How accurate are TDEE calculators?

Close for most people, and further off for some. The resting part of the math lands within 10% of a lab measurement for about four people in five. The bigger error is the activity level you pick. So use a calculator for a starting range, then let four weeks of your own meals and weigh-ins give you the real number.

Two calculators can give answers several hundred calories apart for the same person, which makes the whole thing feel like guesswork. Most of a gap like that comes from the activity setting. The resting formulas sit much closer together: in the 1990 study that built Mifflin-St Jeor, the formula many calculators use, the older Harris-Benedict formula ran about 5% high.

The simple version

A calculator is a weather forecast for your body. It’s right about the season and often off about the day. Your scale is the thermometer.

The numbers

• In a 2013 study of 337 adults, Mifflin-St Jeor came within 10% of measured resting burn for 82% of them: 87% of people without obesity and 75% of people with obesity.
• A 2005 review of the four most used formulas found Mifflin-St Jeor the most reliable. It also warned that older adults and US minority groups were underrepresented in the research behind all of them.
• 10% of a 1,700 calorie resting burn is 170 calories. Multiplied up for activity on the usual calculator scale, 1.2 to 1.9, that’s about 200 to 320 calories a day either way, before any error in the activity level.
• Activity is the bigger lever. On that scale each level up adds 0.175 times your resting burn, about 300 calories at a 1,700 resting burn. Our calculators pick the level from your steps, in bands from a 2004 review of pedometer studies.

What to do

• Use a calculator that starts from your steps, like the calories from your steps calculator, and take its answer as a range.
• Eat at the middle for four weeks, weigh in most mornings, then measure your real TDEE.