Using TDEE for Weight Loss — How to Set Your Calorie Deficit

To set a calorie deficit from your TDEE, subtract a daily amount from your maintenance calories, then check that the result is not so low that it falls under your resting energy (BMR) or the calculator's safe-minimum flag. A 500 kcal deficit is the usual starting point, but it is a very different cut for a person with a TDEE of 2,800 than for someone with a TDEE of 1,400. This post is about sizing the deficit to the person, and about why it shrinks as you lose weight.

Worked example: sizing the deficit

Example: a 38-year-old woman, 70 kg (154 lb), 165 cm, light exercise one to two days a week (factor 1.375). Mifflin-St Jeor BMR = 10 x 70 + 6.25 x 165 - 5 x 38 - 161 = 1,380 kcal. TDEE = 1,380.25 x 1.375 = 1,898 kcal. Each deficit below is checked against her BMR and against the calculator's 1,200 kcal safe-minimum flag for women (the flag is 1,500 for men).

The implied loss uses the calculator's rule of thumb of about 3,500 kcal per lb, so 500 kcal a day is roughly 1 lb a week. The CDC states that people who lose weight at a gradual, steady pace of about 1 to 2 pounds a week are more likely to keep it off than people who lose it faster. In this example the 500 kcal deficit sits only 18 kcal above her BMR, so for her it is close to the practical limit rather than a safe default.

When the standard 500 does not fit

Take a 50-year-old woman, 58 kg, 160 cm, sedentary (factor 1.2). BMR = 580 + 1,000 - 250 - 161 = 1,169 kcal; TDEE = 1,403 kcal. A 500 kcal deficit would mean 903 kcal, which is 35.6% of her TDEE and 297 kcal under the 1,200 flag. The most she can cut before reaching the flag is 203 kcal, about 14.5% of TDEE. For a small or inactive person, the lever is usually more movement rather than less food, and the realistic rate is slower. Compare a 90 kg, 178 cm, 45-year-old man at factor 1.2: TDEE is 2,151 kcal, and a 500 kcal deficit lands at 1,651 kcal, 151 kcal above the men's 1,500 flag.

Why the deficit shrinks as you lose weight

Back to the first example, eating a fixed 1,398 kcal per day. Each week we recompute Mifflin-St Jeor with her new weight and apply the 3,500 kcal per lb rule. The right-hand column shows what a straight-line 1 lb per week would predict.

Assumptions: the 3,500 kcal per lb rule holds, the activity factor and height stay fixed, and weight is the only thing that changes. The deficit falls from 500 to 334 kcal, so the plan loses about 5.5 lb less than the straight-line rule by week 32. Real life adds further effects, and a dynamic model of weight change (Hall et al., The Lancet, 2011) reports that bodyweight responds slowly to a change in intake, with half-times of about one year.

Recalculate at checkpoints

Rather than waiting for a plateau, re-enter your weight every 10 lb or so. Her recalculated maintenance and 500 kcal target look like this:

Every 10 lb lost removes about 62 kcal from TDEE in this model, because Mifflin-St Jeor drops BMR by 10 kcal per kg and the factor multiplies it. By 30 lb the 500 kcal target is within 11 kcal of the 1,200 flag, so the sensible response is a smaller deficit, not a deeper cut.

Common mistakes

• Applying the same 500 kcal to everyone. As a share of TDEE it ranged from 23% to 36% in the examples above.
• Picking an activity factor that is too high. The calculator advises choosing the lower option when unsure; one step is 12.7% of TDEE at 1.375 to 1.55.
• Cutting further when weight stalls without recalculating first. A lower weight lowers TDEE.
• Reading the 3,500 kcal rule as exact. It is a rough conversion, not a promise.