Devices that measure metabolic rate can tell dieters how many calories they need to eat each day to lose weight, according to experts.

Metabolic rate is a number that represents how many calories you would burn in a single day with minimal physical activity, dietitian Vicki Bovee told Reuters Health. The number is influenced by body weight, gender, age, and other factors, and can range from 1,000 to close to 3,000.

Knowing their metabolic rate can be important for dieters, according to Bovee, who measures the metabolic rate of every person who visits her obesity and weight management clinic.

“Not everyone is built the same,” she said, and once people know how many calories they need to maintain their normal body weight, they can figure out exactly how many calories they need to cut in order to start losing weight.

Losing one pound per week typically means cutting 500 calories per day, she said in an interview, meaning that people who need 2,600 calories to stay the same weight need to cut intake down to 2,100 daily calories to lose weight at that rate.

“It’s a lot easier to know how many calories you are supposed to be eating, rather than guessing,” according to Bovee, who is based at the University of Nevada in Reno.

Metabolic rate measurements can be particularly useful for people who are trying to cut calories but find they are not losing weight, perhaps because they are not cutting calories enough given their intake needs, Bovee said.

One way to measure metabolic rate is to use a handheld device available at gyms and doctors’ offices, which people breathe into for 10 minutes, Bovee said. The device works by calculating how much oxygen people take in to maintain normal body functions, which is a reflection of their metabolic rate, she explained.

The handheld test typically costs between $50 and $100, and dieters can visit http://www.metabolicfingerprint.com and enter their zip code to find locations that administer the test.

Other, non-handheld devices can also measure metabolic rates. People who don’t want to seek out a device can also use standardized charts that estimate metabolic rates using body weight and height, but Bovee said the charts are not nearly as accurate.