Most diets don't fail because clients lack willpower. They fail because the maths was wrong from day one — and no amount of motivation fixes wrong maths. After 15 years of building plans for over 800 clients, the pattern is identical every time. Get the numbers right and the diet runs itself. Get them wrong and the client is fighting hunger and frustration for nothing.
Here's the framework I use for every fat-loss client, and the three places most plans go off the rails.
1. The BMR formula matters — and most calculators use the wrong one.
Your basal metabolic rate (BMR) is what you'd burn lying in bed all day. It's the foundation of every calorie target. The Harris-Benedict formula from 1919 is still in half the calculators online — it overestimates BMR by 5–15% for most modern adults. Mifflin-St Jeor (1990) is the validated gold standard and the one I use:
Men: 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
Women: 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
For a 75 kg, 178 cm, 35-year-old man: 750 + 1113 − 175 + 5 = 1,693 kcal BMR. That's your floor. Everything else gets built on top.
2. Activity multipliers — not "I train 4 times a week".
This is where most plans break. People over-rate their daily activity because they're proud of their gym sessions. But a 5×/week lifter who sits at a desk all day is moderately active, not very active. The ACSM scale:
- Sedentary (desk job, little movement): BMR × 1.2
- Lightly active (1–3 days/week light exercise): × 1.375
- Moderately active (3–5 days/week moderate): × 1.55
- Very active (6–7 days/week hard training): × 1.725
- Extra active (2×/day, physical job): × 1.9
Be honest. Most people who think they're "very active" are actually moderate. Pick the lower of two if you're between brackets.
For our 75 kg example training 4 days a week with a desk job, that's 1,693 × 1.55 = 2,624 kcal maintenance.
3. The deficit — aggressive is the enemy of consistent.
A 20% deficit is the upper end of what most people can sustain without crashing. 15% is more realistic for adherence. Past that you start losing muscle, energy and sleep — which kills training quality, which kills the entire programme.
For our example: 2,624 × 0.82 = 2,150 kcal target. Expected loss: about 0.5 kg per week, mostly fat if protein and training are dialled.
The protein number that protects everything.
Calories control weight change. Protein controls what kind of tissue you lose. The 2017 ISSN position paper is clear: during a calorie deficit, lifters need 2.3–3.1 g per kg of bodyweight to preserve lean mass. Most online calculators give you 1.5–2.0 g/kg, which is fine for a maintaining sedentary person and a disaster during an aggressive cut.
For our 75 kg lifter: 165–200 g of protein per day, every day, no matter what else changes. Spread across 4 meals: 20–40 g per sitting. This isn't a suggestion. This is the difference between losing fat and losing muscle.
Safety floors — never cross these.
- Men: never below 1,500 kcal/day without medical supervision
- Women: never below 1,200 kcal/day
- Never below BMR × 1.1 — your body needs more than basal to function
If the maths above gives you a target under those floors, your goal is wrong, not the maths. Push the timeline out and aim for a smaller deficit.
The four-meal rule.
Protein synthesis is best stimulated every 3–4 hours. Four meals is the practical sweet spot — enough to spread protein evenly, few enough to actually fit into a real working day. Six meals can work for hard gainers; three meals leaves protein bunched up. Most clients land at four.
The diet that works isn't the strictest one or the cleverest one. It's the one with the right numbers that you can run for six months without thinking about it.
Putting it together — one worked example.
Female client, 70 kg, 168 cm, 35 years old, lightly active (3 sessions/week), wants to lose 5 kg over 12 weeks.
- BMR: 700 + 1050 − 175 − 161 = 1,414 kcal
- Maintenance: 1,414 × 1.375 = 1,944 kcal
- Target (18% deficit): 1,594 kcal — above the 1,200 floor ✓
- Protein: 70 × 2.5 = 175 g (about 45 g per meal across 4 meals)
- Fat: 70 × 0.7 = 49 g (minimum 35 g)
- Carbs: remainder = ~140 g
Expected: 0.4–0.6 kg loss per week. Hits target in 10–12 weeks. Protein high enough to protect muscle, deficit small enough to sleep through, carbs high enough to train hard. That's the structure.
The boring truth.
The "secret" to fat loss is that there isn't one. The numbers are well-understood. What separates results from frustration is whether the maths is right for you on day one, and whether you have someone to adjust it as your body changes.
Most clients I see arrive having done three or four diets, all of which "worked for a bit" then stalled. The diet didn't stop working — their body adapted, the deficit shrank, and nobody adjusted the numbers. That's the work coaching does. Not motivation. Maths.
Educational only. This article describes the principles I use for healthy adults without medical conditions. If you have diabetes, thyroid issues, or other metabolic conditions, work with your doctor before applying any structured deficit.