ℹ️ This article is educational and not medical or dietary advice. Everyone’s needs differ. Speak to a doctor or registered dietitian before making significant changes to your diet, especially for weight loss.
Calories in, calories out
Body weight is governed, at the simplest level, by energy balance: the calories you take in versus the calories you burn. Eat the same amount you burn and weight holds steady. Eat less and your body makes up the difference from its stores, so you lose weight. Eat more and the surplus is stored, so you gain. Everything else — meal timing, food choices, macros — operates within this frame.
The hard part is knowing how many calories you actually burn. That’s what the Calorie Calculator estimates, by first finding your BMR and then scaling it up for activity.
BMR: your resting baseline
Basal Metabolic Rate is the energy your body uses just to keep you alive at complete rest — heartbeat, breathing, temperature regulation, cell repair, and brain activity. For most people it’s the single largest chunk of daily energy use, often 60–70% of the total.
The most widely recommended way to estimate BMR is the Mifflin-St Jeor equation, published in 1990 and endorsed by the Academy of Nutrition and Dietetics as the most accurate predictive formula for the general adult population:
BMR (men) = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
BMR (women) = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161The only difference between the sexes is the final constant (+5 versus −161), which reflects average differences in body composition. A 30-year-old man at 80 kg and 180 cm has a BMR of about 1,780 kcal/day.
TDEE: adding activity
Almost nobody spends the day at complete rest, so BMR alone understates your needs. Total Daily Energy Expenditure scales BMR by an activity multiplier that captures exercise plus the general movement and digestion of daily life:
- Sedentary (×1.2) — desk job, little or no exercise.
- Light (×1.375) — light exercise 1–3 days/week.
- Moderate (×1.55) — moderate exercise 3–5 days/week.
- Very active (×1.725) — hard exercise 6–7 days/week.
- Extra active (×1.9) — very hard exercise plus a physical job.
That same 1,780 kcal BMR becomes about 2,759 kcal of TDEE at the moderate level. Be honest with this choice — overestimating activity is the most common reason a calorie plan stalls. When unsure, pick the level below the one you’re tempted by.
Setting a calorie target
Once you know your TDEE, your goal sets the adjustment:
- Maintain: eat at TDEE.
- Lose: subtract a deficit. About 500 kcal/day tends to produce roughly 0.5 kg (1 lb) per week, because a kilogram of body fat stores roughly 7,700 kcal. A gentler 250 kcal/day deficit trades speed for sustainability.
- Gain: add a surplus, typically 250–500 kcal/day, ideally paired with resistance training so the gain favours muscle.
Don’t go too low
Aggressive deficits backfire. Eating far below your needs risks muscle loss, fatigue, nutrient deficiencies, and a metabolism that adapts downward. A common safe floor is around 1,200 kcal/day for women and 1,500 for men, and even those should be approached carefully. The Calorie Calculator flags a target when it dips below that floor so you don’t unknowingly set an unsafe goal.
Where macros fit
Macronutrients — protein, carbohydrate, and fat — are the calorie-containing components of food. Protein and carbs provide about 4 kcal per gram; fat provides about 9. A balanced starting split is 30% protein, 40% carbohydrate, 30% fat, which the tool translates into grams at your maintenance level.
But keep perspective: for weight change, total calories do the heavy lifting, and after that, adequate protein is what best preserves muscle in a deficit and supports growth in a surplus. The exact carb-to-fat ratio is largely a matter of preference and performance. Treat the suggested macros as a reference, not a rule.
Putting it into practice
- Estimate your TDEE with the Calorie Calculator, choosing your activity level conservatively.
- Pick a goal and the matching target — a modest deficit or surplus beats an extreme one you can’t sustain.
- Track for a few weeks and watch the trend, not the daily number. Bodyweight fluctuates with water and food in transit.
- Adjust. Formulas are estimates. If the scale isn’t moving as expected after two to three weeks, nudge your intake by 100–200 kcal and reassess.
The numbers are a starting point, not a verdict. For the resting baseline on its own, see the BMR Calculator, and to gauge the calories you burn in a workout, try the Calories Burned calculator. If the distinction between resting and total burn is still fuzzy, BMR vs TDEE, Explained breaks it down.