Free calculator

How big should your hot-water cylinder be?

Size the storage from your busiest hour and the recovery rate of the water-heating system that will serve it.

Heatuneed

How big should your hot-water cylinder be?

Prepared

Volume alone does not tell you whether the hot water will last

A cylinder starts with stored heat and keeps adding heat while taps are open. The useful answer is therefore its first-hour supply: the mixed water already in the tank plus what its heat pump, immersion heater or boiler can recover in that hour. This tool sizes those two parts together and recommends the smallest listed cylinder that clears the peak.

Start from a typical household

Household hot-water demand

Use the normal full-time occupancy, then raise the litres per person if guests or teenagers regularly create the peak.
L
This is water at the delivery temperature, after mixing. Around 40 L is efficient, 50 to 60 L typical, and 70 L or more covers long showers and baths.
%
Estimate the share of the whole day's hot water used within the busiest 60 minutes. Example: if the household uses 200 L/day and 120 L during back-to-back morning showers, enter 60%. Use 100% only when nearly all daily use could happen in one hour.
°C
Winter mains water sets the hardest sizing and recovery condition.
°C
The temperature at the shower or tap after the mixing valve, not the cylinder thermostat.

Cylinder and storage temperature

L
Pick a real product size or click a row in the size table. The result recommends the smallest listed size that clears the design peak.
°C
Hotter storage produces more mixed water from the same tank, but increases standing loss and scald risk. Follow local hygiene rules and use suitable temperature controls and mixing protection.

Water-heating system

The selection provides a starting recovery output. Replace it with the manufacturer's useful output when you have a product in mind.
kW
Thermal power actually reaching the stored water. For a heat pump, use heat-pump-only output unless backup is intentional. For an indirect cylinder, use the lower of the boiler output and coil rating.

Your inputs

Your cylinder size

Updated live as you change the household, temperatures or heating output.

{selected} L covers the busiest hour

The smallest listed size that clears the design peak is {size} L. Your selected cylinder supplies {supply} L against {demand} L required.

Recommended cylinder —
Selected first-hour supply —
Design busiest-hour demand —
Mixed hot water per day —
Usable water from storage —
Recovered in one hour —
Selected peak coverage —

First-hour capacity

Busiest-hour design demand —
Selected cylinder supply —

What each cylinder size can supply

The first-hour figure combines usable stored water with one hour of the entered system output. Click a row to test that volume. The star marks the smallest listed size that clears the design peak.

Cylinder First hour Peak covered Result

Recovery and first-hour supply use the useful heating output you entered. For a heat pump, any resistance backup is excluded unless you deliberately include it in that figure.

This is an early cylinder-sizing screen, not a plumbing design. It assumes 90% of nominal tank volume is usable, a 15% peak reserve, ideal mixing and constant output. Real first-hour rating, stratification, coil performance, ambient temperature, backup elements, hygiene cycles and controls are product-specific. Verify the manufacturer's first-hour rating and local plumbing and legionella requirements with a qualified installer.

Calculated with the free tool at heatuneed.com/hot-water-system-calculator/

How the calculation works

The model puts storage and recovery on one consistent mixed-water demand, so a fast heater can reduce the required storage without hiding a genuine busiest-hour shortfall.

1

Build the mixed-water demand

Occupants times litres per person gives the daily volume at the tap temperature. The busiest-hour percentage finds the back-to-back peak, then a 15 percent design allowance covers normal uncertainty.

2

Convert stored heat into usable litres

Water stored above the delivery temperature is mixed with cold water. The temperature-rise ratio converts nominal cylinder litres into mixed litres, after allowing 10 percent for unusable volume and imperfect stratification.

3

Add one hour of recovery

Heating output divided by the energy needed for one litre at the tap gives the litres recovered per hour. Added to storage, this is the first-hour supply used to choose the smallest listed cylinder that clears the peak.

4

Choose the first size that passes

Each listed cylinder is checked against the reserved busiest-hour demand. The first one whose stored and recovered litres cover that peak becomes the recommendation; the final product still needs its manufacturer-rated first-hour performance checked.

Common questions

Because tank size is only the stored half of first-hour supply. A heater with more thermal output replaces hot water while it is being used, and a hotter tank produces more mixed water through a thermostatic valve. Product selection should still use the manufacturer's tested first-hour rating, because real stratification and controls are not ideal.

Often yes. Compressor-only recovery is much slower than gas or electric resistance recovery, so extra storage helps it ride through a morning peak without bringing on an inefficient backup element. The table shows that trade directly: lower the entered heat-pump output and the recommended volume rises.

Use the useful thermal output actually reaching the cylinder. For a heat-pump water heater, use heat-pump-only output unless you intentionally want backup resistance counted. For an immersion heater, use its element rating. For an indirect cylinder, use the lower of the available boiler output and the coil's rated transfer output.

It creates more mixed litres from the same tank, not free capacity. The heater must put in more heat, standing loss rises and scald risk becomes more serious. Follow the appliance instructions and local hygiene rules, and use a correctly specified thermostatic mixing valve where required.

Use 100% when most of the household's showers or baths can happen back to back. Use a lower share only when hot-water use is genuinely spread across the day. If unsure, keep 100%: underestimating the peak directly undersizes the cylinder.

Verify the manufacturer's tested first-hour rating, recovery conditions, coil rating where applicable, physical dimensions, plumbing code and legionella controls. For a heat-pump water heater, also check its air-volume, condensate, noise and electrical requirements.

Planning the rest of the building's heat system too?

Heatuneed scans rooms with LiDAR and calculates heat loss through walls, windows, doors, floors and roofs — the evidence needed to size space heating alongside the hot-water system.

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