Deficitly
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How accurate are TDEE calculators?
A good TDEE calculator is usually within about 10% either way for most people: roughly ±190 calories at a TDEE of 1,890. The bigger error is often the activity level: picking one level too high adds 240–310 calories a day. The most accurate TDEE is your own: eat a steady amount for 3–4 weeks and see what your weight trend does. The box below does the math.
Where the error comes from
| Source | Typical size | How to reduce it |
|---|---|---|
| BMR equation | ±10% for most people | Mifflin-St Jeor; Katch-McArdle if you know your body fat |
| Activity level label | 240–310 kcal per level | Use your step count and real workouts |
| Watch calories | Often 25–90% off | Use steps from your watch, not its calories |
| Counting rest twice | ~1 kcal per kg per exercise hour | Count only the extra calories of exercise |
| Food counting | Often 10–30% under | Weigh food for a while; trust the scale trend |
The equations: good, not exact
Resting burn is the biggest part of TDEE, and it's estimated from your sex, age, height and weight. The best of the equations, Mifflin-St Jeor, lands within about 10% for most people but misses some by more, especially very muscular people (underestimated) and people with a lot of body fat. Knowing your body-fat percentage helps: the Katch-McArdle equation works from lean mass instead. The equations are compared on TDEE formula.
The activity level: the biggest guess
Most calculators ask you to choose "sedentary", "lightly active" or "moderately active". One step between those labels is worth 240 calories a day for a 70 kg woman and 310 for an 85 kg man, and many people pick one level too high: "moderately active" really means about 17,000 steps a day, or 8,000 plus around 4–5 hours of exercise a week. See what each activity level means in steps.
Fitness watches
Watches count steps fairly well, but their calorie numbers are much less reliable. In a Stanford study of seven wrist devices, none estimated energy expenditure within 20%, and the average error ranged from 27% to 93% depending on the device. Many also include the calories you'd burn anyway: a 75 kg (165 lb) person burns about 190 extra calories in an hour of weight training, but the total including resting is about 265. Eating back a watch's total is a common reason weight loss stalls: see should you eat back exercise calories?
How Deficitly tries to be accurate
- Mifflin-St Jeor for resting burn, or Katch-McArdle when you enter your body fat.
- Your average daily steps instead of an activity label, turned into calories from your height and weight.
- Workouts by type, minutes and times per week, counting only the extra calories above resting (MET − 1).
- Steps taken during workouts aren't counted twice.
- A range (±10%) instead of one falsely exact number, and it errs slightly low for very active people on purpose.
The formulas and sources are on how Deficitly calculates.
How to find your real TDEE
- Start with a calculator estimate and eat about that much, or simply eat as you normally do.
- Track roughly what you eat every day for 3–4 weeks. Consistency matters more than perfection.
- Weigh yourself several times a week, first thing in the morning, and look at weekly averages.
- Work it out: TDEE ≈ average calories eaten + (weight lost in kg × 7,700 ÷ days). For example: you ate 1,800 a day for 28 days and your average weight fell by 1.2 kg: 1,800 + 1.2 × 7,700 ÷ 28 = 2,130 calories a day.
- Use that number from now on, and repeat every couple of months or after a big change in weight or activity.
Leave out the first week of a new diet: cutting carbs or salt drops water weight quickly, which makes the first week look like far more fat loss than it is.
What makes a TDEE calculator accurate?
- It asks for sex, age, height and weight, and uses Mifflin-St Jeor (or Katch-McArdle with body fat).
- It measures your activity (steps, actual workouts) instead of one label for your whole life.
- It counts exercise without double-counting the calories you'd burn anyway.
- It shows a range, and tells you to check against your weight trend.
No calculator can know your exact metabolism. The "most accurate" one is the one you correct with your own results.
Why the calculator and the scale disagree
Day to day, the scale moves with water, salt, food in your gut and, for women, the menstrual cycle; swings of 1–2 kg are normal (see why your weight fluctuates). Fat loss shows up as a trend over weeks. If after 3–4 weeks the trend doesn't match the plan, adjust your calories by 100–200 and give it another few weeks (common reasons: calorie deficit but not losing weight). For what to aim for, see how many calories to eat to lose weight.
Sources
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc 2005;105(5):775–789.
- Shcherbina A, Mattsson CM, Waggott D, et al. Accuracy in wrist-worn, sensor-based measurements of heart rate and energy expenditure in a diverse cohort. J Pers Med 2017;7(2):3.
- Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 1990;51(2):241–247.