How Many Calories Do I Need a Day? BMR and TDEE Math

Step 1: calculate your BMR with Mifflin-St Jeor

The calorie calculator uses Mifflin-St Jeor by default, the equation most dietitians start with. It works in kilograms and centimeters:

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

For the example: 150 lb is 68.04 kg and 5 ft 6 in is 167.64 cm. So BMR = 680.4 + 1,047.8 − 175 − 161 = 1,392 calories a day. The tool converts US units for you and prints this exact working under the result, so you can check each term.

Step 2: multiply by an activity factor to get TDEE

Your total daily calories are BMR times an activity multiplier. The Activity level menu offers five:

• Sedentary, desk job with little exercise: 1.2
• Lightly active, light exercise 1 to 3 days a week: 1.375
• Moderately active, 3 to 5 days: 1.55
• Very active, hard exercise 6 to 7 days: 1.725
• Extra active, physical job plus hard daily training: 1.9

Lightly active gives 1,392 × 1.375 = 1,914 calories a day. Most people overrate their activity, so if you are unsure between two levels, start with the lower one.

Calories to lose or gain weight

The tool turns TDEE into a goal table using the rule of thumb that about 3,500 calories equal a pound of body weight (7,700 per kilogram in metric mode). For the example:

• Maintain: 1,914 calories a day
• Mild loss, about 0.5 lb a week: 1,664
• Loss, about 1 lb a week: 1,414
• Extreme loss, about 2 lb a week: 914, which the tool flags as below 1,200 and not advised without medical supervision

The floor is 1,200 calories for women and 1,500 for men. Weight change is rarely linear, so recheck your numbers every few weeks. The result also suggests balanced macros at maintenance, 30% protein, 40% carbs and 30% fat, which is 144 g protein, 191 g carbs and 64 g fat here.

Gaining works the same way in reverse: the table adds 250 calories for about half a pound a week and 500 for about a pound. A slower gain paired with strength training usually adds more muscle and less fat than a large surplus. In metric mode the steps become 275 and 550 calories, matching 0.25 kg and 0.5 kg a week, because the tool assumes roughly 7,700 calories per kilogram of body weight. Protein, fiber and regular meals make any target easier to keep.

Harris-Benedict or Katch-McArdle instead?

The Formula menu has two alternatives. The revised Harris-Benedict equation, an older formula, tends to run a little higher: 1,445 BMR and 1,986 TDEE for the same person. Katch-McArdle ignores sex and age and uses lean body mass instead: BMR = 370 + 21.6 × lean mass in kg. At 30% body fat the example has 47.6 kg of lean mass and a BMR of about 1,399.

Katch-McArdle is only as good as your body fat estimate, so use it when you have a reliable measurement. None of these formulas apply during pregnancy or breastfeeding, or to children and teens who are still growing.

Why your real calorie needs may differ

Every equation predicts an average person with your measurements. Muscle mass, genetics, sleep, some medications and health conditions shift the result, and so does how honestly activity is rated. Treat the number as a starting estimate: eat close to it for two or three weeks, track your weight trend, and adjust by 100 to 200 calories if the trend does not match your goal.

Share link and Copy result save your inputs and the goal table, so you can rerun the same numbers after your weight or routine changes. Again, this is general information and not medical advice; a doctor or registered dietitian can account for your health history.

Body composition matters too; the Body Fat Calculator and BMI Calculator give extra context.

Step-by-step

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