How this calorie calculator works
- Resting burn (BMR) from the Mifflin-St Jeor equation: 10 × weight (kg) + 6.25 × height (cm) − 5 × age, then +5 for men or −161 for women.
- Maintenance (TDEE) = BMR × an activity factor from 1.2 (sedentary) to 1.9 (very hard daily training or a physical job).
- Goal: subtract or add energy. Using the common 7,700 kcal per kg (3,500 per lb) rule, 500 kcal a day less is about 0.5 kg a week, at first.
For a 35-year-old woman, 165 cm and 68 kg, lightly active: BMR = 680 + 1,031 − 175 − 161 = 1,375 kcal; maintenance ≈ 1,375 × 1.375 = 1,891 kcal; a 0.5 kg/week loss target ≈ 1,340 kcal.
Choosing a realistic target
Slower is easier to keep up and protects muscle, especially if you eat enough protein and do some resistance training. Many dietitians start with a 300–500 kcal deficit and adjust after a few weeks of real-world weigh-ins, looking at the weekly average instead of day-to-day swings.
A formula feels less like magic once you can nudge each input and watch the output move; ahaboo teaches the same way, with a narrated explainer that shows how US tax brackets really apply.
If you want to turn your number into a plate, the macro calculator splits it into grams of protein, carbohydrate and fat.
When not to use a calorie calculator
Pregnancy and breastfeeding change energy needs; so do many medical conditions and medicines. If you have or have had an eating disorder, calorie targets can do harm. Please speak to your doctor or a registered dietitian instead.
Questions people ask
How many calories should I eat a day?
Your maintenance calories are your BMR multiplied by an activity factor. For many adults that lands between about 1,600 and 3,000 kcal. The calculator shows your personal estimate plus targets for gentle weight loss or gain.
How big should my calorie deficit be?
A deficit of roughly 500 kcal a day is the traditional starting point, often quoted as about 0.5 kg (1 lb) a week at first. Larger deficits are harder to sustain and can cost muscle. Your doctor or a registered dietitian can set a target that suits your health.
What is the lowest calorie intake that is safe?
General guidance, including from Harvard Health, is not to go below about 1,200 kcal a day for women or 1,500 for men without medical supervision, because it becomes hard to meet nutrient needs. The calculator flags targets below these levels.
Which formula does this calorie calculator use?
The Mifflin-St Jeor equation (1990), which the Academy of Nutrition and Dietetics found the most accurate of the common equations for healthy adults. Estimates are usually within about 10% of measured values.
Why am I not losing weight at the calculated calories?
Formulas estimate averages; food logging often misses 10–30% of intake; water weight hides fat loss for days; and your burn falls as you get lighter. Give any target two to four weeks and adjust by 100–200 kcal based on the trend.
Should I eat back exercise calories?
The activity level already includes your usual exercise. If you pick “sedentary” and log workouts separately, add back only part of them, since trackers tend to overestimate calories burned.
Sources
- Mifflin MD, St Jeor ST et al. (1990). A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 51:241–247.
- Frankenfield D et al. (2005). Comparison of predictive equations for resting metabolic rate. J Am Diet Assoc 105:775–789.
- NIH / NIDDK Body Weight Planner. Dynamic model showing why weight change slows over time (Hall KD et al., Lancet 2011).
- Harvard Health Publishing. Guidance on minimum daily calorie intake without medical supervision.
Every formula on VitalMetrics is listed with its original reference on Sources & methods.