Glycol Correction Factor Calculator

🥶 Glycol System Inputs

Glycol Type

Glycol Concentration (%)

Operating Fluid Temp (°F) Average loop temperature; properties are evaluated here (CHW ~44°F, ice storage ~25°F, HHW 140–180°F)

Water-based GPM (baseline) GPM sized for pure water

📊 Glycol Correction Results

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Corrected GPM (glycol)

—

Flow Correction Factor (Cf)

—

Freeze Point (°F)

—

Pump Head Multiplier (at corrected GPM)

Fluid Properties at Operating Temp

Specific Gravity (vs 60°F water)—

Specific Heat (BTU/lb·°F)—

Kinematic Viscosity—

Heat Constant K (replaces 500)—

Pipe Friction Ratio (same GPM)—

Pump Power Multiplier—

How to use: Multiply your water-based GPM by Cf to get glycol GPM (or use Q = K × GPM × ΔT in place of 500). Multiply the pipe-friction part of your water-based pump head by the pump head multiplier — it already includes the higher glycol flow. Coils and control valves have their own glycol corrections; take those from the manufacturer.

📚 Glycol Selection Guide

Application	Glycol Type	Typical Concentration	Why
CHW plant, no freeze concern	EG or PG	20–25%	Minimum needed; lower viscosity penalty
CHW + outdoor piping	EG	30–35%	Protection to ~0°F design temp
Ice storage / low temp	EG	30–40%	25°F leaving temperature
Food/bev, indirect contact	PG	30–40%	Low toxicity (inhibited PG)
Extreme cold climate	EG	40–50%	Protection to about −30°F
Never exceed	—	50%	Higher conc. hurts heat transfer significantly

Inputs, results, and method — as an Excel workbook or plain CSV.

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About This Calculator

This glycol correction calculator adjusts a water-based hydronic design for ethylene or propylene glycol antifreeze. Pick the glycol type, concentration, and operating temperature and enter your water-based GPM; the tool returns the corrected glycol flow, the flow correction factor Cf, the freeze point, and the pump-head multiplier at the corrected flow, along with specific gravity, specific heat, and viscosity at that temperature.

Glycol carries less heat per gallon and flows with more friction than water, so a glycol loop needs higher flow and more pump head for the same duty — and the penalty grows as the fluid gets colder. Use the corrected GPM and the pump-head multiplier to re-check pipe size and pump selection, and use the freeze point to confirm protection for your coldest piping.

Formula & Method

Flow correction	Cf = 1 ÷ (SG × c_p) (glycol at the operating temperature; SG and c_p relative to the 60°F water behind the 500)
Glycol flow	GPM_glycol = GPM_water × Cf
Heat constant	K = 60 × ρ(lb/gal) × c_p = 500 ÷ Cf (Q = K × GPM × ΔT, BTU/hr)
Friction, same GPM	(ν_g / ν_w)^0.25
Pump head mult.	(ν_g / ν_w)^0.25 × Cf^1.75 (at the corrected GPM)
Pump power mult.	Cf × head mult. × (ρ_g/ρ_w)

Heat carried = GPM × 500 × (SG · c_p relative to water) × ΔT, so holding duty and ΔT constant means GPM_glycol = GPM_water × Cf; 500 = 8.33 lb/gal × 60 min/hr × 1.0 BTU/lb·°F for water. Density, specific heat, and viscosity are Dow’s typical data for inhibited glycols by volume percent (DOWTHERM SR-1 ethylene glycol and DOWFROST HD propylene glycol), interpolated in temperature and, for 20, 25 and 35%, between the neighbouring tabulated concentrations. The heat-capacity correction is referenced to 60°F water, the basis of the 500, so K is the true constant for the glycol at its temperature; the friction and power ratios compare with water at the same temperature (40°F for colder loops). Freeze and burst points come from the same Dow product data. The friction terms assume turbulent flow in smooth pipe (Blasius friction factor, head ∝ ν^0.25·V^1.75) — right for copper and PEX; in commercial steel the viscosity penalty is smaller and the multiplier runs roughly 5–10% high; laminar or transitional flow in cold, viscous glycol — small coil circuits, low velocities — can cost much more. Confirm against your fluid supplier’s tables for final design.

Frequently Asked Questions

Why does a glycol system need more GPM than water?

Glycol carries less heat per gallon than water because the product of its specific gravity and specific heat is lower than water's. To move the same BTU per hour you need more flow. The flow correction factor is Cf = 1 divided by (SG times Cp), with SG and Cp relative to the 60°F water behind the 500 constant, and corrected GPM equals the water-based GPM times Cf. At chilled-water temperatures a 30 percent mix raises required flow by roughly 7 percent for propylene glycol and 10 percent for ethylene glycol; 50 percent raises it by about 18 to 20 percent.

How much does glycol increase pump head and pressure drop?

Glycol is more viscous than water, which raises friction loss in the piping. In turbulent pipe flow, friction head at the same GPM scales with kinematic viscosity to about the 0.25 power, so 30 percent ethylene glycol at 44°F (about 3.1 cSt against 1.45 cSt for water) adds about 21 percent. The glycol loop also needs more flow, and friction rises with flow to about the 1.75 power, so the pump head at the corrected flow is about 1.44 times the water design for that mix. This tool reports both. These are smooth-pipe (Blasius) figures; in commercial steel pipe the viscosity penalty is somewhat smaller. Cold, viscous glycol in laminar or transitional flow can cost much more, so check coils and small branches with the manufacturer’s data.

Should I use ethylene or propylene glycol?

Ethylene glycol has lower viscosity and better coil heat transfer, so it is preferred where toxicity is not a concern, such as sealed CHW or outdoor piping. Inhibited propylene glycol has low toxicity and is used where incidental contact with potable water or food is possible, but it is more viscous, so it carries a larger pump-head and coil penalty; its flow penalty is similar or slightly smaller at the same concentration, and it needs a few percent more concentration for the same freeze point. This tool shows the freeze point for each by concentration.

What glycol concentration should I use?

Use the lowest concentration whose freeze point is at least 5°F below the coldest temperature the fluid will see (Dow’s guidance), typically 20 to 35 percent for most chilled water systems and 30 to 50 percent for cold climates or low-temperature loops. Avoid exceeding 50 percent because higher concentrations sharply hurt heat transfer and raise viscosity. Size on the freeze point, not the burst point, for any loop that must keep circulating.

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