Preliminary numbers for estimating and for planning a fill. The fluid maker's data for the product in the system, the system designer and a refractometer reading come first. The corrosion inhibitor is not checked here.

Maintained by BuildSolver · Updated November 3, 2026

Calculator · Hydronics

Glycol freeze protection calculator

A hydronic loop that sees 5°F needs glycol in the water or the pipes can freeze and split. Dow's DOWFROST table asks for 37.7 percent propylene glycol concentrate by volume for freeze protection at 0°F, the row 5°F below that low, and 25.1 percent if ice slush is acceptable and has room to expand. In a 100 gallon system that is 37.7 gallons of concentrate and 62.3 gallons of water. This page reads Dow's tables for propylene and ethylene glycol, interpolates between rows without extrapolating, and shows that the glycol also carries less heat per gallon, about 6 percent more flow for the same load at the default mix. For the boiler itself see the boiler size calculator, and for the pipe the hydronic pipe size calculator.

Pick propylene or ethylene glycol, the type in the system, and freeze protection for a loop that has to circulate in the cold or burst protection for one that can sit as slush.
Enter the lowest temperature the fluid will see, keep a margin of at least 5°F below it, and enter the gallons the system holds.
The result shows the percent by volume, the gallons of concentrate and water, the freezing point of the mix and the heat a gallon carries. Enter the concentration measured in the system to see what to drain and replace.

Percent by volume of Dow DOWFROST (propylene glycol) or DOWTHERM SR-1 (ethylene glycol) concentrate for a design temperature from 20 to -60°F. Other brands, pump head and the inhibitor are not covered. The fluid maker's data comes first.

Glycol type

Use the type already in the system, and do not mix types or brands without the fluid maker's word. Dow calls DOWFROST low in acute oral toxicity; check the safety data sheet and the local rules for either fluid.

Protection wanted

Freeze protection keeps the fluid flowing and is the choice for a loop that has to circulate in the cold. Burst protection accepts ice crystals and slush, and holds only if the slush has room to expand.

Lowest temperature the fluid will see, °F

The coldest outdoor or buried temperature the system can reach with the fluid in it, such as the design low of the climate or the coldest night the pipes are exposed to.

Margin below that, °F

Dow says to pick a table temperature at least 5°F (3°C) below the expected lowest ambient. The design temperature is the lowest temperature less this margin, and has to be between 20°F and -60°F, the rows of Dow's table.

System volume, gallons

All the fluid the system holds: boiler or chiller, piping, emitters or coils, expansion tank and buffer tank. Piping tables and the maker's data for each component give the gallons.

Average fluid temperature for the heat figures

Dow prints the specific heat, density and viscosity at these temperatures. Pick the one nearest the average of the supply and return, for example 149°F for a 160°F supply and a 140°F return.

Concentration in the system now, % by volume (optional)

Percent of the concentrate by volume, from a refractometer or hydrometer reading for the same glycol type. Enter 0 for plain water. Leave blank when filling a new system.

Example result for the default inputs

DOWFROST (propylene glycol), freeze protection to 0°F, 100 gal system. Dow data sheet 180-01314-1019.

Freeze protection

37.7 % by volume

Concentrate

37.7 gal

Water

62.3 gal

Calculation details
Design temperature	5°F less a 5°F margin = 0°F. That is a row of Dow's table, 0°F, so the page reads it directly with no interpolation.
Freeze protection	37.7 percent DOWFROST by volume. Dow: keeps the fluid flowing with no ice crystals.
Burst protection	25.1 percent DOWFROST by volume. Ice crystals can form; the system is protected as long as the slush has room to expand.
Concentrate and water	100 gal x 37.7% = 37.7 gal of DOWFROST concentrate and 62.3 gal of water. Mix before filling, and measure the result; volumes are taken as additive.
Freezing point of this mix	-0.2°F from Dow's freezing point table, as a check on the protection table. That is about 36.0 percent propylene glycol by volume.
Specific heat and density at 149°F	0.931 Btu/lb°F and 62.91 lb/ft3 at 36.0 percent propylene glycol by volume, interpolated between the 30, 40 and 50 percent blocks of Dow's table.
Heat carried per gpm	60 x 8.409 lb/gal x 0.931 = 470 BTU/h per gpm per °F, against 500 for water. For the same load and temperature drop the flow is about 6 percent higher than with water.
Viscosity at 149°F	1.07 cP, from Dow's table. It rises steeply as the fluid cools; the pump head and friction are not worked here, so ask the fluid maker or use its pressure drop chart.

For estimation purposes only. Consult a licensed engineer for final designs.

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How the concentration and volume are worked

Water freezes at 32°F, and ice splits copper, steel and plastic pipe. Glycol in the water lowers the temperature at which ice forms, and Dow prints the percent of its concentrate, by volume, that gives freeze protection and burst protection at each temperature. The page reads that table, adds the gallons for the system, and shows what the glycol costs in heat carried per gallon. The concentrate is a corrosion-inhibited fluid, so the percent is of the concentrate and not of pure glycol.

Design temperature: Tdesign = Tlowest - marginUnder both tables Dow says that for an extended margin of protection you should select a temperature in the table at least 5°F (3°C) lower than the expected lowest ambient. That is the margin field. With a 5°F low and the 5°F margin the design temperature is 0°F. The tables run from 20°F to -60°F, and the page refuses a design temperature outside them.
Concentration: pct = Dow freeze or burst column, interpolated at TdesignAt 0°F DOWFROST needs 37.7 percent for freeze protection and 25.1 percent for burst protection, and DOWTHERM SR-1 needs 34.6 and 23.1 percent. Between two rows the page interpolates linearly, so a 2°F design temperature gives 36.2 percent of DOWFROST between the 30.4 percent of the 10°F row and the 37.7 percent of the 0°F row.
Concentrate and water: Vconcentrate = Vsystem x pct / 100, Vwater = Vsystem - VconcentrateA 100 gal system at 37.7 percent takes 37.7 gal of concentrate and 62.3 gal of water. The volumes are taken as additive, so the mix has to be checked with a refractometer after it is made.
Freezing point of the mix: interpolation in Dow's freezing point tableAs a check on the protection table, Dow's freezing point table gives -0.2°F for 37.7 percent DOWFROST. Dow's protection columns and its freezing table agree within about a degree and a half across the rows, which the tests check.
Heat carried per gpm: BTU/h per gpm per °F = 60 x (density in lb/ft3 / 7.48052) x specific heatCaleffi's constant for water, 500, is "rounded off from 8.33 x 60", and Caleffi says to use the fluid's own density and specific heat for an antifreeze solution. From Dow's table at 149°F, 40.0 percent glycol by volume carries 465 BTU/h per gpm per °F, 7 percent less than water, so the same load needs about 7 percent more flow. The default mix, 36.0 percent glycol, needs about 6 percent more. Dow prints the properties at 30, 40 and 50 percent glycol, and the page interpolates between those blocks.
Drain and replace: Vdrain = Vsystem x (pct - now) / (1 - now)A volume balance for a mix that is too weak: drain the volume, fill with concentrate. For 100 gal at 30.0 percent now and 37.7 percent wanted, it is 11.0 gal. The result holds if the volumes add, and it assumes the concentrate goes in at full strength.

Seven design temperatures through the same function

Every row is a 100 gallon system with a 5°F margin below the lowest temperature, so the design temperature is 5°F lower than the one in the label. Rows one, two, four, five, six and seven land on a row of Dow's table and give its printed figure; row three is between the 10°F and 0°F rows and is interpolated. The last column is the concentrate for freeze protection.

Glycol concentration for freeze and burst protection at seven design temperatures
Fluid and lowest temperature	Design, °F	Freeze, % by volume	Burst, % by volume	Concentrate, gal per 100
Propylene glycol, low 15°F (Dow's 10°F row)	10	30.4	20.9	30.4
Propylene glycol, low 5°F (0°F row)	0	37.7	25.1	37.7
Propylene glycol, low 7°F (between rows)	2	36.2	24.3	36.2
Propylene glycol, low -5°F (-10°F row)	-10	44.0	29.3	44.0
Propylene glycol, low -15°F (-20°F row)	-20	48.2	31.4	48.2
Ethylene glycol, low 15°F (10°F row)	10	26.2	17.8	26.2
Ethylene glycol, low 5°F (0°F row)	0	34.6	23.1	34.6

The same loop protected to -20°F needs 48.2 percent against 37.7 percent at 0°F. If the mix on hand measures 30.0 percent, it is 7.7 points short, and draining 11.0 gallons of the 100 and filling with concentrate corrects it.

Standards and data sources

Dow, DOWFROST Heat Transfer Fluid, Technical Data Sheet (form 180-01314-1019) - the freeze and burst protection columns of propylene glycol by volume from 20°F to -60°F, the note on the 5°F margin and on inhibitor levels below 30 percent glycol, the freezing point table, and the specific heat, density and viscosity at 30, 40 and 50 percent glycol by volume.
Dow, DOWTHERM SR-1 Fluid, Technical Data Sheet (form 180-01312-0417) - the same three tables for inhibited ethylene glycol: freeze and burst columns, the freezing point table, and the properties at 30, 40 and 50 percent glycol by volume.
Dow Glycol Heat Transfer Fluids (D. Cohen), Freeze and Burst Protection, published by ChemPoint - what the two columns mean: freeze protection keeps the fluid flowing without ice crystals, and at burst protection the fluid stays flowable as a slush-liquid mixture and the system is protected as long as expansion volume is accommodated.
Caleffi, idronics issue 16, Circulation in hydronic systems (2015) - the heat conveyance formula q = 500 f (delta T), the 500 as 8.33 x 60 for water near 60°F, and the instruction to use the fluid's own density and specific heat for an antifreeze solution.

Limitations of this calculator

This page gives preliminary numbers for estimating, for checking a quote and for planning a fill. The system designer, the fluid maker's data and the installer's gauge readings come first. Nothing here approves a design or a fluid.
The tables are Dow's for two inhibited fluids, DOWFROST (propylene glycol) and DOWTHERM SR-1 (ethylene glycol), and the percent is of the concentrate by volume, not of pure glycol. Another brand, DOWFROST HD, an uninhibited or automotive antifreeze and a different glycol content all have their own numbers. Use the maker's table for the fluid in the system.
Dow's tables run from 20°F to -60°F and the page does not extrapolate beyond them. Between two rows it interpolates linearly, which is an approximation, and Dow calls the figures typical examples that may not suit your situation.
The page does not size or test the corrosion inhibitor. Dow's note says inhibitor levels should be adjusted for solutions of less than 30 percent glycol, and a mix diluted by a leak, a top-up with water or water left in the system loses inhibitor with the glycol. The fluid maker's test kit or lab analysis checks it.
The gallons assume the concentrate and the water add to the system volume. Water left in a drained system, pockets of air, and the accuracy of the volume estimate move the real percent. Measure the finished mix with a refractometer or hydrometer made for the glycol type and correct for temperature as the instrument says.
The heat figure uses Dow's specific heat and density at 30, 40 and 50 percent glycol and at 104, 149 and 194°F, and compares it with the 500 of water. Viscosity is shown, but the page has no friction method for glycol, so the pump head, the pipe size and the heat exchanger performance are not worked. Cold glycol is much thicker than the figures at 104°F or above.
Burst protection allows ice crystals to form and depends on room for the slush to expand. The page does not size the expansion volume, and a loop that has to circulate at the lowest temperature needs freeze protection.
Open-loop systems, potable water, food contact, refrigerants, steam and process fluids above 250°F are outside the page. Local codes can limit which glycol is allowed in a given system and call for backflow protection and labeling. The code official decides.

Common mistakes with glycol mixes

Picking the freezing point of the lowest temperature, with no margin. Dow's note asks for a table temperature at least 5°F below the expected lowest ambient. For a loop that sees 15°F, DOWFROST for freeze protection is 24.6 percent with no margin and 30.4 percent with the 5°F margin. The weaker mix has its freezing point at the lowest temperature itself, with nothing in reserve for a colder night.
Using the burst percent on a loop that has to circulate. At 0°F Dow's burst column is 25.1 percent DOWFROST and its freeze column 37.7 percent. The weaker mix is allowed to turn to slush, and slush does not pump. A boiler loop or a snow melt loop that runs in the cold needs the freeze figure. Pick one.
Adding the glycol to a system that still holds water. 100 gal of a 37.7 percent mix poured into a system with 5 gal of water still in it is 105 gal at 35.9 percent, whose freezing point in Dow's table is about 2.5°F instead of -0.2°F. Drain and measure first, then mix to the volume the system holds, and check the finished fluid with a refractometer.
Topping up with plain water. Water lost through a leak or a relief valve takes glycol out with it, and water put back dilutes what is left. Repair the leak, measure the concentration, and enter it in the concentration field above to see how much to drain and replace.
Sizing the loop on 500 BTU/h per gpm per degree. The 500 is for water. A 40.0 percent glycol mix at 149°F carries 465, 7 percent less per gallon, so the same flow delivers that much less heat, and the same load needs about 7 percent more flow. Cold glycol is also thicker than water, which raises the pump head. Plan for it.
Mixing brands or glycol types in one system. Each fluid carries its own inhibitor package, and the percent in a table belongs to one product. Adding another brand or an automotive antifreeze to a system filled with an inhibited heat transfer fluid leaves a mix that neither table describes. Ask the fluid maker before topping up with anything else.

Common questions about glycol protection

What percent propylene glycol do I need to protect a system down to 0°F?

Dow's DOWFROST table gives 37.7 percent by volume for freeze protection at 0°F and 25.1 percent for burst protection. Dow's note says to pick a table temperature at least 5°F below the expected lowest ambient, so a loop that sees 5°F uses the 0°F row. The percent is of the concentrate, not of pure glycol. Read the row.

How much glycol do I need for a 100 gallon system?

For freeze protection to 0°F, 37.7 percent of 100 gal is 37.7 gal of DOWFROST concentrate and 62.3 gal of water. To 10°F it is 30.4 gal, and to -20°F it is 48.2 gal. Mix it before it goes into the system, count the water already in the pipes, and measure the result with a refractometer. Measure it.

What is the difference between freeze protection and burst protection?

Dow describes freeze protection as keeping the fluid flowing with no ice crystals. At burst protection ice crystals can form, and the fluid stays flowable as a slush-liquid mixture, so the system is protected as long as the expansion volume is accommodated. Burst protection takes less glycol but is only for systems that can sit as slush and have room to expand. A loop that has to circulate in the cold needs freeze protection.

Propylene glycol or ethylene glycol: which needs less?

At 0°F the DOWTHERM SR-1 table gives 34.6 percent for freeze protection, against 37.7 percent for DOWFROST, so ethylene glycol needs a little less by volume. The choice usually turns on toxicity and local rules rather than the percent. Dow describes DOWFROST as low in acute oral toxicity; read the safety data sheet of either fluid and the local requirements, and keep to the type already in the system. Brands differ.

How does glycol change the heat a hydronic system delivers?

It lowers the specific heat and raises the density, so a gallon carries less heat. From Dow's table at 149°F, 40.0 percent glycol carries 465 BTU/h per gpm per °F against 500 for water, 7 percent less, and 50.0 percent carries 450. The same load needs more flow, and the glycol is also more viscous when cold, so the pump head goes up. Size the pump for it.

How do I check the concentration in a system?

With a refractometer or a hydrometer made for the glycol type, read at the temperature the instrument is calibrated for. Dow's freezing point table lists the refractive index and the degree Brix beside each concentration of DOWFROST, so a reading can be matched to the percent. Check the inhibitor with the fluid maker's test as well. This page cannot see what is in the pipes.

Is more glycol always safer?

No. Dow's burst column stops rising at 36.6 percent for DOWFROST and 31.4 percent for DOWTHERM SR-1, so a stronger mix adds no burst protection in the table. A stronger mix also carries less heat per gallon and is thicker when cold, and Dow's ethylene glycol table shows freezing points rising again at very high concentrations. Use the percent for the temperature, with the margin, and no more.

Does this replace the fluid maker or the system designer?

No. This page is for estimating and for checking a quote or a fill. The fluid maker's data for the product in the system, the designer's calculation for the pump and the heat exchangers, and the installer's refractometer reading come first.