Weight Change Simulator
How many days to your target weight? The Mifflin-St Jeor formula gives an accurate BMR, then activity + running are added for a realistic timeline.
Added on top of the activity level
Recommended: 300–500. Too much means muscle loss
Basal metabolic rate (BMR)
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kcal / day
Total energy expenditure (TDEE)
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kcal / day
Recommended intake
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Running calories (wk)
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To lose
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Time to goal
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Check your BMI too For reference — BMI is less reliable with high muscle mass
Current BMI
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Target BMI
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Current class
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Normal range
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Why are these formulas accurate?
Mifflin-St Jeor formula (1990) — the current dietitian-association standard
- Male:
BMR = 10 × weight(kg) + 6.25 × height(cm) - 5 × age + 5 - Female:
BMR = 10 × weight(kg) + 6.25 × height(cm) - 5 × age - 161 - Error ±10%. About 5% more accurate than the Harris-Benedict (1919) formula.
TDEE (total daily energy expenditure)
TDEE = BMR × activity factor + running calories- Activity factor 1.2 (sedentary) to 1.9 (very active)
- Running is added separately — folding it into the activity factor double-counts it
Running calories
- This calculator applies a simple
~1.0 kcal/km/kg(assuming flat ground and average pace) - For accurate pace and grade, use the calorie calculator
7,700 kcal = 1 kg of fat — the golden ratio
- 1 g of fat = 9 kcal, but fat cells are 86% pure fat → losing 1 g ≈ 7.7 kcal
Estimated days = (amount to lose(kg) × 7700) ÷ daily deficit(kcal)- In practice the first 1–2 weeks are fast as water drops, then a plateau follows (the scale-gets-angry phase)
Caution — losing too fast backfires
- Daily deficit over 500 kcal → muscle loss + lower BMR + rebound
- WHO recommendation: 0.5–1 kg per week or less
- Runners should be more conservative — muscle mass directly affects pace
Sources: Mifflin MD et al. (1990) Am J Clin Nutr · ACSM Position Stand on Weight Loss · WHO obesity guidelines
Dieting alone usually fails
78% quit by week two. Join a runner crew with the same goal and you get social pressure — "if I skip today, the group chat will roast me." Surprisingly, it is the most effective diet aid.
