A TDEE calculator gives you a starting estimate. Your real TDEE is the number of calories that keeps your weight steady, and you can measure it at home with a kitchen scale, a bathroom scale and four weeks of honest tracking.
This guide shows the method, the formula and a full worked example.
Why formulas are only a starting point
Every TDEE formula predicts your resting metabolism from age, sex, height and weight, then multiplies it by an activity factor. A 2005 review by Frankenfield and colleagues found that even the best equation, Mifflin-St Jeor, predicted resting metabolic rate within 10% of measured values for most people, but not for everyone. Some people were further off.
The activity factor adds more uncertainty. Two people who both call themselves “moderately active” can differ by hundreds of calories a day because of walking, standing and fidgeting.
So treat the TDEE calculator result as a first guess. Your own data will correct it.
The formula
Body weight changes when energy in and energy out do not match. A common working estimate is that 1 kg of body weight change equals about 7,700 kcal (about 3,500 kcal per lb).
Real TDEE ≈ average daily intake − (weight change in kg × 7,700 ÷ number of days)If you lost weight, the weight change is negative, so the formula adds calories back. That makes sense: you burned more than you ate.
Step-by-step method
- Pick a steady intake. Use your calculator result or your normal eating. You do not need to diet for this test.
- Weigh and log your food for 28 days. Use a kitchen scale and a food app. Include oils, drinks, sauces and snacks. Missing items are the biggest source of error.
- Weigh yourself every morning. Same scale, after the toilet, before food or drink, in the same clothes.
- Use weekly averages, not single days. Daily weight can jump 1–2 kg from water, salt, carbs and digestion. Weekly averages smooth this out.
- Compare week 1 with week 4. Using the first and last weekly averages over 21 days (week 1 midpoint to week 4 midpoint) gives a cleaner trend than day 1 vs day 28.
- Run the formula.
Worked example
Sara logs her food for four weeks. Her average intake is 1,900 kcal a day.
| Week | Average morning weight |
|---|---|
| 1 | 72.4 kg |
| 2 | 72.1 kg |
| 3 | 71.9 kg |
| 4 | 71.6 kg |
From the middle of week 1 to the middle of week 4 is 21 days. Her weight changed by 71.6 − 72.4 = −0.8 kg.
Energy from that change: −0.8 × 7,700 = −6,160 kcal. Per day: −6,160 ÷ 21 = −293 kcal.
Real TDEE ≈ 1,900 − (−293) = about 2,190 kcal a day.
Her calculator said 2,100 kcal. Her real number is about 90 kcal higher, so she can eat slightly more and still lose at the same pace.
How to read your result
| What happened | What it means | What to do |
|---|---|---|
| Weight flat (±0.2 kg over 3 weeks) | Your intake ≈ your TDEE | Use this intake as your maintenance number |
| Weight fell | TDEE is above your intake | Use the formula; keep going if the pace suits you |
| Weight rose | TDEE is below your intake | Use the formula; check for unlogged food first |
Common mistakes
- Testing for less than 3 weeks. One or two weeks are dominated by water changes.
- Starting a new diet or training plan at the same time. New training and lower carbs both shift water weight. Hold your routine steady.
- Including the first week of a big diet change. Cutting carbs can drop 1–2 kg of water in days. Start counting from week 2 of any change.
- Guessing portions. Use a scale for at least the test period.
- Women: comparing different points of the menstrual cycle. Water retention changes across the cycle. A 4-week window covers one full cycle, which helps.
When to repeat the test
Repeat after every 4–5 kg of weight change or when your activity changes a lot, such as a new job, a new sport or an injury. TDEE falls as body weight falls; see why your TDEE drops as you lose weight.
Limits of the method
The 7,700 kcal per kg figure is an average. Early weight loss is partly water and glycogen, so the formula can overstate your TDEE in the first weeks of a diet. Research by Hall and colleagues shows the real energy content of weight change varies between people and over time. The method is still far more personal than any equation.
If you want to set a deficit from your real number, Tuff Search explains how to choose a safe calorie deficit.
FAQ
How many weeks do I need? At least 3, ideally 4. Longer is more accurate.
Can I use my fitness watch instead? Watches estimate calories burned and can be off by a wide margin. Your scale trend is direct evidence.
Do I have to eat the same every day? No. The average matters, so weekends count. Just log everything.
Sources
- Frankenfield D, et al. (2005). Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults. J Am Diet Assoc 105(5):775–789. https://pubmed.ncbi.nlm.nih.gov/15883556/
- Hall KD, et al. (2011). Quantification of the effect of energy imbalance on bodyweight. Lancet 378:826–837. https://doi.org/10.1016/S0140-6736(11)60812-X
This article is general information for healthy adults, not medical advice.