GPM / Tons / ΔT Calculator

Solve the hydronic heat transfer equation for flow, capacity, or temperature difference.

Solve For
GPM
—
gal/min
BTU/hr
—
BTU/hr
Fluid Factor Cf
—
× 500 (water = 1.000)
Formula
Tons = GPM × ΔT × 500 × Cf / 12,000

Where:
 · GPM = flow rate in gallons per minute
 · ΔT = supply/return temperature difference (°F)
 · 500 = 8.34 lb/gal × 60 min/hr × 1 BTU/lb·°F
 · Cf = fluid factor, (ρ·cp)glycol at the mean fluid temp ÷ (ρ·cp) of 60°F water (1.0 for water)
 · 12,000 = BTU/hr per ton

Typical ΔT: CHW = 10–12°F  |  HHW = 20–40°F  |  Condenser = 10°F
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About This Calculator

This hydronic capacity calculator ties together flow rate, cooling or heating load, and the supply-to-return temperature difference (ΔT) using the standard water-side heat equation. Pick which quantity to solve for — GPM, tons, or ΔT — enter the other two, choose the fluid, and the tool returns the answer along with the equivalent load in BTU/hr.

It is the quick check engineers use to size pumps and coils: how much flow a chiller or boiler needs, how many tons a measured flow and ΔT represent, or what temperature difference a given flow will produce. A fluid selector applies a glycol factor, evaluated at the mean fluid temperature, so glycol systems return the higher flow they need for the same load.

Formula & Method
LoadQ (BTU/hr) = 500 · Cf · GPM · ΔT
TonsTons = GPM · ΔT · 500 · Cf ÷ 12,000
GPMGPM = Tons × 12,000 ÷ (500 · Cf · ΔT)
ΔTΔT = Tons × 12,000 ÷ (500 · Cf · GPM)

This is the standard hydronic heat balance, Q = m·cp·ΔT, in GPM form. The constant 500 = 60 min/hr × 8.34 lb/gal × 1.0 BTU/lb·°F for water near 60°F; 12,000 BTU/hr = 1 ton of refrigeration. For 180–200°F heating water the exact constant is about 485–487 (hot water is lighter), so the 500 convention overstates hot-water capacity by roughly 3%. The fluid factor Cf = (ρ·cp)glycol at the mean fluid temperature ÷ (ρ·cp) of the 60°F water behind the 500, so 500·Cf is the glycol’s true constant; properties are from Dow’s typical data for inhibited glycols by volume percent (DOWTHERM SR-1 ethylene glycol, DOWFROST HD propylene glycol) — the same data and method as our Glycol Correction calculator. At 45°F it is about 0.92 for 25% ethylene glycol and 0.84 for 50% (0.95 and 0.85 for propylene glycol).

Frequently Asked Questions
What is the 500 factor in the GPM, tons, and Delta-T formula?
The 500 is the water-side heat constant. It comes from 60 minutes per hour times 8.34 pounds per gallon times 1.0 BTU per pound per degree Fahrenheit, which together convert GPM and a temperature difference into BTU per hour. So heat flow in BTU per hour equals 500 times GPM times Delta-T for plain water.
How do I convert tons to GPM for chilled water?
Multiply tons by 12,000 to get BTU per hour, then divide by 500 times the Delta-T to get GPM. At the common chilled water design of a 10 degree Fahrenheit rise, one ton needs about 2.4 GPM. This tool does the same math and lets you solve for GPM, tons, or Delta-T from the other two values.
Why does glycol change the result?
Glycol antifreeze carries less heat per gallon than water for the same temperature change, because its specific heat is lower (partly offset by a higher density). This calculator multiplies the 500 constant by a fluid factor from Dow’s glycol data at the mean fluid temperature: at 45°F about 0.92 for 25 percent ethylene glycol and 0.84 for 50 percent, or 0.95 and 0.85 for propylene glycol. A 50 percent ethylene glycol loop therefore needs about 20 percent more flow than water for the same load.
What Delta-T should I assume?
Typical design temperature differences are about 10 to 12 degrees Fahrenheit for chilled water, 20 to 40 degrees Fahrenheit for hot water heating, and about 10 degrees Fahrenheit for condenser water. A larger Delta-T reduces the required flow and pump energy but may require more coil surface, so it is a design trade-off.
Related Tools
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