Universal Propylene & Ethylene Glycol Calculator for HVAC & Hydronic Systems

• System Volume (V): The total liquid capacity of the loop in gallons: all piping, plus expansion, buffer, and storage tanks, plus a safety factor.
• Present Concentration (P): The glycol percentage in the loop now, measured with a refractometer.
• Desired Concentration (D): The target percentage for freeze or burst protection at your lowest expected temperature, plus 5°F of margin.
• Source Strength (S): The glycol percentage of the product you're adding, printed on the label (for example, 95% concentrate or a 70% blend).

Gallons per foot = 0.0408 × ID², where ID is the pipe's inside diameter in inches.

Example: 150 ft of 2" Schedule 40 steel (2.067" ID) = 0.0408 × 2.067² × 150 = 26.1 gallons.

Gallons of glycol product = V × D ÷ S. Fill the rest with distilled or deionized water.

Example: a 200-gallon system at 40% using 95% concentrate = 200 × 40 ÷ 95 = 84.2 gallons of product + 115.8 gallons of DI water.

G = V × (D − P) ÷ (S − P), where G is the gallons of system fluid to drain, then replace with the same amount of glycol product.

Example: a 200-gallon loop testing at 25%, target 40%, using 95% concentrate = 200 × 15 ÷ 70 = drain and replace 42.9 gallons.

The common formula G = V × (D − P) ÷ (100 − P) assumes 100% glycol, and most online glycol calculators use it. No inhibited glycol ships at 100%. In the example above, that formula calls for 40 gallons. Using 95% concentrate, the loop ends at about 39%. Using a 70% blend, it ends at about 34%, well short of the 40% target. The correct amount with a 70% blend is 66.7 gallons. This calculator asks for the strength of the glycol you're actually using, so the dosage is right for concentrates and pre-mixed blends.

Yes. This calculator uses standard specific gravity and volume formulas that work with most inhibited Propylene and Ethylene glycol fluids. All percentages are percent glycol by volume. If your product is a pre-diluted blend, enter the glycol percentage of the blend, not 100%. Always verify specific requirements with your manufacturer's technical data sheet (TDS).

The Concentration Adjustment section is designed to match the way techs actually test systems:

• System Volume (Gal): The total gallons in the loop, either calculated in Section 1 or known from drawings.
• Source Glycol %: The glycol strength of the product you're adding, printed on the drum or tote (for example, 95% concentrate or a 55% blend).
• Current % (Testing): The actual concentration in the system now, measured with a refractometer.
• Target % (Desired): The concentration you want to end up with, based on required freeze protection.

No. Protection depends only on the final glycol concentration in the system. A system at 35% has the same freeze and burst protection whether it was filled from a 95% concentrate, a 70% blend, or a 50% blend. A stronger product simply takes fewer gallons to reach the same target, so compare products by cost per gallon of glycol, not per pail. When diluting, use distilled or deionized water. Minerals in tap water use up corrosion inhibitors.

No. Freeze protection improves as glycol is added, but only up to about 60–70%. Beyond that, the glycol itself starts to freeze out of the solution, and the freeze point rises again. Undiluted ethylene glycol freezes at about 9°F, so concentrate stored outdoors or in cold lines can freeze. Propylene glycol behaves differently: near-pure PG thickens into a glass-like solid instead of freezing cleanly.

Concentrations above 60% also reduce heat transfer and become very thick in the cold, which strains pumps. For circulating systems, Vapco recommends staying at or below 60% glycol. Most systems need only 30–50%.

Take a clean sample of system fluid and place it on the refractometer lens. Read the glycol percentage according to the instrument’s scale for either Propylene or Ethylene glycol. Make sure you read the scale for the correct glycol type, since the two read differently. Then:

• Select the correct Fluid Type (Propylene or Ethylene) in the Refractometer Check section.
• Enter the percentage reading into the Refractometer Reading (%) field.
• The calculator will estimate Freeze Point, Burst Protection, and Boiling Point based on published industry reference data.

This same reading can be entered as the Current % (Testing) in the Concentration Adjustment section to determine how much to drain and refill.

Vapco strongly advises against concentrations below 30%. Below that level, corrosion inhibitors become too diluted to protect system metals, and glycol loses its bacteriostatic properties. This allows biological growth (bacteria, algae, and slime) to flourish, which can clog pumps and foul heat exchangers. If a system must run below 30%, inhibitor levels should be boosted and a professional biocide monitoring and treatment program is required. For standard low-maintenance operation, keep concentrations at 30% or higher.

Freeze point is the temperature where ice crystals begin to form. Above it, the fluid is fully liquid and the system operates normally.

Burst protection covers the range below the freeze point. As the fluid cools, water freezes out first as ice crystals. The glycol left behind gets stronger and stays liquid, so the fluid becomes a flowable slush instead of solid ice. The slush takes up more room than the liquid did, and the extra volume moves into the expansion tank or air space. As long as that room exists, pipes and components don't crack. The system can't run, but it survives undamaged. Burst protection requires less glycol than freeze protection for the same temperature.

Some products advertise "pumpable" or "flow" temperatures below the freeze point. There is no industry test standard for these ratings, so design to the freeze point when the system must operate.

Protect to the freeze point, at least 5°F below the lowest expected temperature, if any of these apply:

• The system has to run in cold weather (snow melt, outdoor loops, process cooling).
• The system shuts down for winter but has to restart while it's still cold.
• The loop has to survive a power outage or pump failure.
• There's little or no room for expansion.

Burst protection is enough only when the system stays shut down the entire time it's below its freeze point and has room for the slush to expand. A closed chilled water system that sits idle all winter and restarts in spring is a typical example.

Use Propylene Glycol where low toxicity is required: food and beverage processing (NSF HT1), systems with possible incidental contact with potable water, and residential applications. Use Ethylene Glycol for industrial and commercial systems where toxicity is not a concern. It offers better heat transfer and lower viscosity at the same concentration. Check local codes, as some jurisdictions restrict ethylene glycol. Never mix the two types in the same system.

Use the Concentration Adjustment section of this tool. By entering your Total System Volume, your Current Test Percentage (via refractometer), and your Desired Target Percentage, the tool will provide the exact amount of fluid to drain and replace with either water or glycol concentrate.

The Safety Factor % in Section 1 is there to account for unknowns: extra piping, coils, and components that weren’t measured exactly. A default of 10% is typical for most jobs. For systems with large heat exchangers, multiple risers, or where drawings are incomplete, counter staff or contractors may choose a slightly higher factor to avoid under-filling.

The Capacity Margin (Safety) multiplies the entered BTU/hr load to account for real-world conditions, future expansion, and unknowns. A value of 1.15 (15% margin) is common. Using higher margins (up to 2.0) may be appropriate for very critical loads or uncertain designs, but always coordinate with the system engineer or manufacturer recommendations.

The flow and tonnage calculations use standard HVAC formulas (BTU/hr = GPM × factor × ΔT and 12,000 BTU/hr per ton). The factor is adjusted for glycol type and approximate concentration. This provides a practical engineering estimate for sizing discussions, but it should not replace a full heat load design. Always confirm final equipment selection with a licensed engineer or the OEM’s design tools.

Inside sales and counter staff can use the calculator as a guided script. Ask the contractor:

• “What kind of glycol is in the system now—Propylene or Ethylene?”
• “What percentage did your refractometer just read?”
• “Does the system need to run in the cold, or does it sit idle over winter?”
• “What percentage do you want to target for your lowest ambient?”
• “About how many feet of each pipe size do you have?”
• “Do you have any expansion or buffer tanks, and what are their gallon ratings?”
• “Do you know the system BTU load and design ΔT?” (if they want flow/tonnage guidance)

With these answers, the counter person can walk through each section on-screen and print or email a complete summary for the contractor.