The best time to find out your candy thermometer isn’t reading correctly is before it’s in a boiling pot of fudge.

Kimberly Holland
April 03, 2019
ToscaWhi / Getty Images

Candy thermometers aren’t incredibly expensive. You can buy one for well under $10.

But cost does not equate to import in the kitchen. Indeed, this little gadget pulls a heavy load. If it’s right, your frostings, fudges, and toffees will turn out spectacularly. You may even become famous for them among friends and the office break-room population.

If the candy thermometer is wrong, however, you could be living in infamy, with fudges that crumble when cut and frostings that shatter when sliced.

The wrong time to discover your candy thermometer isn’t working, or is measuring a few degrees off, is when the probe is popped into a vat of boiling sugar and cream. But, as it turns out, that’s precisely when most of us realize it.

Before you spend the effort (and the ingredients) to make caramel or buttercream only to watch it fail, test your thermometer to make sure it’s still reading the right temp. Here’s how.

How to Test a Candy Thermometer

1. Fill a pot with water, and place the thermometer in the pot. Make sure the bulb is fully immersed but not touching the bottom or sides of the pot. Heat on high to boiling.

2. When the boiling is constant and rolling, make sure it’s reading 212°F or 100°C (at sea level). 

3. Set a timer for five minutes, then check the thermometer again. Is it still 212°F? Good news! Your thermometer is accurate.

Not showing 212°F? Read on to find out how to calibrate the difference.

RELATED: 15 Fabulous Fudge Recipes

How to Calibrate an Inaccurate Thermometer

If you’ve had your thermometer for some time (such as the one I inherited from my grandmother), or even if it’s brand-new off the shelf, odds are, it isn’t spot-on accurate. That doesn’t mean you need to toss it. Instead, learn how to calibrate a candy thermometer so you can use it when cooking.

When the water is boiling, read the thermometer. If it’s not 212°F (100°C), what is it? If it’s 216°F, you know your thermometer reads hot—4°F hotter. If it’s showing 205°F, your thermometer reads 7°F cold.

You can account for that when you’re cooking by adding or subtracting the variance in the temp you need to reach.

For example, if your recipe calls for bringing your caramel frosting to a soft boil, you’d normally want to reach 235°F. If your thermometer reads 7 degrees cold, you actually want to reach 242°F.

If your thermometer reads 4°F hot, you’re looking for a reading that’s at 239°F.

WATCH: How to Make Beef Fat Fudge:


For People Living in High Altitudes

The closer you get to the clouds, the lower your boiling temperature falls. Contact your local extension office to find out what the temperature for boiling is in your area. Use that to adjust your final temperature goal.

Another way to know: For every 500-foot increase in elevation, the temperature of boiling decreases 0.5°C. If you live in Albuquerque, New Mexico (about 5,300 feet), your boiling temp is likely around 202°F (94°C), or 10 degrees cooler than sea level. 

You need to adjust the temperature gpa; accordingly. For example, if your fudge calls for a temp of 238°F (115°C), you’ll want to aim for 228°F—if your thermometer is correct when in boiling water. If it isn’t, you’ll need to calibrate for that amount, too.

Candy Thermometer Tips

Test regularly: Use the pot of boiling water test at least once a year, or more frequently if you use the thermometer regularly. If you have a big holiday baking marathon coming up, be sure to test before you start any of those recipes.

Write it down: Can’t remember if your thermometer is off by four degrees, or 14? Make a little note to yourself and tape it on the inside of a cookbook or cabinet door. Update it when you do new readings.

Know when to replace: If you start getting different results with every batch of fudge or pan of toffee, the thermometer may be busted, despite your valiant calibrations. Look for a new one.

Store upright: Candy thermometers are fragile, so don’t toss them into your gadget drawer. Instead, place it upright in a glass jar that has a base of rice or beans to protect the bulb. You can place your other food thermometers in here, too.