Glycol Calculator

A glycol calculator returns how many gallons of glycol concentrate and water to mix for a hydronic heating system. It multiplies the total system volume by the target glycol percentage to get the concentrate. The rest is water. A 50 gallon system at 30% glycol needs 15 gallons of concentrate and 35 gallons of water.

Last reviewed September 2026Method & sources Report an error

Glycol Concentrate Needed

15.00 gal

Breakdown
Glycol Concentrate Needed	15.00 gal
Water Needed	35.00 gal

How to Calculate Glycol

Total system volume times the target glycol percentage gives the volume of glycol concentrate needed. The remainder is water. Exact freeze-protection temperature at a given percentage varies by glycol type (propylene vs. ethylene) and manufacturer formulation. Always check your specific product's data sheet for its exact freeze-point table. Don't rely on a single universal figure. Freeze points vary enough between products that a generic table risks under- or over-protecting your system.

Example: 50 gallon system, targeting 30% glycol concentration

A 50 gallon system, targeting 30% glycol concentration: 1. Glycol concentrate: 50 × 0.30 = 15 gal 2. Water: 50 − 15 = 35 gal

Glycol: Compare Scenarios

Scenario	Glycol Concentrate Needed	Water Needed
Small system (10 gal, 30% glycol)	3.00 gal	7.00 gal
Medium system (50 gal, 30% glycol)	15.00 gal	35.00 gal
Large system (200 gal, 30% glycol)	60.00 gal	140.00 gal

What Is Glycol?

A glycol calculator converts a hydronic heating system's total volume and a target glycol concentration into gallons of glycol concentrate and water to mix. The same math applies to other closed-loop systems. Propylene glycol is the common non-toxic choice. Ethylene glycol is more toxic and is sometimes used in non-potable industrial systems. At the same concentration, its freeze point differs somewhat.

Glycol: Pros and Cons

Pros

Straightforward, exact mixing math once you know your system volume and target percentage
Works for any system size and any target concentration within the typical 10-60% range
Explicitly flags that exact freeze-point behavior varies by product, rather than asserting one universal temperature table

Cons

Doesn't determine what percentage you need. That depends on your climate's lowest expected temperature and your specific glycol product's freeze-point data, which you should get from the manufacturer's data sheet
Doesn't account for fluid already in the system. If you're partially refilling rather than starting from empty, adjust the volumes for what's already present
Assumes you know your system's total volume. If you don't, it can typically be estimated from the boiler, piping, and any buffer tank's combined capacity, or measured by draining and refilling with a known quantity

Glycol: What a Pro Checks

Check your specific glycol product's data sheet for its exact freeze-point table. Don't rely on a generic percentage-to-temperature figure. Exact freeze protection varies by glycol type (propylene vs. ethylene) and manufacturer formulation.
Choose propylene glycol for a residential system where any human or pet contact is possible. It's the common, much less toxic choice. Ethylene glycol offers somewhat better heat transfer but is more toxic.
If you're topping off a system that already has some fluid in it rather than filling from empty, adjust the volumes for what's already present instead of using the full system volume.

Frequently Asked Questions

How much glycol do I need for my heating system?

Multiply total volume by the glycol percentage. A 50 gallon system at 30% concentration needs 15 gallons of glycol concentrate and 35 gallons of water.

What percentage of glycol do I need for freeze protection?

It depends on your climate's coldest expected temperature and your specific glycol product. Check the manufacturer's data sheet for its exact percentage-to-freeze-point table, since this varies between propylene and ethylene glycol and between formulations. As a general, not precise, sense of scale, higher percentages protect to lower temperatures.

Should I use propylene or ethylene glycol?

Propylene glycol is the common choice for residential hydronic systems since it's much less toxic. Ethylene glycol offers somewhat better heat transfer and freeze protection at the same concentration. It is more toxic, though, so it's a less common choice wherever human or pet contact is possible.

How do I find my system's total volume?

Add up the volume of the boiler, all the piping in the system, any buffer or expansion tank, and any other components holding fluid. Or, if you're refilling from empty, track the volume you add as you fill the system.

Can I mix glycol myself, or should I use pre-mixed glycol?

Both are available. Concentrate requires you to mix with water to your target percentage, which this calculator helps with. Pre-mixed solutions come at common percentages. Pre-mixed is simpler but less flexible if you need a specific concentration that isn't offered pre-mixed.

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