The Macro Split Calculator estimates your daily calorie needs using the Mifflin-St Jeor equation and splits that target into protein, fat, and carbohydrate gram targets. It is built for anyone tracking macros for fat loss, lean building, or general health — no more guessing how many grams of each nutrient to eat.
Simply enter your age, sex, height, and weight, select your activity level and calorie goal, and choose protein and fat targets that fit your training style. The calculator does the rest: it computes your basal metabolic rate, applies an activity multiplier, and distributes the remaining calories to carbs. The result screen shows an easy-to-read macro breakdown plus the step-by-step math behind it.
Use the preset buttons to quickly test common scenarios — Cut, Maintain, or Bulk — and tune the protein and fat settings from there. Ideal for athletes, dieters, and fitness professionals who want a fast, transparent macro starting point.
⚙️ Your Details
🎯 Goal & Activity
🥩 Protein & Fat Targets
How to Use This Macro Calculator
Start by entering your age, sex, height, and weight. You can switch height and weight units to centimeters or inches and kilograms or pounds — the calculator normalizes everything to metric before calculating. Choose the activity level that best reflects your average weekly movement, not your weekend exercise only. The calorie goal dropdown applies a percentage adjustment to your maintenance calories: subtract 20% for fat loss, leave at 0% to maintain, or add 10% for a lean bulk.
Then set your protein and fat targets. The defaults — 1.8 g of protein per kg of bodyweight and 0.8 g of fat per kg — are reasonable for most active adults. Athletes often prefer the 2.4 g/kg protein preset, while less active people can use the Balanced preset at 1.6 g/kg. Click Calculate Macros to see your daily calorie target and how many grams of protein, fat, and carbs fit within it. Use the Cut, Maintain, and Bulk quick buttons to compare different calorie scenarios without retyping your details.
- Check your results weekly and adjust calories by 5–10% if weight is not trending in the intended direction.
- Weigh yourself under similar conditions at least three times a week before trusting a trend.
- Treat the macro targets as starting points, not rigid rules.
The Math Behind the Macro Split
The calculator estimates your basal metabolic rate (BMR) with the Mifflin-St Jeor equation, one of the most widely used predictive formulas in clinical and sports nutrition. The equation is:
BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + modifier
The modifier is +5 for men and −161 for women. BMR represents the energy your body burns at complete rest. To estimate total daily energy expenditure (TDEE), the calculator multiplies BMR by an activity factor ranging from 1.2 for sedentary lifestyles to 1.9 for highly active athletes. Your calorie goal is then applied to TDEE — for example, a 20% deficit multiplies TDEE by 0.8.
Once the calorie target is fixed, macro gram targets are calculated from your bodyweight. Protein and fat grams are multiplied by their energy densities: protein provides 4 kcal/g, carbohydrate provides 4 kcal/g, and fat provides 9 kcal/g. The remaining calories after protein and fat are assigned to carbohydrates, which keeps the split flexible and calorie-accurate. For a deeper look at the equation, see the Mifflin-St Jeor equation on Wikipedia.
Worked Example
Sarah is a 30-year-old woman who is 165 cm tall, weighs 65 kg, and exercises lightly three days a week. Her BMR is 10 × 65 + 6.25 × 165 − 5 × 30 − 161 = 1,370 kcal/day. With light activity (factor 1.375), her maintenance calories are 1,884 kcal/day. If she chooses a 20% fat loss goal, her target becomes 1,507 kcal/day.
Sarah’s macro split with a 1.8 g/kg protein target and 0.8 g/kg fat target:
Protein = 65 × 1.8 = 117 g → 468 kcal (31%)
Fat = 65 × 0.8 = 52 g → 468 kcal (31%)
Carbs = (1,507 − 936) ÷ 4 = 143 g (38%)
Notice that protein and fat happen to contribute the same number of calories in this example. The exact percentages change whenever you change your weight-based targets or calorie goal, which is why a gram-based calculator is more useful than a rigid 40/30/30 split.
Standard Protein and Fat References
These ranges are general starting points from sports nutrition guidelines. Beginners and older adults often benefit from the higher end of the protein range to preserve muscle during fat loss.
| Goal | Protein (g/kg) | Fat (g/kg) | Notes |
|---|---|---|---|
| General health | 1.2–1.6 | 0.6–1.0 | Minimum protein for active adults |
| Fat loss | 1.8–2.2 | 0.8–1.0 | Higher protein supports muscle retention |
| Muscle gain | 1.6–2.2 | 0.8–1.2 | Surplus calories provide the rest |
| Athlete / high volume | 2.0–2.4 | 1.0+ | More fat can support hormone function |
If you enter a very low calorie target and high protein/fat values, the calculator will warn you that carbs would drop below zero. In that case, increase calories or lower the grams per kg inputs.
Factors That Affect Accuracy
Mifflin-St Jeor is accurate for about 70% of healthy adults within roughly 10–15% of measured energy expenditure, but individual variation is real. Lean body mass is a major driver of BMR; the equation does not see body composition, so a muscular person may be under-estimated and a highly sedentary person may be over-estimated. Activity multipliers are also broad estimates. A desk worker who does 10,000 steps daily may sit closer to 1.4 than 1.2, while a recreational lifter doing one hour of training three times a week may still need the Light or Moderate multiplier.
For best results, use your average weekly weight trend rather than a single day's reading. If weight loss stalls for two weeks, reduce calories by 5–10%; if you are losing over 1% of body weight per week while resistance training, consider raising calories or protein to protect lean mass.
Frequently Asked Questions
Why are carbs the remainder instead of a fixed percentage?
Because protein and fat targets are anchored to bodyweight, making carbs the remainder automatically adjusts to your calorie goal. A bodybuilder in a surplus gets a higher carb intake for training performance, while someone in an aggressive cut gets a lower carb intake without having to manually recalculate percentages. This approach is flexible, sustainable, and easy to scale.
Should I eat exactly these macro numbers every day?
No. The calculator provides a target. Hitting protein within 10–15 g and calories within 100 kcal of the goal on most days is enough for progress. Fat intake can be more variable between days, and carbs are the most flexible energy source.
How quickly should I adjust my calories?
Give any new target at least two or three consistent weeks. Daily water changes, glycogen shifts, and menstrual cycle fluctuations can obscure real fat loss. Track your daily weight and look at the weekly average before making another change.