How we know · 25 decisions · 42 sources

Research, not opinions.

Strong · several good trials agreeGood · solid but with gapsEstimate · few trials, expert consensus

The decisions

Your routeLose about 0.5% of your weight a weekLosing slowly keeps more muscle. In one trial, athletes who lost slowly even added some.
Strong

What the studies found

Elite athletes who lost about 0.7% of their weight a week gained 2.1% lean mass and lost 31% of their fat. Those losing 1.4% a week lost 21% of their fat and their lean mass did not change. Across training studies, an energy deficit held back lean-mass gains, though strength still improved.

What it means for you

0.4 kg a week is 0.5% of your 77 kg, inside the 0.5 to 1% a week these studies and contest-prep guidance support.

Sources

In the code: src/utils/roadmap.ts FAT_LOSS_FRACTION_BW_PER_WEEK

Your routeThe faster pace needs fat to spareFat stores can only release so much energy a day. Diet harder than that and the gap comes out of muscle.
Estimate

What the studies found

A model built from underfeeding studies puts the most the fat store can supply at about 290 kJ (69 kcal) per kg of fat per day, and says a deficit beyond that takes fat-free mass straight away. It is a model, not a trial, so we plan under it, at 48.5 kcal per kg of fat.

What it means for you

With the fat you carry, the deepest deficit we plan is about 650 kcal a day, so the fastest pace offered to you is 0.5% a week.

Sources

In the code: src/utils/lossRate.ts FAT_ENERGY_RELEASE_KCAL_PER_KG_PER_DAY

Your routeRecomp keeps the deficit under 500 kcalIn training studies, a deficit of about 500 kcal a day was enough to stop muscle gains.
Strong

What the studies found

A meta-regression of resistance-training trials found an energy deficit of about 500 kcal a day prevented lean-mass gains, and the authors advise against deficits larger than that when muscle is the goal. Strength gains were not affected by the deficit.

What it means for you

Your recomp runs about 450 kcal under maintenance, below that line, so building muscle while losing fat is still realistic.

Sources

In the code: src/utils/roadmap.ts RECOMP_DEFICIT_CEILING_KCAL_PER_DAY

Your routeEase in at about 0.5% a weekIf you already train, a small deficit can trim fat while your muscle keeps growing.
Estimate

What the studies found

In a 10-week trial of 30 trained people on high protein, a moderate deficit lost 2.9 kg of fat and gained 1.0 kg of lean mass, while eating at maintenance lost 1.4 kg of fat and gained the same lean mass. The fat-loss difference between the two groups was not statistically significant.

What it means for you

You chose to ease in, so the scale moves about 0.4 kg a week. This is one small trial, so treat the pace as a best estimate.

Sources

In the code: src/utils/roadmap.ts EASE_FRACTION_BW_PER_WEEK

Your routeFor men, 10 to 15% body fat is the sweet spotRaters liked men at moderate leanness and muscle best. The very leanest bodies did not score highest.
Good

What the studies found

Across six studies, men with moderate muscularity were rated most attractive. When observers set fat and muscle on male bodies themselves, they chose levels within the healthy range. A 2025 study of 283 raters in three countries, using real DXA scans, reportedly found the peak at about 13 to 14% body fat.

What it means for you

Your goal of 13% is measured against that band. These are ratings of photos by mostly Western raters, not a promise about real life.

Sources

In the code: src/utils/attractivenessTargets.ts ATTRACTIVE_BF_RANGE

Your routeNatural muscle tops out near an FFMI of 25In one study, drug-free athletes topped out at a muscle-for-height score of about 25, so goals past it are flagged.
Estimate

What the studies found

Among 157 male athletes, the 74 who did not use steroids had fat-free mass indexes reaching a well-defined limit of 25.0, while many users went past it. The authors called the limit preliminary, and some people exceed it naturally.

What it means for you

Your goal works out to an FFMI of about 22.4. Near or past 25 we show it as uncommon, not impossible.

Sources

In the code: src/utils/roadmap.ts FFMI_CEILING

Your routeBuild about 0.10 kg of muscle a weekMuscle grows slowly and slows each year you train, and people differ, so the time is a range.
Estimate

What the studies found

In 49 trials, resistance training alone added about 1.1 kg of lean mass over roughly 13 weeks, and extra protein added about 0.3 kg more. Men gain more muscle in absolute terms, but women gain about the same relative to the muscle they start with. How fast gains slow over the years is our assumption, not a measurement.

What it means for you

From your training history and frame, about 0.10 kg a week, give or take about a third.

Sources

In the code: src/utils/roadmap.ts GAIN_DECAY_K, RATE_SPREAD, FFMI_UNTRAINED

Your routeBuild up to 18%, trim back to 12%This range is your choice of how lean to stay. No study shows one range builds more muscle than another.
Estimate

What the studies found

When 34 men overate by 40% for 8 weeks, those who started with more body fat gained more weight and more fat. In a trial of 61 trained adults, dieting in blocks with breaks and dieting straight through kept the same lean mass and strength, and the block dieters felt less hungry.

What it means for you

You chose the balanced way, so you build up to about 18% and trim back to 12%.

Sources

In the code: src/utils/operatingBands.ts OPERATING_BANDS

Your routeWomen's plans stop at 22% body fatVery lean women risk losing their period, so plans keep a safety margin above that point.
Estimate

What the studies found

Female physique athletes measured off-season averaged 23.4% body fat by DXA. Among those not on hormonal contraception, 27% had no periods, against none of the regular gym-goers. The study is small and cross-sectional, so 22% is a safety margin, not a proven threshold.

What it means for you

The leanest your plan takes you is 12%.

Sources

In the code: src/utils/operatingBands.ts OPERATING_BANDS (female floors), leanStopFor

Your routeExpect some fat with the muscleEven a planned surplus adds fat alongside muscle. A bigger one adds more fat, not more muscle.
Estimate

What the studies found

Elite athletes given a planned surplus gained more weight than those eating freely, but the extra was fat: fat mass rose 15% against 3%, and lean mass gains did not differ. An off-season review advises a surplus of about 10 to 20% and slower gains for advanced lifters.

What it means for you

Your plan assumes about 0.5 kg of fat for every kg of muscle, so a build ends a little softer and the next trim takes it off.

Sources

In the code: src/utils/syntheticNutritionAnswers.ts FAT_PER_LEAN_KG

Your routePhases end on your weight, not a body-fat readingBody-fat readings are too rough to see a few kilos of change. A weight trend can.
Good

What the studies found

Even a DXA scan, repeated on consecutive days in 21 trained athletes, varied by about 1.3 kg of fat mass, and home body-fat scales are less precise still. In one man weighed daily for 26 years, weight varied by about 0.5% from one day to the next, small enough for a trend to show real change.

What it means for you

This phase ends when your weight trend reaches about 73.6 kg. You still see body fat, it just does not decide.

Sources

In the code: src/utils/phaseTransition.ts (weight-based detection), roadmap.ts exitWeightKg

Your routeDecisions wait for a month of weigh-insA full month evens out water swings, including the menstrual cycle, before a phase is called done.
Estimate

What the studies found

Across 612,613 menstrual cycles logged in an app, the average cycle was 29.3 days, so a 28-day average covers most of a cycle. On real smart-scale data, exponentially weighted averages were among the best ways to fill gaps between weigh-ins.

What it means for you

We need at least three weigh-ins over a week before showing a trend, and phase decisions use your last 28 days.

Sources

In the code: src/utils/phaseTransition.ts TREND_DECISION_DAYS, TREND_EWMA_DAYS, TREND_MIN_READINGS

Training16 hard sets a week for side deltsMore weekly sets grow more muscle, with smaller gains from each extra set at the top.
Good

What the studies found

A 2026 analysis of 67 studies found more weekly sets meant more muscle and strength, with returns tapering, more so for strength. Counting sets where a muscle only assists as half a set fitted the data best. An earlier analysis put each extra weekly set at about 0.4% more growth.

What it means for you

16 sets a week is where your experience and choices put side delts. Sets where it only assists count as half.

Sources

In the code: src/utils/workoutGenerator/data/volume-landmarks.json, check.ts positionRange/targetsFor

TrainingEach muscle about twice a weekSplitting the same weekly sets over two days grows muscle at least as well as one big day.
Good

What the studies found

An early analysis of 10 studies found training a muscle twice a week grew it more than once. A larger 2019 analysis of 25 studies found that with weekly sets matched, frequency made no meaningful difference. So frequency mainly helps you fit the sets in.

What it means for you

Your 4-day plan trains most muscles twice a week, which spreads the sets into sessions you can finish.

Sources

In the code: src/utils/workoutGenerator/generateProgram.ts TEMPLATES

TrainingMost sets in the 5 to 15 rep rangeLight and heavy weights build similar muscle when sets are hard. Heavier sets build more top-end strength.
Good

What the studies found

Across 21 studies with sets taken to failure, light loads (60% of max or less) and heavy loads built similar muscle, while heavy loads gave bigger one-rep-max gains.

What it means for you

Big barbell lifts sit at 5 to 10 reps for strength and size; isolation moves sit at 10 to 15.

Sources

In the code: src/utils/workoutGenerator/data/rep-ranges.json

TrainingStop most sets a rep or three short of failureMuscle grows more as sets get close to failure, but going all the way adds little.
Good

What the studies found

Muscle growth improved as sets ended closer to failure, while strength gains were similar across a wide range of effort, though the authors call this exploratory. A separate analysis of 15 studies found no evidence that training to true failure beats stopping short.

What it means for you

Each block starts a few reps short of failure and edges closer each week, and any exercise done as a single set goes to failure.

Sources

In the code: src/utils/workoutGenerator/data/rir.json

Training2 to 3 min rests on the big liftsResting more than a minute keeps your reps up. Past about 90 seconds, growth evens out.
Good

What the studies found

A meta-analysis of 9 studies found a small growth benefit to resting more than 60 seconds, and no clear difference past 90. In trained men, 3-minute rests built more strength and thigh muscle than 1-minute rests. For maximum strength in trained lifters, a review favours rests over 2 minutes.

What it means for you

Your pace rests the big lifts for 2 to 3 minutes. A quicker pace trades a little growth for time.

Sources

In the code: src/utils/workoutGenerator/data/rest.json, src/data/restPaceCopy.ts

TrainingA lighter week every 5 weeksA planned easier week lets fatigue clear. Coaches agree on it, but trials are few.
Estimate

What the studies found

Expert coaches agree a deload is a planned stretch of reduced training to clear fatigue, and a practical review notes they are commonly placed every 4 to 8 weeks. In one trial, a full week off mid-programme did not change muscle growth but slightly reduced strength gains.

What it means for you

Your lighter week is the last of each 5-week block: about half the sets, same weights, more reps in reserve.

Sources

In the code: src/utils/workoutGenerator/data/deload.json, generateProgram.ts deloadSets

TrainingYour training stays the same while you cutCutting back sets during a diet did not help trained men keep muscle.
Estimate

What the studies found

Trained men dieting for 6 weeks did either 5 or 3 sets per exercise. Thigh muscle thickness showed no difference between groups, and the small loss of lean mass was similar in both.

What it means for you

Your sets are the same in every phase. Only the food changes.

Sources

In the code: src/utils/goalsProfile.ts phaseToVolumeTier

Meals170 g of protein a dayProtein helps muscle grow, and you need more when eating less, so cuts get the most.
Good

What the studies found

Across 49 trials, protein added about 0.3 kg of lean mass to training, with no further gain above about 1.6 g per kg of body weight a day (the top of its confidence range was 2.2). When lean athletes cut calories, a review of 6 studies put needs higher, at 2.3 to 3.1 g per kg of lean mass.

What it means for you

170 g is about 2.2 g per kg of your weight. It rises as you get leaner on a cut, and never goes down.

Sources

In the code: src/utils/nutritionMacros.ts phaseProteinGrams

MealsEat 2,180 kcal, about 450 under maintenanceA deficit big enough to lose fat, capped where studies saw muscle gains stop.
Good

What the studies found

Across resistance-training trials, a deficit of about 500 kcal a day stopped lean-mass gains, while strength still improved. So no cut here asks for more than 500, and leaner people get less.

What it means for you

Your deficit is about 450 kcal a day, which plans for about 0.4 kg a week.

Sources

In the code: src/utils/nutritionMacros.ts phaseCaloricTarget

MealsNever below 1,950 kcalEating too little for your training disrupts hormones, so calories never drop under a floor set by your lean mass.
Estimate

What the studies found

In 29 regularly menstruating women, hormone pulses were normal at 30 kcal per kg of lean mass a day and disrupted below it. That study was small, short and in women who did not exercise, and the IOC 2023 consensus treats low energy availability as a health risk for athletes.

What it means for you

Your floor is 1,950 kcal: 30 kcal per kg of your lean mass plus what your training burns. We apply it whatever your sex.

Sources

In the code: src/utils/nutritionMacros.ts EA_FLOOR_KCAL_PER_KG_FFM

MealsMaintenance of about 2,630 kcalYour burn is estimated from your size, age, daily activity and training, so treat it as a starting point.
Estimate

What the studies found

The Mifflin-St Jeor equation was built from measured resting energy in 498 adults. A systematic review found it predicted resting burn within 10% for more people than any other equation, but still with notable individual errors. Weight training is listed at 3.5 to 6.0 METs.

What it means for you

About 2,630 kcal from your body, your day and your training sessions. Your weigh-ins show whether it is right for you.

Sources

In the code: src/utils/nutritionMacros.ts tdeeFor, LIFESTYLE_MULTIPLIERS, TRAINING_SESSION_METS

MealsEat about 5% above maintenanceA small surplus feeds muscle growth. Eating more adds fat faster, not muscle.
Estimate

What the studies found

Athletes given a bigger planned surplus gained more fat than those eating freely, with no difference in lean mass gained. An off-season review suggests about 10 to 20% over maintenance and gaining 0.25 to 0.5% of body weight a week, with advanced lifters going slower.

What it means for you

Your 5% sits at or below that range, because the more training you have behind you, the less extra food you can turn into muscle.

Sources

In the code: src/utils/nutritionMacros.ts phaseCaloricTarget (bulk, lean_bulk)

MealsMacros worked out from weighed ingredientsEvery recipe is added up from USDA values per 100 g of each ingredient, not guessed.
Good

What the studies found

USDA's FoodData Central brings together lab-analysed foundation foods, the long-standing SR Legacy tables, survey foods and branded products, and aims to be reliable and transparent. Each ingredient here carries values per 100 g from it.

What it means for you

Your plan converts every amount to grams first, then adds it up, so swapping a meal recalculates the day.

Sources

In the code: src/data/ingredients.ts macros_per_100g, src/utils/computeMacros.ts ingredientMacros

Numbers with a dotted underline are examples. In the app they're your own.

The reference files

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Nine reference documents, one per topic, with every source cited in full. These are the files the app's rules were written from. The page renders the files themselves, so they can never drift apart.

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