What This Calculator Does
The Goal Body Fat Weight Calculator estimates the body weight you would reach at a chosen target body fat percentage, assuming you keep all of your lean body mass (muscle, bone, organs, water) and only lose fat. It also tells you how much fat you would need to lose to get there. This is a universal physics-of-composition tool — it applies to anyone regardless of country.
How to Use It
Enter your current body weight, your current body fat percentage, and the target body fat percentage you want to reach. The calculator computes your lean body mass, then divides it by the fraction of your goal weight that should be lean to find your projected goal weight. The difference between your current and goal weight is the fat you would shed.
The Formula Explained
First, lean mass is fixed: \( \text{Lean Mass} = \text{Weight} \times (1 - \text{BF\%}) \). At your target, lean mass should make up \( (1 - \text{Target\%}) \) of your total weight, so \( \text{Goal Weight} = \text{Lean Mass} \div (1 - \text{Target\%}) \). Finally, \( \text{Fat to Lose} = \text{Current Weight} - \text{Goal Weight} \). The model assumes lean mass stays constant, which is the goal of a well-managed cut.
$$\begin{gathered} W_{goal} = \frac{L}{1 - \dfrac{\text{Target BF (\%)}}{100}} \\[1.5em] \text{where}\quad L = \text{Weight (kg)} \times \left(1 - \frac{\text{Current BF (\%)}}{100}\right) \end{gathered}$$
Worked Example
Suppose you weigh 80 kg at 25% body fat and want to reach 15%. Lean mass = \( 80 \times (1 - 0.25) = 60 \) kg. Goal weight = \( 60 \div (1 - 0.15) = 60 \div 0.85 \approx 70.59 \) kg. Fat to lose \( \approx 80 - 70.59 = 9.41 \) kg. Your current fat mass is \( 80 \times 0.25 = 20 \) kg.
$$L = 80 \times (1 - 0.25) = 60 \text{ kg}$$ $$W_{goal} = \frac{60}{1 - 0.15} = \frac{60}{0.85} \approx 70.59 \text{ kg}$$
FAQ
Does lean mass really stay the same? In an ideal cut, yes. Aggressive diets can cost lean mass, so use the result as a target, not a guarantee.
Which body fat number should I use? Use the most accurate measurement you have (DEXA, calipers, BIA). Garbage in, garbage out.
Can target be higher than current? The math still works but is meant for fat loss, where the target is lower than current.
