A candy thermometer is a type of large, glass mercury thermometer that measures candy syrups during cooking. A candy thermometer does for sugar what a meat thermometer does for meats, with the only difference being that the candy thermometer can read higher levels of temperature. In general, a candy thermometer is designed to measure temperatures as low as 40°F (about 4.4°C) and as high as 400°F (about 204.4°C).

Using a candy thermometer involves placing the thermometer in a standing or hanging position in a pan while the sugar is cooking. When using a candy thermometer, the bulb part of the instrument should not touch the bottom of the pan. If it does, the candy thermometer will yield a very high temperature rating.

There are several candy thermometer styles to choose from. One of the older styles of candy thermometer comes with a liquid-based feature. Another candy thermometer has a digital display. With this type of thermometer, the chef can identify the various stages of sugar when heated. These include sugar soft-ball, soft crack, hard crack, very hard ball, thread and caramelized sugar. Each stage of the sugar cooking process is shown next to its appropriate temperature on the candy thermometer.

The rectangular metal candy thermometer is favored by most in the culinary field. A rectangular thermometer is encased in a matching metal frame designed to prevent the thermometer bulb from touching the bottom of the pan. If this type of candy thermometer slips into the pan and the bulb touches the base, it does not display a reading. This helps ensure the proper reading is measured every time.

A candy thermometer is a vital tool in candy making. This is because of the precise timing necessary during the cooking period. With a candy thermometer, one can ensure a certain type of consistency at the end of the candy making process. Without it, the candy may crumble too easily, crack, or otherwise fail to hold its proper shape.