Why calibrate
Dial thermometers are easily put out of adjustment by rough handling, as the UC Davis-hosted calibration paper puts it. NDSU Extension says every thermometer should be calibrated at least once a month and again any time it is dropped or exposed to extreme temperatures. Penn State Extension sets the target accuracy at plus or minus 2°F. A thermometer outside that range turns a reading into a guess, which matters most for the official safe temperatures below, where a few degrees are the margin.
Method 1: ice water (32°F / 0°C)
This is the method to use first. Water with ice in it sits at the freezing point, 32°F or 0°C, at any elevation. NDSU Extension recommends it for high altitudes because there is nothing to correct for.
- Make a slush. Fill a tall glass with crushed ice and add cold water, about half and half by NDSU's description. Penn State says to fill the glass to the top with crushed ice, then add water and stir.
- Use crushed ice, not cubes. The UC Davis-hosted calibration paper reports that water with ice cubes or large chunks will probably sit at 34 to 35°F rather than 32°F. Keep the water slightly below the top of the ice.
- Insert the probe. Put the sensing area at least 2 inches into the center of the slush. Do not let it touch the sides or bottom of the glass.
- Wait. Give it 15 to 30 seconds, as Penn State advises, until the reading stops moving.
- Read and adjust. It should show 32°F (0°C). If it does not, adjust it while the stem is still in the slush.
Method 2: boiling water
The boiling method checks the thermometer near the top of its range. Bring a pot of water to a full rolling boil, insert the stem without touching the pot, wait for the reading to settle and compare it with the boiling point for your elevation. Keep your hand out of the steam.
Water only boils at 212°F (100°C) at sea level under standard air pressure. Higher up, the air pressure is lower and water boils cooler. NDSU Extension gives a simple rule: lower the 212°F setting by 1°F for each 550 feet above sea level. A paper hosted by UC Davis food safety gives standard-atmosphere formulas for the same thing. The table compares both.
| Elevation | Formula | °C | NDSU rule |
|---|---|---|---|
| 0 ft (0 m) | 212.0°F | 100.0°C | 212.0°F |
| 1,000 ft (305 m) | 210.2°F | 99.0°C | 210.2°F |
| 2,000 ft (610 m) | 208.4°F | 98.0°C | 208.4°F |
| 3,000 ft (914 m) | 206.6°F | 97.0°C | 206.5°F |
| 4,000 ft (1,219 m) | 204.8°F | 96.0°C | 204.7°F |
| 5,000 ft (1,524 m) | 203.0°F | 95.0°C | 202.9°F |
| 6,000 ft (1,829 m) | 201.1°F | 94.0°C | 201.1°F |
| 7,000 ft (2,134 m) | 199.3°F | 92.9°C | 199.3°F |
| 8,000 ft (2,438 m) | 197.4°F | 91.9°C | 197.5°F |
| 9,000 ft (2,743 m) | 195.5°F | 90.8°C | 195.6°F |
| 10,000 ft (3,048 m) | 193.6°F | 89.8°C | 193.8°F |
The two agree within 0.2°F up to 10,000 feet, so the simple rule is enough. At 5,000 feet, a well calibrated thermometer in boiling water should read about 203°F, not 212°F. Adjusting it to 212°F there would make it read about 9°F too high from then on.
Weather matters too. The same paper shows that at a barometric pressure of 29.0 inches of mercury water boils 1.5°F lower than at the standard 29.921, and at 30.8 inches 1.4°F higher. That is close to Penn State's whole 2°F tolerance, which is another reason the ice method is the better reference. The guide on high altitude cooking and baking covers what lower boiling points change in the kitchen.
Adjusting a dial thermometer
NDSU Extension says a bimetal stem thermometer should have a calibration nut under the dial. Penn State describes the adjustment: keep the stem in the ice water or boiling water, hold the nut with a small wrench or pliers, and turn the dial head until the needle points to the correct temperature. Test it again afterwards to confirm the new reading.
Dial thermometers measure along a length of the stem, not at the tip; the UC Davis-hosted paper puts the sensing coil at about 3 inches of the stem. NDSU Extension says to insert them to the dimple on the stem or at least 2 inches, which is also why the ice slush needs to be deep enough to cover that length.
One check is not a full check. The UC Davis-hosted paper notes that a dial thermometer adjusted at 32°F could still be about 2 percent off at 150°F. Testing in boiling water as well tells you whether it holds near cooking temperatures.
Digital thermometers
Digital models vary. NDSU Extension says some can be calibrated; Penn State says digital thermometers generally cannot and should be checked for accuracy now and then with the same water tests. The manual tells you whether yours has an adjustment. If it has none and reads outside the tolerance in ice water, the practical options are to correct each reading by the known error or to replace it. NDSU adds that refrigerator and freezer thermometers that read wrong usually cannot be recalibrated and must be replaced.
If your thermometer only shows Celsius and a recipe or chart gives Fahrenheit, the temperature converter does the conversion. Ice water is 0°C and boiling water at sea level 100°C; at 5,000 feet the boiling point is about 95.0°C.
What a few degrees mean
The FDA's safe minimum internal temperatures are the readings a food thermometer exists to confirm. The food safety temperatures guide has the full list. If a thermometer reads 3°F high, it shows each target before the food actually reaches it:
| Food | FDA minimum | A 3°F-high thermometer shows |
|---|---|---|
| Beef, pork, veal, lamb steaks and roasts | 145°F (63°C), plus 3 minutes rest | 148°F at the true 145°F |
| Ground meats | 160°F (71°C) | 163°F at the true 160°F |
| Poultry, leftovers, casseroles | 165°F (74°C) | 168°F at the true 165°F |
Put the other way: when it shows 165°F, the food is at 162°F. A calibrated thermometer removes that gap. The meat cooking time calculator estimates how long a roast needs, and the thermometer confirms when it is there. For oven settings in Celsius or gas marks, use the oven temperature converter.
Quick checklist
- Crushed ice, cold water, glass full to the top.
- Probe at least 2 inches in, not touching the glass.
- Wait 15 to 30 seconds; it should read 32°F (0°C).
- Adjust the nut while the stem is in the water.
- Optional second check in boiling water, against the boiling point for your elevation.
- Repeat monthly and after every drop.
Frequently asked questions
What should a food thermometer read in ice water?
It should read 32°F (0°C) in a slush of crushed ice and cold water, about half and half. NDSU Extension says to put the sensing area at least 2 inches into the middle of the slush without touching the sides or bottom, and to wait for the reading to settle.
What temperature does water boil at my elevation?
212°F at sea level, about 1°F less for every 550 feet up by the NDSU Extension rule. At 5,000 feet that gives 202.9°F, and the standard-atmosphere formula gives 203.0°F. The weather moves it by up to about 1.5°F either way.
Which is better, the ice method or the boiling method?
The ice method, for most kitchens. Ice water is 32°F at any elevation, while boiling water changes with altitude and air pressure. NDSU Extension recommends the ice point method for high altitudes for that reason. The boiling method is useful as a second check near cooking temperatures.
Can a digital thermometer be calibrated?
Some can. NDSU Extension says some digital models can be calibrated, while Penn State Extension says digital thermometers in general cannot and should have their accuracy checked from time to time with the same water tests. The manual says which kind you have.
How often should I calibrate a food thermometer?
NDSU Extension says at least once a month, and any time the thermometer has been dropped or exposed to very hot or cold temperatures. Penn State Extension says dial thermometers may need it as often as daily.
Sources
- NDSU Extension FN712, Keep Hot Foods Hot and Cold Foods Cold: A Foodservice Guide to Thermometers and Safe Temperatures, reviewed June 2025 (ice point method with 50/50 slush and 2-inch depth; boiling point method; 1°F lower per 550 feet; calibrate monthly and after drops; calibration nut; some digital models can be calibrated)
- Penn State Extension: Thermometer Use and Calibration (wait 15 to 30 seconds; adjust the nut with a small wrench or pliers; target plus or minus 2°F; digital thermometers generally checked rather than calibrated)
- O. Peter Snyder, Calibrating Thermometers in Boiling Water: Boiling Point / Atmospheric Pressure / Altitude Tables, hosted by UC Davis food safety (altitude and barometric formulas and tables; ice cubes give 34 to 35°F; crushed ice slush)
- U.S. Food and Drug Administration: Safe Food Handling (safe minimum internal temperatures: 145°F with 3-minute rest, 160°F, 165°F)