How to Set Your Calories Using the 10/15/20 Rule

You do not need a calculator to work out your calories. You need a number to start from and a way to tell whether it was wrong.

Most people get this backwards: an afternoon spent comparing Mifflin-St Jeor against Katch-McArdle, picking whichever answer is friendliest, then eating that number for six months without ever checking it against their bodyweight. The equation was never the hard part. The feedback loop is.

The bodyweight multiplier gets you a starting number in seconds. Then your scale tells you what to do about it.

What the bodyweight multiplier actually is

Take your bodyweight, multiply it by a number, eat that many calories: that is the entire method. The multiplier depends on what you are trying to do:

The version people remember is the simplified one: 10x to cut, 15x to maintain, 20x to gain, easy to hold in your head at the supermarket. It is also, at the top end, wrong for most of the people repeating it.

Why a rough multiplier beats a better equation

Surely an equation weighing height, age, and body fat beats one that only knows what you weigh? Not really: bodyweight already does most of the work. In a 2023 cohort of athletes, body mass alone explained 62% of the variance in resting metabolic rate for men and 69% for women, more than fat-free mass managed, with each extra kilogram worth about 19-21 kcal per day (Jagim et al., 2023).

The elaborate equations are less accurate than their decimal places suggest: a systematic review of 29 studies, 1,430 athletes, and 100 prediction equations found that most formulas significantly mis-estimate resting metabolic rate in athletes, with even the best performer landing within 10% of a person's true measured value only 80.2% of the time (O'Neill et al., 2023). Measured directly by indirect calorimetry, these general-population formulas substantially underestimate resting metabolic rate specifically in bodybuilders (Sordi et al., 2022).

Energy expenditure also varies within the same person by a coefficient of variation of about 11.8%, drifting further as time passes (Black & Cole, 2000), and more than a third of people under-report what they eat by over 25% when measured against doubly-labelled water (Schoeller, 2002).

Your true maintenance moves. Your logging is probably wrong. A more precise formula fixes neither. Only the feedback loop does.

Why 20x bodyweight makes most lifters fat

The simplified rule falls over here. Take a 180 lb lifter, 81.6 kg, at a 2,700 kcal maintenance (15x). Here is what each gaining multiplier actually represents as a surplus:

The off-season nutrition literature for bodybuilders recommends a surplus of 10-20%, with a target gain of 0.25-0.5% of bodyweight per week, and tells more advanced lifters to be more conservative on both (Iraki et al., 2019).

17x sits inside the recommended band, 18x sits at the ceiling, and 20x is a third over maintenance, roughly half again the biggest surplus anyone recommends.

There is a direct test. An 8-week randomised trial in trained lifters compared a small surplus (5%) against a large one (15%). The large surplus group gained significantly more fat by skinfold thickness and no additional muscle thickness at any site measured, biceps, triceps, or quads (Helms et al., 2023).

Strength is the honest complication. Squat 1RM did not differ between groups, but bench press did: the large surplus group added 13.4 kg against 7.3 kg on the small surplus. So the extra calories bought something. They just did not buy measurable muscle, and they came with the fat.

That is the trade you are actually making at 20x, and the surplus that produced it, 15%, is smaller than the 20% you get at 18x and less than half the 33% at 20x.

For most resistance-trained lifters, 15-17x bodyweight is the honest starting range through lean gaining; reach for 18-20x only if very active, struggling to gain, or enhanced.

The cutting side has the same problem in reverse

Faster is not better going down either. Elite athletes were randomised to lose weight slowly, targeting 0.7% of bodyweight per week, or quickly, targeting 1.4%. Both groups lost almost exactly the same bodyweight, 5.6% against 5.5%. What differed was what that weight was made of: fat mass fell 31% in the slow group against 21% in the fast group, and lean mass rose 2.1% in the slow group while the fast group's did not move (Garthe et al., 2011).

Dieting harder cost them fat loss and lean mass at the same time. Worth noting the fast group only achieved 1.0% per week rather than the 1.4% they were aiming for, so the gap between sensible and reckless may be narrower than the study design suggests.

This is where the 0.5-1.0% per week guidance comes from: the standard evidence-based review of contest preparation recommends exactly that range, drifting toward the slower end as you get leaner (Helms et al., 2014).

Protein does real work here, independent of the calorie number: resistance-trained men in a severe 40% deficit for four weeks who ate 2.4 g/kg of protein gained 1.2 kg of lean mass while losing 4.8 kg of fat, against 0.1 kg of lean mass and 3.5 kg of fat at 1.2 g/kg (Longland et al., 2016), and a shorter aggressive cut found 2.3 g/kg preserved significantly more lean mass than 1.0 g/kg (Mettler et al., 2010).

Set protein first. The multiplier decides the rest.

Your daily weight lies. Your 7-day average does not.

The multiplier gives you a hypothesis. The scale tests it, but only if you read the scale properly, and almost nobody does.

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That is four weeks of a textbook cut at 0.7% per week. The daily readings span 2.3 kg, which is more than the 2.2 kg actually lost over the entire month. Every one of those grey dots is a morning where someone could have concluded their diet was working, or that it had stalled, and been wrong either way. Only the green line was ever telling the truth.

Day-to-day bodyweight has a standard deviation of about 0.53% over a one-day interval, rising to 0.69% across a week (Schneditz et al., 2023): for an 82 kg lifter, roughly 400-500 g of pure noise on any given morning, since a genuine day-to-day change of 0.5% in fat, protein, and mineral is physiologically improbable. Almost all of it is water. An older cohort weighing young women daily for 68 days found 86.6% of day-to-day changes fell within 0.5 kg and 98.2% within 1.0 kg (Robinson & Watson, 1965).

Bodyweight also runs on a weekly rhythm: across 4,657 measurements in 80 adults, weight rose on weekends and fell through the week (Orsama et al., 2014).

Glycogen is the largest lever this audience pulls deliberately: every gram of stored muscle glycogen binds 3-4 g of water, and restoring around 500 g of glycogen pulled in 2.2 litres of total body water in the classic measurements (Olsson & Saltin, 1970). Refeed on Saturday and you can be 2 kg heavier on Sunday with zero fat change, as anyone who has run a peak week carb load knows; creatine does a smaller version during loading.

Weigh daily anyway: daily weighers do better. A randomised trial found daily weighers lost 6.1 kg more over six months than those weighing less often (Steinberg et al., 2015), and in 9,768 smart-scale users tracked for nearly three years, only daily weighing was associated with a downward weight trend across every BMI group (Vuorinen et al., 2021).

Weigh every morning, the same way, and ignore every individual number. Only the 7-day average counts.

Women should lean on the trend harder than men. Bodyweight runs about 0.45 kg higher during menstruation than in the early follicular week, driven by extracellular water (Kanellakis et al., 2023), and self-reported fluid retention peaks at the start of flow with a second smaller rise around ovulation (White et al., 2011). Compare the same cycle week to the same cycle week where you can.

Your 7-day average has not moved. Now what?

This is the part people get wrong, usually by panicking.

First, check whether enough time has passed. One 7-day average absorbs the weekend rhythm and most of the water noise, but a single week-to-week comparison still sits inside the roughly 0.7% noise band. You need two full weeks of averages before a change means anything. A 100-200 kcal daily deficit only produces about 0.1-0.3 kg of real tissue change per week, smaller than the noise you are trying to see through.

Work through it in order:

Is the trend within target? For a cut, 0.5-1.0% of bodyweight per week. For maintenance, flat. For a lean bulk, 0.1-0.25% per week. For an 82 kg lifter that is 0.41-0.82 kg per week cutting, and 0.08-0.20 kg per week gaining. Gaining targets are small enough to be hard to see over one week, hence the two-week rule.

If the trend is off, adjust by 100-200 kcal per day, not more. On a 2,700 kcal maintenance that is a 3.7-7.4% change. It feels too small. It is not. People who slash 500 kcal the moment the scale stalls overshoot, tank their training, and end up cycling between over-restriction and rebound.

If you are cutting and nothing moved in two weeks: drop 100-200 kcal, or add 1,000-2,000 daily steps, then wait another two weeks. Prefer adding activity if your intake is already low.

If you are cutting faster than 1% per week: this is not winning. Add 100-200 kcal back. You're on track to lose the lean mass Garthe's fast group failed to gain.

If you are lean bulking and gaining more than 0.25% per week: cut 100-200 kcal. Past that rate you are buying fat, not muscle.

If you are lean bulking and nothing moved: add 100-200 kcal. Be honest about whether you actually ate what you logged: a third of people under-report by more than a quarter.

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That under-reporting figure is worth sitting with before you touch your calories. If your logging is 25% light, the number you are adjusting is not the number you are eating, and no amount of arithmetic downstream will fix it. Weigh things for a fortnight before concluding your metabolism is unusual.

Two weeks. One adjustment. Repeat. That is the whole protocol, and it beats any calculator because it uses your body instead of a population average.

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When your appetite is lying to you

Here is the part that generic nutrition advice cannot help you with, because the audience it was written for is not taking anything.

Most calorie guidance quietly assumes hunger is informative. Eat when hungry, stop when full, adjust by feel. For a large share of lifters right now, that signal has been pharmacologically overwritten in one direction or the other.

GLP-1 agonists suppress intake hard. Semaglutide reduced total daily ad libitum energy intake by 24%, about 726 kcal per day, in a crossover trial (Blundell et al., 2017). Tirzepatide cut ad libitum lunch intake by an estimated 525 kcal versus placebo by blunting reward-region brain activation to high-fat and high-sugar food, without changing dietary restraint at all (Martin et al., 2025). That last detail matters: this is not willpower, it is gut-brain signalling. If you are running semaglutide, tirzepatide, or retatrutide, a 700 kcal suppression is larger than the entire activity adjustment in most TDEE formulas. You can drift into a far steeper deficit than you planned and feel fine about it, right up until your lifts fall apart. Our GLP-1 bloodwork guide covers what to monitor alongside it.

MK-677 does the exact opposite. It is an orally active ghrelin receptor agonist (Nass et al., 2008), and ghrelin is the primary hunger hormone. This appetite has nothing to do with your energy balance. Lifters on MK-677 routinely describe being ravenous in a surplus. Trust the trend, not the stomach. Worth watching your glucose and insulin too, for reasons covered in how much MK-677 raises blood sugar.

Clenbuterol moves the other side of the equation. In young healthy men it raised resting energy expenditure by 21% and fat oxidation by 39% (Jessen et al., 2020). Your multiplier was calibrated for a body that is no longer burning what it was.

The practical conclusion is the same in every case: when appetite is driven by a drug rather than energy balance, the 7-day trend is not just the better feedback signal. It is the only one you have left.

Where the multiplier breaks down

It is a starting estimate. These are the situations where it will be furthest off:

Your NEAT is not average. Non-exercise activity thermogenesis varies by up to 2,000 kcal per day between similarly sized people, and changes in it explained most of the variance in fat storage during controlled overfeeding (Levine et al., 1999). A tradesman and an office worker at the same bodyweight need different multipliers.

You have been dieting a long time. Sustained restriction produces adaptive thermogenesis beyond what weight loss alone predicts: six years after The Biggest Loser, contestants' resting metabolic rate sat 704 kcal per day below baseline, about 499 kcal per day of that pure metabolic adaptation (Fothergill et al., 2016). Extreme case, but the direction applies to any long cut: your multiplier from twelve weeks ago is no longer your multiplier.

Your thyroid panel looks off mid-cut. A falling free T3 with a relatively normal TSH during a sustained deficit is an adaptive response, not thyroid disease, the same low-T3 pattern discussed in low T3 on steroids. It signals your energy expenditure has fallen and your calorie target needs revisiting, not your thyroid needing medicating.

You are enhanced. This is the biggest caveat: every study cited above was conducted in natural lifters, none administered testosterone, anabolic steroids, or growth hormone. Growth hormone shifts substrate partitioning toward fat oxidation independent of the calorie arithmetic (Meinhardt et al., 2010). Enhanced lifters plausibly tolerate a larger surplus, but no trial has established what it should be: start at the natural numbers and let your trend data find your window. The same linear-scaling problem applies if you are very light or very heavy: lean athletes need the higher band, heavier lifters the lower one.

Putting it together

1. Set protein first. 1.8-2.4 g per kg of bodyweight, independent of the multiplier.
2. Pick your multiplier. 10-12x per lb to cut, 14-16x to maintain, 16-18x to lean bulk. In kilos, 22-26x, 31-35x, 35-40x. Unsure? Start at 15x per lb (33x per kg) and find out.
3. Weigh daily, first thing, after the toilet, before food. Record it and ignore it.
4. Compare 7-day averages, never individual days.
5. Wait two full weeks before judging. One week is inside the noise.
6. Adjust by 100-200 kcal per day, then wait another two weeks. Targets: 0.5-1.0% loss per week cutting, flat for maintenance, 0.1-0.25% gain per week lean bulking.
7. If you are on a GLP-1, MK-677, or a beta-agonist, distrust your appetite entirely and let the trend drive every decision.

The multiplier is a guess. A guess plus a working feedback loop beats a precise-looking calculation with no feedback loop, every time.

Key takeaways

• Bodyweight alone explains 62-69% of resting metabolic rate variance in athletes, more than fat-free mass does, making a simple multiplier a legitimate starting point rather than a shortcut.
• The best resting metabolic rate equation available still misses one athlete in five by more than 10%, so extra formula complexity buys precision you cannot use.
• 20x bodyweight is a 33% surplus for a typical lifter, well above the 10-20% the literature supports; a trial comparing 5% against 15% found the larger surplus added fat and bench press strength, but no extra muscle thickness anywhere measured.
• 15-17x per lb (33-37x per kg) is the realistic starting range for most resistance-trained lifters.
• Cutting slowly beat cutting fast on both counts: for the same total weight lost, the slow group shed 31% of its fat mass and added 2.1% lean mass, while the fast group shed 21% and added none.
• Daily weight carries roughly 0.5% standard deviation of pure water noise, plus a weekend rhythm, plus 2 kg swings from glycogen. Only the 7-day average is readable.
• Give any change two full weeks before adjusting, then move by 100-200 kcal, not 500.
• GLP-1 agonists cut intake by 500-700 kcal per day and MK-677 amplifies hunger, so appetite is a corrupted signal for many lifters and the trend is the only honest feedback.
• Every rate and surplus figure here comes from natural-lifter research; no controlled trial has established equivalent numbers for enhanced lifters.

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References

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