To calculate your calories, estimate your basal metabolic rate with the Mifflin-St Jeor formula, multiply by an activity factor to get your maintenance intake, then subtract 300 to 500 kcal for fat loss or add 200 to 400 kcal for muscle gain. For macros, set protein at 1.6 to 2.2 g per kg of body weight, fat at 20 to 35 percent of calories, and fill the rest with carbohydrates. The numbers are a starting point; adjust after two to three weeks based on how your weight actually moves.
Step one: estimate your basal metabolic rate
Your basal metabolic rate (BMR) is the energy your body uses at complete rest to keep you alive. The Mifflin-St Jeor equation is the most accurate of the common formulas for the general population and is the one most dietitians use. It takes weight in kilograms, height in centimetres and age in years.
For men: BMR = 10 x weight + 6.25 x height - 5 x age + 5. For women: BMR = 10 x weight + 6.25 x height - 5 x age - 161. The result is in kilocalories per day. If you only know imperial units, 1 kg is 2.2 lb and 1 inch is 2.54 cm, so a 165 lb person at 5 ft 10 in is 75 kg and 178 cm.
Every formula is an estimate. Two people with the same stats can differ by 10 percent or more in real metabolic rate, which is why the final step, adjusting based on results, matters more than the decimals.
Step two: apply an activity multiplier
Multiply your BMR by an activity factor to estimate total daily energy expenditure (TDEE), the number of calories that keeps your weight stable. Be honest here. Most people overestimate their activity, and an office worker who trains three times a week is moderately active at most, not very active.
| Activity level | Multiplier | Who it describes |
|---|---|---|
| Sedentary | 1.2 | Desk job, little or no exercise, under 5,000 steps a day |
| Lightly active | 1.375 | Light exercise 1 to 3 days a week, or an active daily routine |
| Moderately active | 1.55 | Moderate exercise 3 to 5 days a week, 7,000 to 10,000 steps |
| Very active | 1.725 | Hard training 6 to 7 days a week, or a physical job plus training |
| Extra active | 1.9 | Very hard daily training plus a physical job, or an athlete in season |
Step three: set a deficit or a surplus
For fat loss, subtract 300 to 500 kcal from your TDEE. This produces about 0.25 to 0.5 kg (0.5 to 1 lb) of weight loss per week, which is fast enough to see progress and slow enough to keep your muscle, your energy and your training quality. Larger deficits work in the short term but raise hunger, cost muscle and are rarely sustained. Do not drop below roughly 1,200 kcal for women or 1,500 kcal for men without medical supervision.
For muscle gain, add 200 to 400 kcal above TDEE. Muscle is built slowly, around 0.5 to 1 kg a month for a beginner and less later, so a large surplus mostly becomes fat. Aim to gain about 0.25 kg a week; if the scale moves faster than that, trim the surplus.
For maintenance or body recomposition, eat at TDEE with a high protein intake and let training shift the ratio of muscle to fat over time.
Step four: split your macros
Protein comes first because it drives muscle repair and keeps you full. Set it at 1.6 to 2.2 g per kg of body weight per day. The lower end is sufficient for most people; the upper end is useful in a deficit, when protein also protects muscle. Protein and carbohydrate each provide 4 kcal per gram, fat provides 9 kcal per gram.
Fat comes second. Keep it between 20 and 35 percent of total calories, with 25 percent a sensible default. Going lower than 20 percent for long periods can affect hormone levels and makes food less satisfying. Carbohydrates take whatever calories remain. They fuel training, so people who lift or run hard benefit from the higher carbohydrate intake that falls out of this method.
- Protein: 1.6 to 2.2 g/kg of body weight, then multiply grams by 4 to get calories.
- Fat: 20 to 35 percent of total calories, then divide calories by 9 to get grams.
- Carbohydrates: total calories minus protein and fat calories, then divide by 4 to get grams.
Worked example: a 75 kg person
Take a 30-year-old man who weighs 75 kg (165 lb), is 178 cm tall and trains three to four days a week with a desk job. BMR = 10 x 75 + 6.25 x 178 - 5 x 30 + 5 = 750 + 1,112.5 - 150 + 5 = 1,717.5 kcal. He is moderately active, so TDEE = 1,717.5 x 1.55 = 2,662 kcal, which rounds to about 2,650 kcal a day for maintenance.
The table shows how his numbers change for each goal. Protein is held at 1.8 g/kg, or 135 g, and fat at 25 percent of calories. A woman of the same weight, height and age would have a BMR of 1,551.5 kcal and a maintenance intake of about 2,400 kcal, with the same method applied from there.
| Goal | Calories | Protein | Fat | Carbohydrates |
|---|---|---|---|---|
| Fat loss (-500) | 2,150 kcal | 135 g (540 kcal) | 60 g (540 kcal) | 268 g (1,070 kcal) |
| Maintenance | 2,650 kcal | 135 g (540 kcal) | 74 g (665 kcal) | 361 g (1,445 kcal) |
| Muscle gain (+300) | 2,950 kcal | 135 g (540 kcal) | 82 g (740 kcal) | 418 g (1,670 kcal) |
When and how to adjust
The calculation gives you a starting point, and your scale gives you the answer. Weigh yourself in the morning, three or more times a week, and compare weekly averages rather than single days, because water and salt can move the scale by a kilogram overnight.
After two to three weeks, compare the trend to your target. If you wanted to lose 0.4 kg a week and lost nothing, reduce intake by 150 to 200 kcal. If you are losing faster than 1 percent of body weight per week, add calories back. If you are trying to gain and the scale has not moved in three weeks, add 150 kcal. Make one change at a time and give it another two weeks before judging again.
Recalculate when your weight changes by about 5 kg, because a lighter body burns fewer calories, and expect maintenance to drift down a little during a long diet as activity drops. BirFit calculates these targets from your profile and builds the daily meal plan around them, so the arithmetic is done before you open the fridge.
Questions
Do I have to track macros, or are calories enough?
Calories decide whether you lose or gain weight; macros decide how much of that change is muscle versus fat and how you feel doing it. If tracking everything is too much, track calories and protein only. Hitting a protein target and a calorie target gets you most of the benefit, and the carbohydrate and fat split can fall where it falls.
Should protein be set from body weight or from calories?
From body weight. A percentage of calories gives too little protein on a diet and too much on a large surplus. Use 1.6 to 2.2 g per kg; if you carry a lot of body fat, use your goal weight instead of your current weight so the number does not become unrealistically high.
How fast should I lose fat?
About 0.5 to 1 percent of body weight per week is the usual recommendation. For a 75 kg person that is roughly 0.4 to 0.75 kg a week. Faster than that increases muscle loss and makes the diet hard to sustain; slower is fine if it fits your life better.
Why has my weight loss stopped on the same calories?
Three common reasons: you weigh less so your maintenance has dropped, you move less than you did at the start because dieting reduces spontaneous activity, or portions have drifted upward without you noticing. Recheck tracking for a week, and if it is accurate, reduce by 150 to 200 kcal or add 2,000 steps a day.