How to use this calculator
- Enter loop flow. Use measured or planned flow through the coil, heat exchanger or hydronic branch.
- Enter Delta T. Use the absolute entering-to-leaving water temperature difference.
- Enter target load. Use the BTU/h or kW load you want the loop to carry.
- Set fluid properties. Use water defaults or enter density and specific heat for the actual glycol mix and temperature.
- Check required flow. Compare entered flow with the back-solved GPM and use the result in pipe, pump and balancing checks.
How it works
The calculator uses the sensible liquid heat-transfer equation:
q = rho · cp · Q · Delta T
where rho is fluid density, cp is specific heat,
Q is volumetric flow and Delta T is the loop temperature
rise or drop.
For water in imperial units, the same relationship is often simplified to: BTU/h = 500 · GPM · Delta F The 500 factor is a rounded water-only shortcut, so the calculator also reports the difference between that shortcut and the entered fluid properties.
Required flow is found by rearranging the formula:
Q_req = q / (rho · cp · Delta T)
and the required temperature difference is
Delta T_req = q/(rho·cp·Q).
Carry the required flow into the pipe size by flow calculator before checking head loss in the pipe pressure drop calculator. For a closed heating or chilled-water loop, use pipe volume and expansion tank sizing after the run is defined.
Worked example
Verified against the live calculator
With 10 GPM of water and a 20°F loop Delta T,
the water shortcut gives 500 × 10 × 20 = 100,000 BTU/h. The
full property calculation with the default water density and specific heat
lands essentially at the same value. If the target load is
150,000 BTU/h at the same Delta T, required flow is about
15 GPM.
Frequently asked questions
What is the BTU per GPM Delta T formula?
For water in imperial units, hydronic heat transfer is commonly estimated as BTU/h = 500 x GPM x Delta F. The full form is q = rho x cp x flow x Delta T, which lets you enter density and specific heat for water or glycol mixtures.
How do I calculate GPM from BTU/h and Delta T?
For the water shortcut, GPM = BTU/h / (500 x Delta F). This calculator uses the full fluid-property equation and reports the equivalent required flow.
Can this be used for chilled-water systems?
Yes, as a sensible heat-transfer screen. Enter the absolute temperature difference across the coil or heat exchanger and the correct fluid properties at operating temperature.
Does this size pumps, pipes or coils?
No. It calculates the heat-transfer relationship between flow, Delta T and load. Use the pipe size by flow calculator, pipe pressure drop calculator and manufacturer coil or pump data for final selection.
Method & assumptions
- Calculates sensible heat transfer only; phase change and latent load are not included.
- Density and specific heat are user-entered so water, glycol and process fluids can be screened without hidden property tables.
- The 500 x GPM x Delta F shortcut is shown only as a water approximation.
- Final hydronic design still needs coil/heat-exchanger data, boiler or chiller performance, pump curves, control valve authority, pipe pressure drop, balancing, glycol concentration, fouling allowance, relief and local code checks.