Calorie Deficit & Weight Loss Calculator
Estimates how many weeks it will take to reach your target weight by modeling your calorie deficit as your body weight—and therefore your TDEE—decreases over time. Use it when planning a structured weight-loss programme with a realistic timeline.
About this calculator
The calculator uses the Mifflin–St Jeor equation to compute basal metabolic rate (BMR): for males, BMR = 10×weight + 6.25×height − 5×age + 5; for females, BMR = 10×weight + 6.25×height − 5×age − 161. Total Daily Energy Expenditure (TDEE) = BMR × activityLevel. As you lose weight, your TDEE falls too, so the formula models the total weeks needed as: weeks = (currentWeight − targetWeight) × 7700 / (TDEE_current − TDEE_target × 0.8). The 7700 kcal/kg figure represents the approximate energy stored in one kilogram of body fat. The 0.8 factor on target TDEE reflects eating at roughly 80% of your new maintenance calories—a moderate deficit that avoids metabolic adaptation. This produces a more realistic timeline than a simple static-deficit calculation.
How to use
Example: 30-year-old male, 85 kg current weight, 75 kg target, 175 cm tall, lightly active (activityLevel = 1.375). Step 1: Current BMR = 10×85 + 6.25×175 − 5×30 + 5 = 850 + 1093.75 − 150 + 5 = 1798.75 kcal. Current TDEE = 1798.75 × 1.375 ≈ 2473 kcal. Step 2: Target BMR = 10×75 + 6.25×175 − 5×30 + 5 = 750 + 1093.75 − 150 + 5 = 1698.75 kcal. Target TDEE = 1698.75 × 1.375 × 0.8 ≈ 1869 kcal. Step 3: Deficit per day = 2473 − 1869 = 604 kcal. Step 4: Total energy to lose = 10 × 7700 = 77,000 kcal. Weeks = 77,000 / (604 × 7) ≈ 18.2 weeks.
Frequently asked questions
How many calories do I need to cut per day to lose 1 kg per week?
One kilogram of fat contains approximately 7,700 kilocalories, so to lose 1 kg per week you need a daily deficit of about 1,100 kcal (7,700 ÷ 7). For most people, this is an aggressive target that requires both dietary restriction and significant exercise. A more sustainable deficit of 500–600 kcal/day produces roughly 0.45–0.55 kg of fat loss per week. Very large deficits risk muscle loss, nutrient deficiencies, and metabolic slowdown, which is why most guidelines recommend losing no more than 0.5–1% of body weight per week.
Why does my calorie target need to decrease as I lose weight?
Your TDEE is directly tied to your body mass because larger bodies require more energy to maintain basic functions and to move. As you lose weight, your BMR drops because you have less metabolically active tissue. This means the same diet that produced a 500 kcal/day deficit early in your journey will produce a smaller deficit later—a phenomenon sometimes called 'metabolic adaptation.' Adjusting your intake progressively downward (or increasing exercise output) as you lose weight is essential to maintain your rate of progress. This calculator accounts for this by comparing your starting TDEE with a reduced target TDEE, giving a more honest timeline estimate.
What activity level multiplier should I use in the calorie deficit calculator?
Activity multipliers (also called PAL factors) are applied to BMR to estimate total daily expenditure. Sedentary (desk job, little exercise) uses 1.2; lightly active (light exercise 1–3 days/week) uses 1.375; moderately active (moderate exercise 3–5 days/week) uses 1.55; very active (hard exercise 6–7 days/week) uses 1.725; and extra active (physical job plus daily training) uses 1.9. Most adults with standard office jobs and casual gym sessions fall into the lightly to moderately active range (1.375–1.55). Overestimating your activity level is one of the most common reasons calculated deficits don't translate into expected weight loss.