How Many Calories Do You Actually Need? BMR, TDEE, and Fixing the Estimate

Every calorie calculator on the internet gives you a number, and most people either treat it as gospel or dismiss it as nonsense. It is neither. It is an estimate with a known error range, built from a formula that is about 90% right for most bodies, and it becomes accurate only after you adjust it against two weeks of real data. Here is what is under the hood, how to use the output, and how to correct it.

Two numbers, not one

BMR (basal metabolic rate) is the energy your body burns at complete rest: breathing, circulation, keeping warm, repairing cells. For most adults it is 1,200–2,000 kcal a day and it accounts for 60–75% of everything you burn.

TDEE (total daily energy expenditure) is BMR multiplied by an activity factor to include movement, exercise and digestion. TDEE is your maintenance intake: eat that and your weight stays where it is.

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The formula

The Calorie Calculator uses the Mifflin–St Jeor equation, which a 2005 review by the American Dietetic Association found the most accurate of the common formulas for the general population:

• Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
• Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161

Example: a 35-year-old woman, 165 cm, 68 kg: 680 + 1,031 − 175 − 161 = 1,375 kcal BMR.

The activity multiplier, honestly

Level	Multiplier	Describes
Sedentary	1.2	Desk job, little deliberate exercise
Lightly active	1.375	Exercise 1–3 days a week
Moderately active	1.55	Exercise 3–5 days a week
Very active	1.725	Hard exercise 6–7 days a week
Extra active	1.9	Physical job plus training

This is where estimates go wrong, because nearly everyone picks one level too high. Three gym sessions a week on top of an office job is “lightly active”, not “moderately”, because the other 165 hours are sitting. Our example woman at 1.375 has a TDEE of about 1,890 kcal. Picking 1.55 would have said 2,130, a 240-calorie gap that turns a planned deficit into maintenance.

Turning TDEE into a target

• Fat loss: TDEE minus 300–500 kcal. A 500 kcal daily deficit is roughly 0.45 kg (1 lb) a week in theory; in practice slightly less as the body adapts. Deficits over 750 kcal are hard to sustain and cost muscle.
• Muscle gain: TDEE plus 200–300 kcal with resistance training. More than that mostly adds fat.
• Maintenance: TDEE, then adjust.

Whatever the goal, do not go below BMR for extended periods, and adults should generally not go under about 1,200 kcal (women) or 1,500 kcal (men) without medical supervision.

The two-week correction

The formula does not know your genetics, your thyroid, your NEAT (how much you fidget and walk) or how much muscle you carry. So treat the output as a starting point:

1. Eat the calculated target for 14 days, tracking honestly.
2. Weigh yourself each morning and average each week (daily readings bounce by 1–2 kg with water).
3. Compare the week-2 average to week 1. Expected loss on a 500 kcal deficit is about 0.4–0.5 kg. If you lost nothing, your true TDEE is roughly 500 kcal lower than the estimate; adjust down 200 and repeat. If you lost 1 kg, adjust up.

After one or two of these cycles the number is calibrated to you, which no formula can do.

What the calculator cannot see

• Body composition. Two people at the same weight and height with different muscle mass have different BMRs. The Katch–McArdle formula uses body-fat percentage instead and is better for lean, muscular people, if you know your body fat.
• Metabolic adaptation. After months in a deficit the body lowers expenditure by 5–15%. Recalculate as you lose weight (every 5 kg) and expect the number to drop.
• Food label error. Labels are legally allowed to be off by 20%. Tracking is approximate; that is fine, consistency matters more than precision.

Pair the result with your BMI and a waist measurement for a rounded picture, and use the percentage calculator for the “what is 20% below 1,890” arithmetic if you would rather not do it in your head.

Frequently asked questions

How many calories should I eat to lose weight?

Your TDEE minus 300–500 kcal, then adjusted after two weeks of real results. For most adults that lands between 1,400 and 2,200 kcal.

Is the calculator accurate?

Mifflin–St Jeor is within 10% of measured BMR for about 80% of people. The activity multiplier is the bigger source of error; choose conservatively.

Do I subtract exercise calories from my intake?

Not if you chose an activity level that already includes your exercise. Counting it twice is a common reason people eat more than they think.

Why has my weight loss stalled at the same calories?

You weigh less, so your TDEE is lower, and metabolic adaptation may have reduced it further. Recalculate and adjust, or take a maintenance break for a couple of weeks.