hour rating. The running cost is a fraction of it, which is the trade.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/water-heater-size-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Water Heater Size Calculator"></iframe>
<p style="font:13px/1.5 system-ui,sans-serif;margin:6px 0 0;color:#64748b">Powered by <a href="https://www.thecalclibrary.com/water-heater-size-calculator" style="color:#64748b">Water Heater Size Calculator</a> from The Calc Library</p>The only condition is that the credit line below the frame stays in place. That one line is what pays for the tool being free — it is how anyone else finds it.
Related calculators
Hot Tub Chemical Calculator
Small volume means the margin for error is tiny.
OpenPipe Flow Rate Calculator
Flow goes with the square of the bore.
OpenSeptic Tank Size Calculator
Sized on the greater of flow and bedrooms.
OpenRing Size Calculator
Finger measurement to US, UK, EU and Japanese sizes.
Open