A free, no-build, single-page, open-science web tool that calculates your daily calories and macros to lose fat at a healthy pace while preserving muscle, with every formula and every threshold linked to the peer-reviewed research it's based on. Explanations are available in two registers, academic and conversational, toggled from a switch at the top of the page, and in six languages.
Open the calculator (GitHub Pages)
- Metric & imperial. kg/cm or lb/ft·in, with automatic value conversion when you switch
- Target weight. Get a weekly loss rate and an estimated finish date
- Inclusive hormonal profiles. Options for trans women, trans men, and non-binary/intersex people, including a months-on-HRT input that gradually shifts the metabolism estimate toward the hormone-matched profile
- Academic and conversational explanations. A toggle at the top of the page switches every hint, warning, and results explanation between a formal register and a plain-language one, without changing any of the numbers
- Six languages. English, Spanish, Dutch, French, German, and Papiamentu, selectable from the top of the page. Every hint, warning, results explanation, and the "How the maths works" section is translated in both the academic and conversational register; the meal plan export follows the selected language too. Academic references are kept in their original language, as in real scholarly practice
- Three independent gram sliders. Protein, fat, and carbs each have their own 1-gram-step slider (bounds scale with your bodyweight) with a status chip, a large pop-up gram readout while you drag, and an explanation of what that level means for muscle retention, hormone health, satiety, or training fuel. Total daily calories are calculated live as the sum of the three, not fixed in advance, so you can watch the number drop as you ease each slider toward its own optimal range, and see how low it goes when all three hit their sweet spot at once
- Suggested target vs. actual total. A separate pace-based target (from your age, activity, and chosen weight-loss rate) is shown alongside your live macro total, with a running comparison and a projected weekly loss/gain that reacts to the sliders in real time
- Muscle-sparing defaults. Protein 2.0 g/kg, fat 0.8 g/kg, deficit capped at 25% of maintenance
- Meal plan export. One click generates a self-contained, printable page (no PDF library, no server) with a formatted overview of your current settings and macros, a curated list of healthy whole-food sources for each macro, and example breakfast/lunch/dinner/snack combinations. Opens in a new tab; use your browser's print dialog to save it as a PDF
- Safety rails. Warnings for very low calories, very low carbs, and under-18 users
- In-app "How the maths works". A numbered, cited walkthrough of every formula, plus a full reference list with DOI links, right on the page
- Animated results (numbers count up, the macro bar morphs, cards reveal on
scroll) that back off automatically for
prefers-reduced-motion - Works offline once loaded, remembers your inputs locally, light & dark mode, no tracking, no dependencies, MIT-licensed
| Step | Formula |
|---|---|
| Resting metabolism (BMR) | Mifflin–St Jeor: 10·kg + 6.25·cm − 5·age + s |
Sex constant s |
Interpolated between −161 (estrogen-dominant) and +5 (testosterone-dominant): s = −161 + 166·b |
| HRT adjustment | Blend factor b shifts linearly from the pre-HRT profile to the hormone-matched profile over the first 6 months of HRT |
| Maintenance (TDEE) | BMR × activity factor (1.2 – 1.9) |
| Suggested target | TDEE − deficit, where deficit is the chosen pace (0.5–1% of body weight/week) × 7700 kcal/kg ÷ 7, capped at 25% of TDEE. This is a reference figure only |
| Protein | User-chosen 1-gram-step slider, bounded to 1.0–3.0 g per kg of reference weight (target weight is used as the reference when BMI ≥ 30); below 1.6 g/kg the tool warns that weight loss will include muscle |
| Fat | User-chosen 1-gram-step slider, bounded to 0.4–1.6 g per kg of reference weight; below 0.6 g/kg the tool warns about hormone production and fat-soluble vitamin absorption |
| Carbs | User-chosen 1-gram-step slider, bounded to 0–6 g per kg of body weight, with a live readout of what that level means for training fuel (thresholds at 0.75 / 2 / 4 g/kg) |
| Actual total calories | protein×4 + carbs×4 + fat×9, calculated live from the three sliders, not fixed in advance |
| Actual deficit / pace | TDEE − actual total, converted to a projected weekly loss (or gain) shown in real time; if it exceeds 25% of TDEE, a warning appears |
| "All sweet spot" notice | Shown whenever protein, fat, and carbs are simultaneously in their optimal zones, alongside the resulting calorie total, the lowest intake achievable without leaving any of the three ranges |
The hormonal-profile handling reflects that metabolism formulas track hormone-driven body composition rather than gender identity: resting energy expenditure measurably shifts within the first months of hormone therapy. The 6-month linear blend is a pragmatic approximation, not a clinical claim.
The calculator itself shows this same table with full narrative context and citations under "How the maths works", in both explanation registers, followed by a numbered reference list with DOI links: nothing above is asserted without a source.
- Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241–247. doi:10.1093/ajcn/51.2.241
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults. J Am Diet Assoc. 2005;105(5):775–789. doi:10.1016/j.jada.2005.02.005
- FAO/WHO/UNU. Human energy requirements: report of a Joint FAO/WHO/UNU Expert Consultation. Rome; 2001. fao.org/3/y5686e
- Wishnofsky M. Caloric equivalents of gained or lost weight. Am J Clin Nutr. 1958;6(5):542–546. doi:10.1093/ajcn/6.5.542
- Hall KD. What is the required energy deficit per unit weight loss? Int J Obes. 2008;32(3):573–576. doi:10.1038/sj.ijo.0803720
- Garthe I, Raastad T, Refsnes PE, Koivisto A, Sundgot-Borgen J. Effect of two different weight-loss rates on body composition and performance in elite athletes. Int J Sport Nutr Exerc Metab. 2011;21(2):97–104. doi:10.1123/ijsnem.21.2.97
- Morton RW, Murphy KT, McKellar SR, et al. Effect of protein supplementation on resistance training-induced gains in muscle mass and strength. Br J Sports Med. 2018;52(6):376–384. doi:10.1136/bjsports-2017-097608
- Helms ER, Aragon AA, Fitschen PJ. Evidence-based recommendations for natural bodybuilding contest preparation. J Int Soc Sports Nutr. 2014;11:20. doi:10.1186/1550-2783-11-20
- Longland TM, Oikawa SY, Mitchell CJ, Devries MC, Phillips SM. Higher dietary protein during an energy deficit promotes greater lean mass gain and fat mass loss. Am J Clin Nutr. 2016;103(3):738–746. doi:10.3945/ajcn.115.119339
- Leidy HJ, Clifton PM, Astrup A, et al. The role of protein in weight loss and maintenance. Am J Clin Nutr. 2015;101(6):1320S–1329S. doi:10.3945/ajcn.114.084038
- Westerterp KR. Diet induced thermogenesis. Nutr Metab (Lond). 2004;1(1):5. doi:10.1186/1743-7075-1-5
- Whittaker J, Wu K. Low-fat diets and testosterone in men: systematic review and meta-analysis. J Steroid Biochem Mol Biol. 2021;210:105878. doi:10.1016/j.jsbmb.2021.105878
- Burke LM, Hawley JA, Wong SHS, Jeukendrup AE. Carbohydrates for training and competition. J Sports Sci. 2011;29(sup1):S17–S27. doi:10.1080/02640414.2011.585473
- Klaver M, de Blok CJM, Wiepjes CM, et al. Changes in regional body fat, lean body mass and body shape in trans persons using cross-sex hormonal therapy. Eur J Endocrinol. 2018;178(2):163–171. doi:10.1530/EJE-17-0496
- Spanos C, Bretherton I, Zajac JD, Cheung AS. Effects of gender-affirming hormone therapy on insulin resistance and body composition in transgender individuals. World J Diabetes. 2020;11(3):66–77. doi:10.4239/wjd.v11.i3.66
It's plain HTML/CSS/JS, no build step:
git clone https://github.com/PeterSlijkhuis/Macro-Calculator.git
cd Macro-Calculator
# open index.html in a browser, or:
python3 -m http.server 8000A workflow (.github/workflows/pages.yml) deploys automatically on every
push via GitHub Actions, with no build step and no branch juggling. The repo
needs to be public (or on a paid plan) for Pages to be available.
This tool provides evidence-based estimates for healthy adults and is not medical advice. Talk to a doctor or registered dietitian before making big dietary changes, especially if you're under 18, pregnant, have a medical condition or eating-disorder history, or are on hormone therapy.
MIT. Use it, fork it, audit it.