What is TDEE, and why do calculators disagree?

The short answer#

Total daily energy expenditure is everything you burn in a day. It is the same quantity this site usually calls maintenance — the intake at which your weight would hold steady.

Calculators disagree about it because they are estimating two things separately, and one of the two is a guess with a very wide range.

The four parts#

Part	Share for most people	Can a form know it?
Basal rate — the cost of being alive at rest	60–70%	Reasonably, within about ±10%
Everyday movement — walking, standing, fidgeting	15–30%	No
Digesting food	~10% of what you eat	Roughly, from the food
Deliberate exercise	often 5–10%	Only if you tell it

The interesting column is the third one. The basal rate is predictable because it tracks size, composition, age and sex — things a form can ask about. Everyday movement is not predictable from any of those, and it is the part that varies most between two people who look identical on paper.

Where the disagreement comes from#

Most calculators do the same two steps:

basal rate × activity multiplier = TDEE

Step one uses one of a handful of equations. Mifflin-St Jeor is the usual default and needs height, weight, age and sex. Katch-McArdle works from lean mass instead, so it needs a body-fat figure but does not care about age or sex — and it is the better choice when you have a reliable reading, because two people of the same weight with different composition have genuinely different resting rates. SmartCrunch prefers Katch-McArdle when Apple Health has a body-composition reading and falls back to Mifflin-St Jeor otherwise, which is rung four of five in its ladder.

Different equations give slightly different answers here — usually within a hundred calories or so.

Step two is where the hundreds appear. The multipliers are conventional:

Description	Multiplier
Sedentary	1.2
Lightly active	1.375
Moderately active	1.55
Very active	1.725
Extremely active	1.9

The gap between “sedentary” and “lightly active” is 15% of your entire expenditure — several hundred calories for most people — and it hangs on which sentence in a dropdown you thought sounded more like you. Two calculators that word those options differently will put the same person in different rows.

You can see both steps and change either in the TDEE calculator.

Why the number is a starting point, not an answer#

None of the above is measured. The basal rate is a prediction about people your size, with real individual scatter around it, and the multiplier is a self-assessment. That is a reasonable place to begin when there is nothing else, and a poor place to still be three months later.

The alternative is to stop estimating and start measuring. Once you have a few weeks of food entries and weigh-ins, the same arithmetic runs backwards: what you ate, against where your weight actually went, gives the expenditure figure that would explain both. That figure absorbs everything the multiplier was guessing at — including the fact that you probably under-record.

SmartCrunch treats that as the top rung and the formula as a fallback, and publishes the four conditions it insists on before it will use it. Most trackers do the opposite: they ask the form questions once, in the first minute, and never revisit the answer.

Using a TDEE number sensibly#

Take the estimate. Do not agonise over the dropdown; pick the row that describes a typical week rather than your best week.
Eat at the implied budget for a fortnight and watch the trend, not the mornings.
If the line moves the way you intended, the estimate was close enough.
If it does not, the estimate was wrong — and you now have exactly the data needed to work out what it should have been.

That loop is the whole method. Everything else is bookkeeping.