Engine Coolant Mix Ratio Calculator

Introduction to glycol concentration and freeze-point depression in engine coolant

A liquid-cooled engine moves waste heat from the combustion chambers into a radiator using a water-based fluid. Water on its own is an outstanding coolant, with roughly twice the specific heat capacity of a 50/50 glycol mixture, but it freezes at 0 °C and boils at about 100 °C at sea level. Freezing is the destructive failure: water expands as it turns to ice, and a block full of ice can crack a cylinder head, split a radiator core or push out a core plug. Adding a glycol to the water depresses the freezing point, raises the atmospheric boiling point a little, and carries the corrosion inhibitor package that keeps iron, aluminium, copper and solder from being attacked.

The important physical fact that most mix charts hide is that freeze-point depression is not linear in concentration. Doubling the glycol does not double the protection. The curve steepens as concentration rises, reaches a minimum at the eutectic composition, and then reverses: past the eutectic, adding more glycol makes freeze protection worse. Dow's tabulated data for aqueous ethylene glycol makes this visible. Freeze points fall from -7.8 °C at 18.1 % glycol by volume to -48.3 °C at 57.8 %, drop below -51 °C somewhere between about 63 % and 74 %, and then climb back to -46.8 °C at 78.9 %, -36.9 °C at 84.3 % and -19.4 °C at 95 %. Neat ethylene glycol freezes near -13 °C. A calculator that draws a straight line from 0 % to 100 % glycol is therefore not merely imprecise; it gives dangerously wrong advice at high concentrations, and it is the reason this page interpolates inside a published table instead of fitting a line.

The second fact worth internalising is that boil-over protection is mostly a pressure effect, not a glycol effect. Going from plain water to a 50 % ethylene glycol mixture buys about 8 °C of atmospheric boiling point. A 103 kPa (15 psi) radiator cap buys about 21 °C on top of that, because it raises the absolute pressure in the system to roughly 205 kPa. This calculator therefore treats the cap rating and your elevation as first-class inputs rather than ignoring them.

How to use the coolant mix calculator on a real cooling system

1. Pick your measurement units. The metric setting works in °C, kPa, metres and litres; the US customary setting works in °F, psi, feet and US quarts. Switching converts the numbers already in the form rather than reinterpreting them.
2. Choose the glycol chemistry. Ethylene glycol is the normal automotive choice; propylene glycol is used where accidental ingestion by people or animals is a serious concern, and it protects less well at the same volume percent.
3. Enter the coldest ambient temperature you expect the parked vehicle to see and a safety margin. Dow recommends picking a table temperature at least 3 °C (5 °F) below the lowest anticipated ambient, so 5 °C is a sensible default and 10 °C is appropriate if you cannot verify the mixture with a refractometer.
4. Enter the radiator cap rating (the gauge pressure stamped on the cap) and your elevation. These do not change the freeze recommendation, only the reported boil-over temperature.
5. Enter the cooling system capacity from the owner's manual and pick the antifreeze product you are actually buying: a nominal concentrate is around 96 % glycol by volume once the inhibitor package is accounted for, and a premix is 50 %.
6. Optionally enter the current glycol concentration measured with a refractometer. If you do, the calculator tells you how much fluid to drain and replace instead of assuming an empty system.
7. Press Calculate coolant mix. Use Copy shareable link to send the exact scenario to somebody else, or Copy result summary to paste the recommendation into a work order.

Formula and interpolation model behind the recommendation

The calculator first converts your inputs to a target freeze point. With as the coldest expected ambient temperature and as the safety margin:

It then searches the tabulated freeze-point curve for the smallest glycol volume fraction whose freeze point is at or below that target. Because the published table is discrete, values between two anchor rows are found by linear interpolation inside a segment, which is accurate to a few hundredths of a degree over the 2 % steps used here, and is not the same thing as fitting one straight line to the whole curve:

Here and are the volume percentages of the two bracketing table rows and , are their tabulated freeze points. The search is restricted to the monotonically decreasing branch of the curve, which ends at 60 % glycol by volume for both chemistries; the calculator refuses to quote a number on the far side of the eutectic rather than pretending the trend continues.

Boiling is handled separately. Ambient pressure is derived from your elevation with the International Standard Atmosphere troposphere relation, in which is geopotential altitude in metres:

The cap rating is a gauge pressure, so the absolute pressure inside a healthy system is . The saturation temperature of water at that pressure is evaluated with the IAPWS-IF97 region 4 backward equation, and the glycol boiling-point elevation read from the same manufacturer table is added on top:

Finally, the fill quantities. For an empty system of capacity filled with a product whose glycol content is percent by volume, the product and water volumes are and the remainder. For a system already holding percent glycol, the volume that must be drained and replaced with the product follows from a simple volume balance:

When the system is already too strong the same balance is used with , which is the case of draining coolant and topping up with distilled water.

Worked example: a -30 °C winter with a 103 kPa cap

Suppose a 6.5 litre cooling system in a region where the coldest expected overnight temperature is -30 °C. You add a 5 °C safety margin, the radiator cap is stamped 103 kPa (15 psi), the vehicle lives at 500 m elevation, and you are buying a standard ethylene glycol concentrate rated at 96 % glycol by volume.

1. Target freeze point: -30 °C - 5 °C = -35 °C.
2. The table brackets -35 °C between 47.6 % glycol (-33.8 °C) and 49.6 % glycol (-36.4 °C). Interpolating inside that segment gives (-35 + 33.8) / (-36.4 + 33.8) = 0.4615, so P = 47.6 + 0.4615 x 2.0 = 48.5 % glycol by volume.
3. Atmospheric boiling point at 48.5 %: interpolating the boil-point column between 47.6 % (107.2 °C) and 52.7 % (108.3 °C) gives 107.4 °C, i.e. a glycol elevation of 7.4 °C over pure water.
4. Ambient pressure at 500 m is 101.325 x (1 - 2.25577e-5 x 500)^5.25588 = 95.5 kPa. Adding the 103 kPa cap gives 198.5 kPa absolute, at which water saturates at about 120.0 °C, so the mixture boils at roughly 127.4 °C.
5. Fill quantities for 6.5 L at 48.5 % using a 96 % concentrate: 6.5 x 48.52 / 96 = 3.29 L of concentrate and 3.21 L of distilled water.

Interpreting that result: 48.5 % is close enough to a standard 50/50 premix that buying premix is the pragmatic answer, and the calculator says so. The value of the number is in the opposite direction. If the recommendation had come back at 33 %, it would tell you that a 50/50 fill is stronger than you need and you are giving away specific heat capacity for nothing. If it had come back at 58 %, it would tell you that a premix jug is not enough and you need concentrate.

Reference data: tabulated freeze and boil points for aqueous glycol

These are the anchor rows the calculator interpolates between, taken directly from Dow's published tables for inhibited glycol heat transfer fluids. The volume percentages are glycol, not product; the boiling points are at 101.325 kPa (760 mmHg). Rows marked "below -51" are the region where the source states freeze points are below -60 °F but does not give a number, and this page will not invent one.

The next table compares the two chemistries at the mix ratios people actually buy. Values are interpolated from the same source tables using the method described above, so they are what the calculator itself would report at sea level with no cap pressure.

Practical notes on mixing, topping up and common mistakes

Concentrate against premix

Antifreeze is sold as concentrate, which is roughly 95 % to 97 % glycol plus the inhibitor package, and as premix, which is nominally 50 % glycol in deionised water. Pouring premix into a system that is already near 50 % barely moves the ratio, and pouring concentrate into a system that is already strong overshoots. Always think in terms of the final concentration in the whole system, which is what the drain-and-replace output on this page computes for you.

Distilled water and scale

Use distilled or deionised water. ASTM D3306 itself specifies coolant performance at 50 volume percent in deionised water, and the calcium and magnesium salts in hard tap water precipitate as scale on the hottest surfaces, which is exactly where you least want an insulating layer.

Mixing thoroughly and verifying

Concentrate and water added separately will stratify for a while. Pre-mix in a clean container where you can, and afterwards verify the result with a refractometer rather than a floating-ball hydrometer; ASTM D1177 is the reference freezing-point method and refractometers correlate with it far better than density balls do, particularly for propylene glycol.

Do not mix inhibitor chemistries

This page models freeze and boil behaviour only. Real coolant selection also depends on additive chemistry (IAT, OAT, HOAT and manufacturer-specific formulations). Mixing incompatible types can gel, drop out silicates, or shorten inhibitor life dramatically. If the history of the system is unknown, flush it and refill with the specification in the service manual.

Limitations, assumptions and where this model stops being valid

• Interpolated table, not a thermodynamic model. The freeze and boil values come from one manufacturer's typical-property tables for inhibited glycol fluids. Dow states explicitly that these are typical numbers and not specifications; different formulations, and glycerin-containing Type V coolants, will shift the curve by a degree or two.
• The recommendation is capped at 60 % glycol by volume. Dow gives 60 % as the maximum recommended concentration for efficient heat transfer, and the numeric freeze data ends there. Beyond it the curve passes through a eutectic and reverses, so the calculator reports that the target is unreachable rather than extrapolating.
• A 30 % floor is enforced. Dow warns that below 25 % to 30 % glycol the inhibitor concentration may be too low for adequate corrosion protection and the solution may be at risk of bacterial contamination, so the calculator never recommends a weaker mix even in a warm climate.
• Boiling-point elevation is treated as pressure independent. The glycol contribution is read at 101.325 kPa and added to the water saturation temperature at system pressure. This is a standard engineering approximation and is good to roughly a degree over normal cap pressures, but it is an approximation.
• The cap is assumed to hold its rating. A weak cap spring, a leaking hose, a failed head gasket pressurising the system with combustion gas, or air trapped in the block will all change real boil-over behaviour far more than the mix ratio does.
• Freeze point is not burst protection. Below the freezing point a glycol solution forms a pumpable slush long before it becomes a solid block; Dow tabulates burst protection separately and it needs much less glycol. This page reports freeze protection, the stricter of the two.
• Ethylene glycol is toxic. It is sweet-tasting and attractive to animals. Clean spills immediately and recycle or dispose of used coolant according to local rules.

Coolant mixing questions answered

Sources

Freeze-point and boiling-point data: The Dow Chemical Company, Engineering and Operating Guide for DOWTHERM SR-1 and DOWTHERM 4000 Inhibited Ethylene Glycol-based Heat Transfer Fluids (Form No. 180-01190), Table 3 "Typical Freezing and Boiling Points of Aqueous Solutions" and Table 4 "Typical Concentrations Required to Provide Freeze and Burst Protection", and the matching DOWTHERM SR-1 Product Information sheet (Form No. 180-01312) — dow.com engineering guide (PDF). Propylene glycol data: The Dow Chemical Company, Engineering and Operating Guide for DOWFROST and DOWFROST HD Inhibited Propylene Glycol-based Heat Transfer Fluids, Table 3. Coolant specification limits: ASTM International, ASTM D3306, Standard Specification for Glycol Base Engine Coolant for Automobile and Light-Duty Service (freezing point -36.4 °C max and boiling point 108 °C min for Type I, -31.0 °C max and 104 °C min for Type II, both at 50 volume percent in deionised water) — ASTM D3306; freezing-point test method ASTM D1177, Standard Test Method for Freezing Point of Aqueous Engine Coolants. Concentrate usage range: SAE International, SAE J1034, Automobile and Light Truck Engine Coolant Concentrate — Ethylene Glycol Type (50 % to 70 % concentrate by volume in water). Water saturation temperature: IAPWS, Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam (IAPWS-IF97), region 4 backward equation . Ambient pressure against altitude: International Standard Atmosphere troposphere relation (ISO 2533).