Use this hot water mixing calculator to estimate how much usable mixed hot water you can get from a hot water cylinder.
A 200 or 250 litre cylinder does not necessarily mean you only have 200 or 250 litres available at the shower or tap. Hot water from the cylinder is normally mixed with colder mains water before you use it.
Change the cylinder size, stored temperature, incoming cold water temperature and target mixed temperature below to see how each one affects the amount of usable hot water.
If you want to calculate how much energy is needed to heat your cylinder, see my Hot Water Calculator.
If you are thinking about getting a heat pump, please visit my Trusted Heat Pump Installers directory where you will find a hand picked list created by me.
You may also want to visit the Heat Pumps Explained section of this website for more heat pump related blogs and articles.
Calculate Your Usable Hot Water
litres
The nominal volume of water stored in the cylinder.
°C
The temperature of the water stored in the cylinder.
°C
The temperature of the cold mains water.
°C
The temperature you want at the shower or tap.
Usable mixed hot water
292 L
from a 250 litre cylinder
Extra usable water from mixing
42 L
Hot water required
85.7%
Cold water mixed in
14.3%
Compare another scenario
Keep the same cylinder size and stored temperature, then compare a different incoming cold water and target temperature.
°C
°C
Original
292 L
10°C cold /
40°C mixed
Comparison
347 L
20°C cold /
38°C mixed
Difference
+55 L
18.9% more usable water
What is usable hot water?
The volume printed on a hot water cylinder is the amount of water physically stored inside it. That is not always the same as the amount of water you can use at a shower or tap.
If the stored water is hotter than the temperature you want to use, cold mains water can be mixed with it. This increases the total volume of water available at the final mixed temperature.
Why does incoming cold water temperature matter?
The colder the incoming mains water, the more hot water from the cylinder is needed to reach the temperature you want.
When incoming mains water is warmer, a smaller proportion of hot water is needed. The same cylinder can therefore provide a greater volume of usable mixed water.
Seasonal example: incoming mains water can be noticeably warmer in summer than in winter. The exact temperature varies by location, pipework and time of year, so use your own measured figure where possible.
Does shower temperature make a difference?
Yes. A lower target temperature means less stored hot water is needed in the final mixture.
For example, someone who prefers a slightly cooler shower during warm weather may get noticeably more usable water from the same cylinder without changing its stored temperature.
Does heating the cylinder hotter give more usable hot water?
Yes. If the stored water temperature is higher, more cold water can be mixed with it before reaching the desired final temperature.
That does not automatically mean hotter storage is the best setting. Higher hot water temperatures can increase heat losses and, with a heat pump, may reduce efficiency. The calculator is intended to show the mixing effect rather than recommend a particular cylinder temperature.
How does the hot water mixing calculation work?
The proportion of hot water needed is calculated from the three temperatures:
Hot water fraction = (Mixed temperature − Cold temperature) ÷ (Hot temperature − Cold temperature)
Using the default example:
(40 − 10) ÷ (45 − 10) = 0.857
So about 85.7% of the final mixed water needs to come from the cylinder.
The usable mixed volume is then:
Usable mixed water = Cylinder volume ÷ Hot water fraction
For a 250 litre cylinder:
250 ÷ 0.857 = 292 litres
So, under ideal mixing conditions, a 250 litre cylinder at 45°C could provide about 292 litres of water at 40°C when mixed with 10°C cold water.
Is this the exact amount of hot water I will get?
No. This is an ideal mixing calculation.
Real hot water systems can behave differently because of cylinder stratification, pipe heat losses, thermostatic mixing valves, cylinder design, sensor position, reheating during use and how completely the stored hot water can be drawn from the cylinder.
The calculator is best used to compare different temperatures and see how those changes affect the theoretical amount of usable mixed water.
More hot water calculators and guides
To calculate the energy needed to heat a cylinder, electricity consumption and approximate reheat time, use the Hot Water Calculator.
You may also find my guide to hot water cylinder sizing and mixing useful.
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