---
title: Fat vs Muscle Loss Calculator: How Much Was Fat? (GLP-1)
url: https://progresslens.app/tools/fat-vs-muscle-loss-calculator/
language: en
updated: 2026-11-13
---

# Fat vs muscle loss calculator

The scale says you lost weight. This shows how much of it was fat and how much was lean mass, from two weigh-ins with body fat readings.

Short answer

Fat lost = start weight × start body fat − current weight × current body fat. Lean mass lost is the rest of the weight lost. Under about 25% of the loss as lean mass is a good result; GLP-1 trials saw roughly 25–40%.

Calculated in your browser. Nothing you enter is sent anywhere.

## Why it matters, especially on GLP-1 medication

Fast weight loss always costs some lean mass. In the STEP 1 trial of semaglutide, a body-composition substudy with DXA scans found that roughly 40% of the weight lost was lean mass; in SURMOUNT-1 (tirzepatide) it was roughly a quarter. Lean mass includes water, glycogen and organs as well as muscle, so it overstates muscle loss, but keeping that share low is the goal. Strength training and enough protein are the two levers most clinicians mention.

## Getting usable numbers

- Use the same method for both body fat readings: the same scale, the same tape method, or two DXA scans. Mixing methods makes the result meaningless.
- Weigh in the morning, after the bathroom, before eating.
- Compare at least 4 weeks apart.

No body fat readings? Arm and shoulder measurements are a practical proxy: if your waist shrinks much faster than your arms and shoulders, most of what you’re losing is fat. ProgressLens tracks waist, chest, shoulders, arms and thighs next to matched photos. Read [GLP-1 progress photos](https://progresslens.app/guides/glp-1-progress-photos/).

## See how far you’ve come.

Take a check-in, tap AI Analyze and get your Progress Report: body-fat range, what changed, fat or muscle, and whether to cut or bulk. On iPhone, iPad and Android, with no account. Your photos stay on your phone.

FAQ

## Frequently asked questions

**How much muscle do you lose on Ozempic?**

In the STEP 1 DXA substudy, about 40% of weight lost was lean mass, which includes but is not only muscle. Resistance training and protein are commonly recommended to reduce it. Ask your clinician.

**What is a good fat-to-muscle loss ratio?**

Most of your loss being fat. Under about 25% lean mass is good for a diet; under 15% is excellent and usually needs strength training and high protein.

**Can I gain muscle while losing fat?**

Yes, especially as a beginner or with higher body fat. The calculator will show lean mass gained if your second reading supports it.

**Is a smart scale accurate enough?**

Its absolute number may be off, but the same scale under the same conditions is consistent enough to see a trend. Use it for both readings.

## More free tools

[Body fat estimatorYour look plus a tape measure, no upload.Open →](https://progresslens.app/tools/body-fat-estimator/)[Bulk or cut calculatorCut, lean bulk or recomp, with a weekly rate.Open →](https://progresslens.app/tools/bulk-or-cut-calculator/)[TDEE calculatorMaintenance calories, and to cut or bulk.Open →](https://progresslens.app/tools/tdee-calculator/)[Macro calculatorProtein, carbs and fat for your goal.Open →](https://progresslens.app/tools/macro-calculator/)[GLP-1 weight loss calculatorOzempic and Mounjaro trial averages for you.Open →](https://progresslens.app/tools/glp-1-weight-loss-calculator/)[Protein calculatorGrams a day for muscle, fat loss or GLP-1.Open →](https://progresslens.app/tools/protein-calculator/)

---
ProgressLens is an AI progress photo app for iPhone, iPad and Android that turns your check-in photos and tape measurements into a Progress Report: your body-fat range, what changed, whether you are losing fat or muscle, and whether to cut, lean bulk, recomp or maintain.
App Store: https://apps.apple.com/app/progresslens/id6806300575
Google Play: https://play.google.com/store/apps/details?id=app.progresslens