Your TDEE Calculator Is Wrong. Here Is How I Found My Real Number.

Every calorie calculator on the internet runs the same formula. You put in your age, height, weight, and an activity level you pick off a dropdown. It hands you back a number and calls it your TDEE, your total daily energy expenditure, the calories you burn in a day.

Mine said 2,842.

My real number, back-calculated from six months of logged data, was 2,410.

That is a 432-calorie gap. Enough to erase most of a deficit and leave you wondering why the scale will not move, when the answer was never your willpower. It was the formula.

Why the formula is wrong, not just imprecise

Mifflin-St Jeor, the equation behind nearly every calculator you have used, was never built to know you. It was built on population averages. It takes your stats, multiplies by an activity factor you selected from a dropdown, and returns the number that fits the average person at your size.

You are not the average person at your size. Nobody is. The formula cannot know your actual training volume, how efficient your metabolism is, how much you fidget, or what your real recovery looks like. It is a starting estimate, not a measurement.

I did not find this out by reading about it. I found it by tracking three numbers every day for eighteen months and watching my results diverge from what the formula predicted.

The data behind the gap

For six months, I logged three things without missing a day: calories eaten, grams of protein, and my morning weight. I trained five days a week.

The standard activity multiplier for that training frequency is 1.55. Applied to my BMR across that period, it predicted a TDEE of 2,842 calories.

Here is what actually happened. I ate close to 1,500 calories a day and lost weight at a rate that implied a 960-calorie daily deficit. Working backward: 1,500 plus 960 is 2,410. That is what my body was actually burning, not 2,842.

Four hundred and thirty-two calories is not a rounding error. It is most of a deficit. Someone trusting the formula's number and eating what they believe is a 500-calorie deficit could, in reality, be sitting close to maintenance and losing almost nothing, then blaming themselves for a math problem that was never theirs to solve.

How to find your own real number

You cannot get this from a calculator. You need your own logged data, and you need AI to do the arithmetic a spreadsheet formula cannot, because the calculation depends on your specific weight trend, not a static input.

Here is the prompt. Paste your own numbers into whichever assistant you use.

I want to calculate my actual daily energy expenditure from real logged data instead of a formula estimate.

My data:
- Daily calorie intake, last 14+ days: [PASTE YOUR DAILY NUMBERS]
- Daily morning weigh-ins, same period: [PASTE YOUR DAILY WEIGHTS]
- Age, sex, height, current weight: [FILL IN]

Do this, in order:
1. Average my daily intake across the full period.
2. Calculate my weight trend using a 7-day rolling average at the start and end, not single-day values, so daily fluctuation does not distort the result.
3. Convert my weight change into a daily surplus or deficit using 3,500 calories per pound. State clearly that this conversion is an approximation.
4. Add or subtract that daily figure from my average intake to estimate my real maintenance calories.
5. Compare that number to what Mifflin-St Jeor predicts for me. Tell me the size and direction of the gap.
6. Tell me how much confidence my sample size supports, and what would make this estimate more reliable.

Show every calculation. Do not round until the final number.

If my data is too short, too inconsistent, or too noisy to support a conclusion, say that instead of giving me a number.

Two weeks of honest logging is the minimum for this to mean anything. Less than that and the noise in day-to-day weight swings drowns out the signal.

What this is not

This is not a claim that formulas are useless. Mifflin-St Jeor is a reasonable starting point when you have no data at all, which is why I still use it on the calculator on this site. It is a floor, not a ceiling. The moment you have two weeks of real numbers, your own data outranks any formula, because a formula describes an average person and your data describes you.

It is also not a shortcut. This prompt does not replace the daily habit of logging calories, protein, and body weight. It only works because that habit produced the data to run it on. AI did the analysis. It did not do the tracking, and it did not do the eighteen months.

That part was mine.