Protein is the one macro you cannot scrimp on during a cut. This calculator sets your protein target from lean body mass first, then fills fat and carbs around your deficit — so you lose fat, not the muscle you worked to build.
iPhone · iOS 17 +
The Science
Net protein balance (NPB) is the difference between muscle protein synthesis (MPS) and muscle protein breakdown (MPB). When NPB is positive, you build or maintain muscle. When it goes negative, muscle tissue is lost. During a calorie deficit, MPB rises — your body senses an energy shortage and accelerates the breakdown of muscle proteins to recycle amino acids for fuel. Eating adequate protein is the primary lever for keeping NPB as close to zero as possible while fat stores shrink.
The question is how much protein is enough. The research is unambiguous on the floor: 0.7–0.8g per pound of bodyweight outperforms lower intakes at preserving lean mass in a deficit. But bodyweight is a blunt instrument. A person carrying 30% body fat has very different muscle mass requirements than someone at 12%, even at identical bodyweights. That is why this calculator uses lean body mass (LBM) — bodyweight minus fat mass — when you supply a body fat percentage.
Phillips & Van Loon (2011) reviewed protein requirements in physically active individuals and concluded that 1.6–2.4g/kg of lean body mass (roughly 0.73–1.1g/lb LBM) is optimal for muscle retention during energy restriction. For practical cutting purposes, this translates to a target of at least 1g per pound of LBM. This calculator uses exactly that as its baseline.
During an aggressive cut — a deficit exceeding 25% of TDEE — there is a compelling case for going higher. Larger deficits amplify MPB, and the extra dietary protein helps offset this. When you select the aggressive or very aggressive deficit options, this calculator bumps the protein multiplier to 1.2g per pound of LBM to provide that additional buffer.
To make this concrete: a 170 lb man at 18% body fat has 139 lb of LBM. At 1g/lb LBM, his protein target is 139g/day. At 1.2g/lb LBM during an aggressive cut, that rises to 167g/day. Both are achievable with whole foods and modest supplementation, and both are well above the minimums that lead to muscle loss.
Fat has a floor too. Below roughly 0.3–0.35g per pound of bodyweight, testosterone synthesis, joint lubrication, and fat-soluble vitamin absorption begin to suffer. This calculator holds fat at 0.35g/lb as a non-negotiable minimum and assigns all remaining calories to carbohydrates, which support training performance and keep muscle glycogen topped up during your sessions.
Calculator
Body fat % is optional but improves protein accuracy. If omitted, protein is estimated at 0.85g × bodyweight.
Example 1 — Moderate cut
170 lb male · 18% BF · TDEE 2,600 kcal
Example 2 — Moderate cut
145 lb female · 24% BF · TDEE 1,900 kcal
Example 3 — Aggressive cut
195 lb male · 20% BF · TDEE 2,900 kcal
Adjustments
A macro plan is not a set-and-forget prescription. Bodies adapt. What produces a 0.5 lb weekly loss in week one may yield nothing by week five. Knowing when and how to adjust is what separates a successful cut from a frustrating plateau.
The most common scenario is a scale that stops moving. If your weight has not changed for two consecutive weeks and you are confident you have been consistent, reduce your daily calorie target by 100–150 kcal. Always take that reduction from carbohydrates first, not protein or fat. Carbs are the most flexible macro — they have the smallest effect on muscle retention and hormone function when trimmed modestly. Protein stays fixed; fat stays at the minimum floor.
The opposite problem — losing too fast — is just as important to address. If the scale is dropping more than 1.5 lb per week, you are almost certainly losing lean mass alongside fat. Add 100–200 kcal back in, again from carbohydrates. Rapid weight loss feels productive but it costs muscle, lowers resting metabolic rate, and often results in a rebound once the cut ends.
For cuts longer than eight weeks, programming one or two refeed days per week at maintenance calories helps blunt the hormonal adaptations that accompany prolonged restriction — particularly the drop in leptin and the rise in ghrelin that make hunger increasingly hard to manage. On refeed days, bring calories up to your TDEE and direct the extra food to carbohydrates. Keep protein and fat unchanged.
Zenith automates this entire adjustment loop. Each week it compares your actual weight trend to the projected loss based on your logged food. If the trend is slower than expected, Zenith nudges your calorie target down by a conservative increment. If it is faster than expected, it nudges up. The adjustments are small and data-driven — no guesswork, no drastic changes that spike hunger or stall performance. You stay in a productive deficit without having to audit your own numbers every Sunday night.
The underlying principle is that cutting is not a sprint. A moderate, consistent deficit maintained over 10–16 weeks with well-calibrated macros produces far better body composition outcomes than an aggressive crash followed by a rebound. Use the numbers from this calculator as your starting point, trust the weekly adjustments, and give the process time to work.
Stop guessing every week
Most people plateau because they set macros once and never update them. Zenith does the math for you — every week, based on your actual results.
Download Zenith FreeSarah Okafor
Certified Fitness Instructor, 8 years coaching · Reviewed May 2026