You put a client's height, weight, age and training days into a calculator, it returns 2,230, and you hand them 2,230 calories and a macro split as if it were a measurement. Four weeks later nothing has moved, and you cannot tell whether the number was wrong, the logging was wrong, or both.
That is the actual problem with calculating a client's targets. The maths is trivial. The trouble is that the output is a prediction with a few hundred calories of error in it, and most coaches treat it as a finding. Everything below is about producing a number you can defend, then correcting it with the client's own data instead of a better equation.
Treat the calculator output as a hypothesis
A TDEE calculator does two things in sequence. It predicts basal metabolic rate from body size and age, then multiplies that prediction by an activity factor the client chooses from a dropdown. Both steps carry error, and the second carries much more than the first.
So the honest reading of a 2,230 output is not 2,230. It is "probably somewhere between 1,950 and 2,500, and I will know which within a fortnight". That framing changes how you set the target and how you talk to the client about it. A target presented as a fact has to be defended when it does not work. A target presented as a starting point that you will adjust from their data is one the client expects you to change, which is exactly what you are going to do.
It also changes what you ask them for at the start. If the number is a hypothesis, the first two weeks are not the diet — they are the measurement that makes the diet possible.
Which equation you use barely matters
Coaches spend more time on this choice than it deserves. Three equations turn up in practice.
Mifflin-St Jeor is the default in most calculators and the one to use if you are picking one:
Men: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
Women: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161
Katch-McArdle uses lean mass instead of total body weight, so it needs a body fat figure:
BMR = 370 + (21.6 × lean body mass kg)
lean body mass = weight kg × (1 − body fat % ÷ 100)
It is the better choice for a client who is very lean or carrying a lot of fat mass, because at those extremes total body weight misdescribes how much metabolically active tissue there is. It is only better if the body fat number is real. A caliper reading or a smart-scale guess with five points of error puts more noise in than the equation takes out.
Harris-Benedict is the oldest of the three and still built into plenty of tools. There is no particular reason to choose it over Mifflin-St Jeor today.
Run the same client through two of them and you will usually see a gap of a few dozen calories. For the client in the worked example below, Mifflin-St Jeor returns 1,439 and Katch-McArdle returns 1,489 — a 50 calorie difference, less than a flat white with full cream milk. That is the entire prize for picking the right equation, and it is dwarfed by what happens in the next step.
The activity multiplier is where the number actually goes wrong
Once you have a BMR, calculators multiply it by an activity factor. The set almost all of them use runs from about 1.2 to 1.9, described with labels like "lightly active" and "very active". Here is what those labels do to a BMR of 1,439:
| Dropdown label | Multiplier | Estimated TDEE |
|---|---|---|
| Sedentary — desk job, no training | 1.2 | 1,727 |
| Lightly active — 1–3 sessions a week | 1.375 | 1,979 |
| Moderately active — 3–5 sessions a week | 1.55 | 2,230 |
| Very active — 6–7 sessions a week | 1.725 | 2,482 |
| Extremely active — physical job plus training | 1.9 | 2,734 |
Top to bottom is a spread of about 1,000 calories from the same client. One step, which is a single click on a dropdown, is 250. And the person choosing between "lightly" and "moderately" active is usually the client, who has no basis for the choice and tends to pick the flattering option.
Worse, the labels describe training sessions, and deliberate training is the smallest movable part of daily expenditure for most general-population clients. Two clients who both train four times a week can differ by 800 calories a day because one is a nurse on their feet for ten hours and the other drives to a desk. The dropdown cannot see that. It is the largest source of error in the whole calculation, and it is the one step you cannot improve by choosing a better formula.
Which is why the calculator is where you start and not where you finish.
Back-calculate maintenance from what the client already eats
The alternative is not more sophisticated. It is just measurement instead of prediction.
Ask the client to log everything they normally eat for two weeks and weigh themselves daily under the same conditions. Do not change their intake. You are not coaching yet; you are reading the system before you touch it.
At the end you have two numbers: average daily calories and the direction their weight moved. The arithmetic uses one working convention — about 7,700 calories per kilogram of body mass. It is an approximation, and it is close enough for this.
- Weight flat. Average intake is maintenance. Nothing else to do.
- Weight down. Multiply the kilos lost by 7,700, divide by 14, and add that to the average intake. That is maintenance.
- Weight up. Same calculation, subtract instead.
Use a weekly average weight rather than two single readings — water, food volume and menstrual cycle swings are much larger than the fat change you are trying to see over a fortnight.
Two objections come up, and both have answers.
Clients under-report. They do, routinely and without meaning to — cooking oil, milk in coffee, the mouthfuls while plating dinner. But a client who under-reports by 15% every day still gives you a usable maintenance figure, because the target you set is expressed in the same under-reported units they will keep logging in. The number is wrong in absolute terms and right in the terms that matter. What breaks the method is inconsistency: someone who tracks meticulously Monday to Friday and stops on the weekend gives you an average that means nothing.
Two weeks is a long time to wait. It is two weeks of the client learning to log, which they need anyway, against four to eight weeks of chasing a target that was never right. You can start them on a rough calculator target immediately and run the measurement underneath it — you just have to account for the deficit when you do the maths.
You need both halves of that data to be visible in one place. In Fitsly, a client's food log week shows their average daily calories against their goal and how many of the seven days they actually logged, which is the number you check before you trust the average.
Set the deficit or surplus as a percentage
With a maintenance figure, the target is a percentage of it, not a round number subtracted from it.
A flat 500 calorie deficit is the most common convention in coaching and the least defensible one. On a 3,000 calorie maintenance it is a 17% deficit — brisk, sustainable, fine. On a 1,700 calorie maintenance it is 29%, which lands a small client on 1,200 calories, where hitting protein is hard, adherence collapses and training quality goes with it. Same subtraction, two completely different diets.
Percentages scale with the client:
| Goal | Deficit or surplus | Expected weekly change |
|---|---|---|
| Fat loss, conservative | 10–15% below maintenance | 0.25–0.5% of body weight |
| Fat loss, standard | 15–20% below | 0.5–0.75% of body weight |
| Fat loss, aggressive or short block | 20–25% below | 0.75–1% of body weight |
| Muscle gain | 5–10% above maintenance | 0.25–0.5% of body weight a month |
| Maintenance or a diet break | At maintenance | Flat |
Most coaches start a general-population fat loss client at 15–20% and only go past that with a lean client, a short block and a reason. Rates as a percentage of body weight rather than in kilos are what keep a 60kg client and a 110kg client on comparable programs. There is more on choosing between those bands, and on when a slower rate is the faster route, in how to set a client's calorie target.
Split the calories: protein first, fat to a floor, carbs with the rest
Now the calorie number becomes macros, in that order, because that is the order of how much the decision matters.
Protein first. It is the macro that protects muscle in a deficit and the one that keeps clients full. Most coaches work between 1.6 and 2.2g per kilogram of body weight, higher in a deficit and for leaner clients, and use goal weight rather than current weight for a client with a lot of fat to lose so the number stays edible. How to set protein targets for clients covers the ranges by goal and how to handle a client who cannot get near them.
Fat to a floor, not a percentage. Fat has a genuine minimum below which hormones, mood and satiety suffer, so set a floor around 0.6–0.8g per kilogram and leave headroom above it. In a deep deficit this matters more, because a percentage split will quietly push fat under that floor exactly when the client can least afford it.
Carbohydrate takes the remainder. Subtract protein and fat calories from the total, divide by 4, and that is carbs. It is the macro training performance responds to most directly, which is why it gets the leftovers by design rather than by neglect — you want the flexible number to be the one you can move when a client is flat in sessions.
The arithmetic is protein and carbs at 4 calories a gram, fat at 9. If the three grams multiply back to the calorie total, the split is done.
A worked example, end to end
A client, 34, 168cm, 72kg, desk job, trains four times a week, wants to lose fat.
Step 1 — the calculator estimate. Mifflin-St Jeor gives a BMR of 1,439. She picks "moderately active", so the calculator says 2,230. If she had picked "lightly active" it would have said 1,979. Write both down; that gap is the error bar.
Step 2 — measure instead. She logs for two weeks and weighs daily. Average intake is 2,120 calories across 12 of 14 days logged. Her seven-day average weight goes from 72.4kg to 72.0kg.
0.4 kg × 7,700 = 3,080 calories
3,080 ÷ 14 days = 220 calorie daily deficit
2,120 + 220 = 2,340 maintenance
Her real maintenance sits above what either dropdown option predicted. That is not an unusual result, and it is the whole argument for measuring.
Step 3 — set the target. A 15% deficit off 2,340 is 1,989. Round it to 2,000. That is a 340 calorie daily deficit, or about 0.31kg a week — roughly 0.43% of her body weight, inside the standard band.
Step 4 — split it.
| Macro | Working | Grams | Calories |
|---|---|---|---|
| Protein | 1.8g × 72kg | 130g | 520 |
| Fat | 0.8g × 72kg, rounded | 60g | 540 |
| Carbohydrate | 2,000 − 520 − 540, ÷ 4 | 235g | 940 |
| Total | 2,000 |
Step 5 — review the data, not the equation. After three to four weeks, compare her average logged intake against her weight trend again. If she is averaging 2,050 and losing 0.3kg a week, the plan is working and you leave it alone. If she is averaging 2,000 and flat for three weeks, her maintenance was lower than measured or her logging has drifted, and how to adjust client macros covers telling those two apart before you cut anything.
Note what step 5 is not: recalculating the equation with her new weight. The measurement replaces the prediction permanently. Once you have real intake and weight data on a client, you never go back to the calculator for them.
Where this stops being your call
All of this is general nutrition guidance for healthy adults, and setting energy and macro targets sits inside a fitness coach's scope. Some of it does not.
Refer to an accredited practising dietitian or the client's doctor, and coach around their advice rather than over it, when a client has:
- a history of disordered eating, or any sign that tracking is making their relationship with food worse
- a diagnosed metabolic condition such as type 1 or type 2 diabetes or a thyroid disorder
- a gastrointestinal condition — coeliac disease, inflammatory bowel disease, significant IBS — where food choice is medical rather than preferential
- a pregnancy, or the postpartum period while breastfeeding
- medication that affects appetite, absorption or body weight, including GLP-1 agonists, corticosteroids and several psychiatric medications
This is not a disclaimer to paste at the bottom of an intake form. It changes what you do: in those cases the calorie and macro numbers come from, or are cleared by, the practitioner, and your job is training, adherence and reporting back what you see. Saying so plainly at the start reads as competence, not as a limitation.
