Body Fat Loss Calculator
Estimates the target body weight you need to reach in order to hit a specific body fat percentage, accounting for potential muscle loss during a cut. Use it when planning a cutting phase to protect lean mass.
About this calculator
This calculator solves for the goal body weight that achieves your target body fat percentage while accounting for the lean mass you may lose during dieting. The formula derives from the relationship: lean mass = totalWeight × (1 − bodyFatPercent ÷ 100). After adjusting lean mass for an expected muscle loss rate, the target body weight is: targetWeight = adjustedLeanMass ÷ (1 − targetBodyFat ÷ 100). In other words, if you know how much lean mass you want to preserve and what percentage of your final weight should be fat, you can back-calculate exactly how much total weight you need to weigh. This is more precise than simply aiming for a scale number, because two people at the same weight can have vastly different body compositions.
How to use
Suppose you weigh 200 lbs, with 25% body fat, a target of 15% body fat, and you expect 2% muscle loss during the cut. Step 1 — Current lean mass: 200 × (1 − 0.25) = 150 lbs. Step 2 — Adjust for muscle loss: 150 × (1 − 0.02) = 147 lbs preserved lean mass. Step 3 — Target body weight: 147 ÷ (1 − 0.15) = 147 ÷ 0.85 = 173 lbs. You need to reach approximately 173 lbs to achieve 15% body fat if 2% of your lean mass is lost.
Frequently asked questions
How do I calculate how much weight I need to lose to reach a target body fat percentage?
Start by calculating your current lean mass: multiply your body weight by (1 minus your body fat fraction). Then divide that lean mass by (1 minus your target body fat fraction) to get your goal body weight. The difference between your current weight and that goal weight is approximately how much total weight you need to lose. Keep in mind this is an estimate — actual results vary based on water retention, glycogen levels, and measurement accuracy.
What is a realistic muscle loss rate during a calorie deficit cutting phase?
Research suggests that a well-managed cut with adequate protein (0.8–1 g per lb of body weight) and resistance training can limit muscle loss to around 0–3% of lean mass over a cutting phase. Aggressive deficits (more than 1,000 calories/day below TDEE) or very low protein intakes can push muscle loss higher. Beginners often experience 'newbie recomposition,' gaining muscle while losing fat simultaneously. Experienced athletes in a lean state tend to lose muscle more easily and should use conservative deficits.
Why is tracking body fat percentage more useful than tracking scale weight alone?
Scale weight includes fat mass, lean muscle, bone, organs, water, and food in transit — it fluctuates by 2–5 lbs daily for reasons unrelated to fat loss. Body fat percentage separates fat mass from lean mass, giving you a clearer picture of actual progress. Someone who loses 5 lbs of fat but gains 3 lbs of muscle has made excellent progress, yet the scale only shows 2 lbs lost. Tracking body fat lets you ensure your deficit is targeting fat, not hard-earned muscle.