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Sources & method

Last updated 20 October 2026

Every number in Month6comes out of a fixed formula, and every formula has a source. Here they are, with the papers behind them. The same list is in the app, under Profile → Sources & method.

Not medical advice

Month6 is a fitness and nutrition app, not a medical service. Nothing here is medical advice, diagnosis or treatment, and no outcome is guaranteed. Talk to a doctor before changing how you eat or train — especially if you are pregnant, under 18, or living with any medical condition, injury or eating disorder.

Your six-month projection

How it’s calculated. Lean mass is projected forward month by month at a rate set by your training experience (and, from 40, your age), capped by a natural fat-free mass index (FFMI) ceiling of 24–25, and turned into weight and body-fat numbers by fixed arithmetic. No model or AI invents these numbers; the same inputs always give the same curve. Two floors sit in that arithmetic: fat loss slows to a stop at 8% body fat for men and 16% for women — inside the American Council on Exercise's "athletes" band, well clear of essential fat — and on a bulk no week is projected to add more fat than muscle.

What that means here. The ceiling comes from the highest FFMI values measured in athletes who had never used anabolic steroids. Rate ranges are taken conservatively — the headline curve sits at 40% of the way from the low to the high rate, not the midpoint, and female rates are set at roughly half the absolute rate. A projection is an estimate, and assumes you hit at least 80% of your plan.

Your projection from age 40

How it’s calculated. From 40, the monthly muscle-gain rate set by your training experience is multiplied by an age factor: 0.65 in your 40s, 0.30 in your 50s, 0.24 in your 60s and 0.16 from 70. Both ends of the rate range scale by the same factor. Under 40 nothing changes.

What that means here. A meta-analysis of 49 resistance-training studies in adults aged 50 to 83 found an average gain of 1.1 kg of lean mass over about 20 weeks, roughly 0.23 kg a month, with smaller gains at older ages. Month6 sets each factor so that a beginner man's headline rate stays at or below what the paper reports for the middle of that age band. A woman's rate is already set at half of a man's, so her curve sits further below the paper. The paper included no one under 50, so the factor for your 40s is Month6's own halfway step, not a figure from the study. Muscle still responds to training at every age, and the curve is drawn for yours.

Your daily protein target

How it’s calculated. Bodyweight in kilograms × a factor set by your goal: 1.8 g/kg to build, 2.0 g/kg to recomp, 2.2 g/kg while cutting.

What that means here. These sit inside the range the evidence supports for people doing resistance training — meta-analysis puts the useful ceiling near 1.6 g/kg for muscle gain, and the ISSN position stand gives 1.4–2.0 g/kg, with more while in a deficit to protect muscle. Past 60 the target does not change: the 1.8–2.2 g/kg Month6 sets is already above the 1.0–1.2 g/kg (and 1.2 g/kg or more for active older adults) that geriatric guidance recommends. Nothing here is a medical prescription; if you have kidney disease or any condition affecting protein intake, ask your doctor first.

Your daily calorie target

How it’s calculated. The Mifflin–St Jeor equation for resting energy expenditure, multiplied by an activity factor stepped by how many sessions a week you train (1.45–1.65), then by your goal's factor: ×1.125 to build, ×1.0 to recomp, ×0.825 to cut.

What that means here. A floor is enforced in code before the number is ever shown: your target is never set below 75% of your estimated daily expenditure, no matter what your goal or a check-in adjustment asks for. Starvation targets are refused by design — including by the coach.

The calculators on this site

How it’s calculated. The calorie and protein calculator runs the same daily-target function as the app, and the FFMI calculator the same index and 24–25 ceiling as the projection, so the sources above apply to both unchanged.

What that means here. One rule exists only on the public calculator, because anyone can open it: it will not set a calorie deficit for a body-mass index below 18.5 — the World Health Organization's threshold for underweight — or for anyone under 18, and shows maintenance instead.

Your carb and fat targets

How it’s calculated. Fat is set first, at a factor of your bodyweight set by your goal — 1.0 g/kg to build, 0.9 g/kg to recomp, 0.8 g/kg while cutting — and never more than 30% of your daily calories. Protein is your protein target. Carbohydrate is whatever calories are left, at 4 kcal per gram.

What that means here. The fat factors sit inside the 0.5–1.5 g/kg range recommended for lifters building muscle, and the 30% cap is the top of the 15–30% of calories recommended while dieting, so a cut never squeezes fat to nothing and never lets it crowd out the carbohydrate your training runs on. Protein stays the number the ring reads; calories, carbs and fat are a budget, and you can switch to protein-only logging in Settings → Goal.

The food and recipe libraries

How it’s calculated. Each of the roughly 300 foods in the library is one typical portion with fixed protein, carbohydrate, fat and calorie estimates, typed from USDA FoodData Central reference entries and rounded (restaurant plates use typical published averages; supplements typical labels), and its calories reconcile with the macros to within 12%. Weighed foods also show a per-100 g line, derived from the same numbers. Each recipe's per-serving protein, carbohydrate and fat are summed from its listed ingredients the same way, and its calories reconcile with 4 kcal per gram of protein and carbohydrate and 9 kcal per gram of fat. Recipes are ranked for you by plain arithmetic — your goal, your daily targets, what you have logged today and the time of day — never by a model.

What that means here. These are estimates of a typical plate, not a lab analysis of yours: a bigger or smaller portion moves the number, and the ×½ / ×2 chips exist for that. Protein is the number the ring reads; calories, carbs and fat inform the daily budget without gating anything. A recipe's gluten-free, dairy-free and low-carb labels are read off its ingredient list by fixed word lists — they are not allergy advice, so check packaged ingredients yourself. Nothing in the library is a diet or a medical prescription. A food you add yourself carries the numbers you typed — from its packet, not from these sources — and is never shared.

Estimated 1RM on your lifts

How it’s calculated. The Epley formula — weight × (1 + reps ÷ 30) — applied to the best working set of each session. Warm-up sets are excluded.

What that means here. It is an estimate of a single-rep max you never have to attempt, and it gets less accurate the more reps a set has. Validation studies put prediction equations like this within a few percent for trained lifts; Month6 uses it to show a trend over time, not to prescribe a one-rep attempt.

Weekly sets per muscle group

How it’s calculated. Working sets per muscle group over a rolling 7 days, against a target range: 10–20 sets for chest, back, shoulders and legs, 6–16 for arms, up to 12 for core.

What that means here. Meta-regression shows muscle growth increasing with weekly set volume across the range studied, which is where the 10–20 band comes from. The range is a guide for spotting a neglected muscle group, not a quota — more is not automatically better, and recovery is part of the plan.

Rep ranges and rest between sets

How it’s calculated. Each exercise has a rep window — narrower and lower on heavy barbell lifts, wider and higher on small isolation and bodyweight moves — and a rest time: 150 seconds on heavy barbell compounds, 90 on other compounds and machines, 60 on isolation and core. Sets start at the bottom of the window; when every set reaches the top, the load goes up by that exercise's smallest step and the reps start again from the bottom (double progression).

What that means here. Muscle growth is similar across a wide range of loads when sets are taken close to failure, so each window is chosen for what the lift tolerates well, not because one range builds muscle and another does not — heavier windows do build more maximal strength. Longer rests between sets of heavy compound lifts produced more strength and muscle than one-minute rests in trained men, which is why the big lifts get the longest breaks.

How hard a set was (RIR)

How it’s calculated. Reps in reserve: how many more reps you could have done. 0 means you hit failure, 3 means you had three left.

What that means here. RIR is a validated way to record effort without needing a max attempt, and it is what progression uses to decide whether to add load. Your own rating is the input — Month6 never infers it.

The 7h+ sleep ring

How it’s calculated. The sleep ring closes when you log seven or more hours.

What that means here. Seven to nine hours is the recommended range for adults in the National Sleep Foundation's consensus review, so seven is the floor rather than the goal. Under-18s need more; if you regularly can't sleep, that is a conversation for a doctor, not an app.

Ages 16–17: conservative defaults

How it’s calculated. Month6 is for ages 16 and up. Under 18, the coach runs in a conservative mode: no aggressive deficits, no maximal-load prescriptions, and technique before load.

What that means here. Supervised, technique-led resistance training is considered safe and beneficial for adolescents in the NSCA's position statement; maximal lifting and aggressive dieting are not part of that. Growth and puberty change what's appropriate, so a young lifter's plan should be checked with a doctor or a qualified coach.