How do I check my kitchen thermometer is accurate?
The ice point test takes two minutes, costs nothing and tells you whether the number you are trusting for cooked chicken is real.
- Difficulty
- beginner
- Time
- 10 min
- Read
- 3 min
Short answer
Pack a glass to the top with crushed ice, add cold water until the ice is just wet, stir, and put the probe into the middle of the slush without touching the sides. It should read 0°C. Within half a degree is fine, one or two degrees is worth allowing for, more than that means recalibrate or replace.
Do this first
Thermometers drift — from being dropped, from heat cycling, from age. That matters most for exactly the readings you rely on for safety, and the check is a two-minute job with things already in your kitchen.
Step by step
- Make a proper ice slush, not iced water.Fill a tall glass or a jug right to the top with crushed or small ice cubes, then add just enough cold water to fill the gaps so the ice is wet all through. Ice cubes floating in a glass of water with clear water between them is not at 0°C, and it gives a false pass. Stir it and let it stand for a minute.
- Take the reading in the middle of the slush.Push the probe into the centre, at least 3cm deep, keeping it away from the glass wall and the base. Stir gently with the probe and wait for the reading to settle. The correct reading is 0°C.
- Interpret the result.Within about 0.5°C: fine, use it as it is. One to two degrees out: usable if you remember the offset — write it on a piece of tape on the handle. More than two degrees: recalibrate it, or replace it if it cannot be adjusted. A thermometer three degrees low will tell you chicken is done when it is not.
- Do the boiling point test as a second check.Bring a pan of water to a vigorous boil and hold the probe in the water, away from the base and sides. It should read 100°C at sea level. This test is less reliable than the ice point — boiling point falls slightly with altitude, and steam and the hot pan interfere — so treat a disagreement between the two as the ice point being right.
- Recalibrate a digital thermometer.Many have a calibration function: a small recessed reset hole to press with a pin, or a button combination held while the probe is in the ice slush. Check the manual — the procedure is model-specific and it usually just tells the unit that what it is currently reading is 0°C.
- Recalibrate an analogue dial thermometer.Most dial thermometers have a hex nut immediately under the dial head. Hold the dial with pliers or a spanner and turn the nut while the probe is in the ice slush until the needle sits on zero. Then check it again.
- Repeat the check periodically and after any knock.Every few months for a thermometer in regular use, and any time it has been dropped, left in a hot oven, or given a reading you did not believe. It is the same two minutes each time.
Tips
- Do this on a brand new thermometer before you trust it. Factory calibration is usually good and is not guaranteed.
- If a thermometer will not hold calibration between checks, replace it — a unit that drifts unpredictably is worse than no thermometer, because you will believe it.
- Crushing the ice matters. Whole cubes leave pockets of water above 0°C, which is exactly the error you are trying to detect.
- Two thermometers that agree with each other but not with the ice slush are both wrong. The ice point is the reference, not the consensus.
Questions people ask
Why is the ice point test better than the boiling test?
Because 0°C is 0°C anywhere. Boiling point changes with atmospheric pressure and altitude, and the steam and the hot pan make it a fiddlier measurement. Use the ice point as the reference and boiling as a sanity check.
My thermometer reads 2°C in ice — is it broken?
Not necessarily. If it is consistently 2°C high, you can either recalibrate it or simply subtract two from every reading. What matters is that the error is consistent — check it twice to be sure.
Does this work for an oven thermometer?
An oven thermometer measures air temperature over a much higher range and cannot be dipped in ice slush usefully. Check an oven thermometer by comparing two of them, or by measuring a pan of boiling water in the oven, and treat oven readings as approximate anyway.