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GPM to BTU Calculator

Find the heat a water flow carries from its flow rate and temperature difference — or the flow a heating or chilled-water load needs — with the hydronic 500 × GPM × ΔT equation.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system
Calculate

Result

Heat transfer rate
100,000BTU/h
Equivalent cooling capacity
8.33tons

10 gpm at 20 °F ΔT carries 100,000 BTU/h (8.33 tons).

  • Water gets lighter as it heats; at typical boiler temperatures the true factor is a few percent below 500. Use “Other fluid” with properties at your temperature for more precision.
Flow rate
10gpm
Temperature difference
20°F
Factor used
500
Show the working
  1. Factor for water = 8.33 lb/gal × 60 min/h × 1 BTU/lb·°F ≈ 500
    500
  2. Q = factor × GPM × ΔT
    500 × 10 × 20 = 100,000 BTU/h

Where the 500 comes from

A US gallon of water weighs about 8.33 lb, there are 60 minutes in an hour, and it takes 1 BTU to warm a pound of water by 1 °F. Multiplying gives 8.33 × 60 × 1 ≈ 500, so:

  • Heat (BTU/h) = 500 × GPM × ΔT
  • Chilled water: tons = GPM × ΔT ÷ 24, because 500 ÷ 12,000 = 1/24.

For glycol and other fluids, multiply 500 by the fluid’s specific gravity and specific heat from the supplier’s data at your operating temperature.

Using it in design

A larger temperature difference moves the same heat with less flow, which means smaller pipes and pumps but a wider swing at the terminal units. Equipment and terminal manufacturers state the temperature differences their products are designed for; size flow to match.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Calculate
Heat from flow and ΔT
Flow rate
10 gpm
Temperature difference (supply − return)
20 °F
Fluid
Water (factor 500)

Result

Heat transfer rate
100,000 BTU/h
Flow rate
10 gpm
Temperature difference
20 °F
Equivalent cooling capacity
8.33 tons
Factor used
500

10 gpm at 20 °F ΔT carries 100,000 BTU/h (8.33 tons).

Assumptions

  • 500 = 8.33 lb per US gallon (water near 60 °F) × 60 min/h × specific heat 1 BTU/(lb·°F), rounded. Other fluids: 500 × specific gravity × specific heat.

Limitations

  • Glycol and other fluid properties vary with concentration and temperature; enter values from the fluid supplier, not a generic table.

Frequently asked questions

How many GPM do I need per ton of chilled water?

At a 10 °F temperature difference, 2.4 GPM per ton; at 12 °F, 2 GPM per ton. In general GPM per ton = 24 ÷ ΔT.

Is 500 exact for hot water?

No. Water is less dense when hot, so at typical boiler temperatures the true factor is a few percent lower. Use the “other fluid” option with properties at your temperature for more precision.

What about glycol?

Glycol mixtures have a lower specific heat than water, so they carry less heat per gallon per degree. Use the supplier’s specific gravity and specific heat for your concentration.

Sources

  1. NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.
  2. Water Density — U.S. Geological Survey Water Science School. Water’s density is roughly 1 g/mL (about 62.4 lb/ft³) and decreases as it warms; the page tabulates density by temperature.

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