Mass Flow Rate Calculator
This mass flow rate calculator lets you work out the water flow through a wet heating system using heat output and the difference between flow and return temperatures.
It applies to any water-based heating system, including traditional boilers and heat pumps. If you just want the calculation, you’re in the right place.
If you want the full explanation of what mass flow rate means, why it matters, and how to measure it properly in the real world, the main guide goes into this in far more detail:
👉 Mass Flow Rate Explained for Heat Pumps
https://energy-stats.uk/mass-flow-rate/
Set the variable to solve for, then enter the other values. Flow conversions update automatically.
When would you use this calculator?
You might use a mass flow rate calculator if you’re:
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Checking circulation in a wet heating system
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Sanity-checking design or commissioning figures
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Looking at radiator sizing and required flow
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Investigating low delta-T issues
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Analysing heat pump performance and COP
This page focuses purely on the calculation itself.
Want the full explanation?
This calculator is just one small part of the picture.
The main article covers:
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What mass flow rate means in real heating systems
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Why low flow causes poor performance and instability
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How mass flow, delta-T and heat output link together
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Real data examples from a Vaillant aroTHERM system
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The difference between calculated flow and measured flow
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Common mistakes homeowners and installers make
If you prefer a clear visual explanation, this Heat Geek video does an excellent job of explaining mass flow rate, delta-T, and why flow matters in real wet heating systems:
https://www.youtube.com/watch?v=i2YSRlasgEQ
👉 Read the full guide: Mass Flow Rate Explained for Heat Pumps
https://energy-stats.uk/mass-flow-rate/
If you’re running or planning a heat pump system, that article is well worth reading.
Vaillant aroTHERM | DT5 | Mass Flow Rate Triangle
Frequently asked questions
What units does the calculator use?
The calculator uses standard heating system units for heat output and temperatures, then calculates mass flow rate using typical water properties.
Is this only for heat pumps?
No. The calculation applies to any wet heating system. It’s especially useful for heat pumps, but the maths is the same for boilers and other water-based systems.
Why is mass flow rate important?
Because it determines how much heat can be moved around the system. In wet heating systems, insufficient flow often leads to poor heat delivery and unstable operation. The full guide explains this in more depth.
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