Calorie deficit math, with a date you can mark on a calendar

The standard calorie deficit math, in plain English

Eat less than you burn. The "less" is the deficit. A 500/day deficit historically meant ~1 lb/week loss. Reality is messier — your body adapts, and the rule of thumb degrades over time.

Why the 3,500-calories-per-pound rule is wrong

Hall et al. (2011) showed metabolic adaptation makes the 3,500 rule overestimate loss by 25–50% over months. The math works for week one. Not for month four.

Calculating your TDEE without a degree in exercise science

Mifflin-St Jeor for BMR, multiply by an activity factor (1.4 sedentary, 1.55 light, 1.7 moderate). That's your TDEE. Subtract 500 for ~1 lb/week loss. Adjust monthly as the body adapts.

From calories per day to a goal date (worked example)

200 lb woman, sedentary office job. TDEE ~1,900. Eat 1,400. Lose ~1 lb/week initially, slowing to ~0.7 lb/week by month three. To 175 lb: about 28–32 weeks.

Calories per day is a recipe. A date on the calendar is a meal.

The deficits that work (and the deficits that backfire)

15–25% below TDEE is the sweet spot. Below 30% and adherence collapses. Aggressive deficits work for two weeks and fail by week five.

How to track without becoming obsessed

Track three weekdays and one weekend. Average. Don't weigh yourself daily — weekly average crushes daily noise. Use the bathroom mirror, the fit of jeans, and energy as proper indicators.

Try the calculator

Calories per day, plus a date you can put on the fridge.