Antifreeze Mix Chart: Freeze & Boil Points by Mix Ratio

The antifreeze mix chart

The full chart, for ethylene glycol and water. The freeze and boil points are by mass percentage from the ASHRAE Handbook and Dow Chemical data, at 14.7 psia (sea level); the by-volume column is the equivalent mix measured by volume. Freeze points below −60°F are shown as below −60°F because the table does not resolve them further.

Source notes: freeze and boil points by mass are the ASHRAE Handbook (Chapter 31) and Dow Chemical ethylene glycol data at 14.7 psia. The by-volume column is the volume equivalent of each mass row. Pure ethylene glycol freezes at 9°F and boils at 387°F — the standard values. Boil points are at sea level; a pressurised cooling system raises them, as the calculator’s cap setting shows.

Mass vs volume: the distinction that matters

The famous 50/50 figure is 50% by volume: half a jug of antifreeze, half a jug of water. But ethylene glycol is denser than water — about 1.11 times as dense — so the same 50/50 by volume is 52.4% antifreeze by mass. The distinction is worth real degrees: 50% by mass freezes at only −28.9°F, while 50% by volume freezes at −35.1°F, because the volume mix carries more glycol. Every figure on this page that says “by volume” is the mix as a driver pours it, and every table row is by mass, with the volume equivalent beside it. If a chart does not say which basis it uses, assume volume for a driver’s chart and mass for an engineer’s table — and check, because the same “50%” is six degrees apart between the two.

How the lookup works

Two interpolations. First, volume to mass: your volume percentage is matched against the volume column, and the mass is interpolated linearly between the two bracketing rows — 50% by volume sits between 47.6% volume (50% by mass) and 52.7% volume (55% by mass), so the mass is 50 plus 5 times 2.4 over 5.1, which is 52.4%. Second, mass to freeze and boil: the freeze and boil points are interpolated linearly between the same two mass rows — at 52.4% by mass, the freeze point is −28.9°F plus −13.1°F times 2.4 over 5, which is −35.1°F, and the boil is 225°F plus 2°F times 2.4 over 5, which is 225.9°F. Between 60% and 75% by mass the freeze point is below −60°F and the table does not resolve it, so the calculator reports “below −60°F” there; between 75% and 80% it estimates from the −60°F anchor at 75% and marks the figure approximate.

The worked example, line by line

Setup: 50% antifreeze by volume, at sea level.

Line 1 — volume to mass. 50% by volume sits between 47.6% volume (50% by mass) and 52.7% volume (55% by mass): 50 + 5 × (50 − 47.6) / (52.7 − 47.6) = 52.4% by mass.

Line 2 — freeze point. At 52.4% by mass, between −28.9°F (50% by mass) and −42.0°F (55% by mass): −28.9 + (−13.1) × 2.4 / 5 = −35.1°F, or −37.3°C.

Line 3 — boil point. At 52.4% by mass, between 225°F (50% by mass) and 227°F (55% by mass): 225 + 2 × 2.4 / 5 = 225.9°F, or 107.7°C.

Line 4 — the cap. A 15 psi radiator cap adds about 40°F, so the same mix boils at about 265.9°F under pressure — published charts give 265°F for 50/50 under a 15 psi cap.

Line 5 — the reverse. To protect to −20°F, the freeze curve is inverted: −20°F needs 46.1% by mass, which is 43.6% by volume.

Every branch, checked

30% by volume: 32.3% by mass — freeze 3.5°F, boil 220.5°F. A weak mix for mild winters: it freezes just above zero, so it is not enough for a hard freeze.

40% by volume: 42.5% by mass — freeze −12.8°F, boil 223.0°F. A middle mix: protection to nearly −13°F, good for most of the northern US.

60% by volume: 62.2% by mass — freeze below −60°F, boil 232.2°F. Maximum cold protection, at the cost of less heat transfer — the mix is thicker and carries heat worse than 50/50.

70% by volume: 71.6% by mass — freeze below −60°F, boil 243.9°F. Still below −60°F, but past the peak: richer than this and the freeze point climbs back up.

Plain water, 0%: freeze 32.0°F, boil 212.0°F at sea level. Water alone is a fine coolant in summer and a cracked block in winter — never run it through a freeze.

Reverse, −40°F: to protect to −40°F you need 51.9% antifreeze by volume — just past the 50/50 mix. The reverse lookup inverts the freeze curve, so it returns the leanest mix that reaches the target: richer mixes also protect, but they cost more and cool worse.

The rules

Never exceed 70% antifreeze. Protection peaks near 68% by mass — around 70% by volume — and then falls: an 80% mix by mass freezes at −52.2°F, a 90% mix at −21.6°F, and a 95% mix at −3.0°F, all warmer than the 50/50 mix. Pure antifreeze freezes at 9°F, warmer than a winter night in most of the US. More glycol past the peak buys a higher boil point and worse freeze protection, worse heat transfer, and a thicker fluid the water pump works harder to move. 50/50 is the standard recommendation for a reason: it protects to −35.1°F, boils at 225.9°F at sea level, and transfers heat nearly as well as water. Propylene glycol is different: a 50% propylene glycol mix protects to about −27°F, less than ethylene glycol’s −35.1°F, so do not use this chart for it — and do not mix the two types in one system. Check the mix you actually have with a hydrometer or refractometer: the number on the jug is the mix you poured, not the mix in the radiator after a top-up with water.

What the defaults assume

The defaults are a 50% mix by volume — the standard 50/50 — at sea level, 14.7 psia, with a 12-quart system at 30% for the top-up. The table is ethylene glycol only; propylene glycol follows a different curve. The interpolation is linear between the 5% mass steps, which is an approximation: the true freeze curve is smooth, so a figure between rows is close, not exact, and the table itself rounds the source data. The cap adjustment is a rule of thumb — about 40°F for a 15 psi cap — not a steam-table calculation. The top-up assumes you drain the same amount of old mix that you add as pure antifreeze, so the system volume stays the same.

Rounding convention, stated plainly: percentages are shown to one decimal — 52.4% by mass — and temperatures to one decimal — −35.1°F, 225.9°F — with the Celsius figures converted from the Fahrenheit ones. The table rows are the published values, shown as printed. “Below −60°F” means the table does not resolve the freeze point further, not that it is exactly −60°F.

Output · formula · meaning

Who uses this

• The winter driver: mixes 50/50 by volume and learns it freezes at −35.1°F and boils at 225.9°F — protection for any winter in the lower 48 states, with the mass-vs-volume distinction spelled out.
• The cold-climate driver: needs protection to −40°F and learns the reverse lookup says 51.9% by volume — just past 50/50 — instead of guessing at a richer mix that cools worse.
• The “more is better” mixer: pours 90% antifreeze and learns it freezes at −21.6°F — warmer than the 50/50 mix — and that protection peaks near 68% and falls past it.
• The top-up DIYer: has a 12-quart system at 30% and learns it takes 3.4 quarts drained and 3.4 quarts of pure antifreeze added to reach 50% — a number, not a guess.

Accuracy & limitations

The table is the ASHRAE Handbook and Dow Chemical ethylene glycol data by mass at 14.7 psia, and the arithmetic is exact for the numbers you enter: the volume-to-mass conversion interpolates the volume column linearly, and the freeze and boil points interpolate the mass table linearly between the 5% steps. But the interpolation is an approximation — the true freeze curve is smooth, not a straight line between rows, so a figure between rows is close, not exact. A cross-check: published volume-based charts list the 50/50 mix at about −34°F (−37°C), while interpolating the ASHRAE mass table gives −35.1°F (−37.3°C) — the same mix within a degree, and this page uses the table interpolation so the calculator and the chart agree. The “below −60°F” zone is unresolved by the table: 60% to 75% by mass all read below −60°F, and the 75% to 80% estimate is anchored at −60°F at 75% and marked approximate. The 15 psi cap adjustment is a rule of thumb — about 40°F — and the published figure for 50/50 under a 15 psi cap is 265°F, against this page’s 265.9°F. The chart is ethylene glycol only: propylene glycol at 50% protects to about −27°F, a different curve, and the two types must not be mixed. Real coolant also carries corrosion inhibitors that shift the numbers slightly, and a hydrometer or refractometer reading of the actual system beats any chart. No studies or statistics are cited on this page beyond the ASHRAE and Dow data, because none are needed: every figure is the table plus arithmetic on the numbers you enter.