Calorie deficit: how to work out your own numbers

Work out your maintenance calories, choose a sensible deficit and split protein, fat and carbs, with two fully worked examples and what to do at a plateau.

The question that actually needs answering is rarely which diet to pick. It is how much to eat, in grams and calories, starting tomorrow morning. What follows is the arithmetic, two worked examples with real numbers, and the order you should do things in — from week one through to the point where the scale stops moving.

This is written for a healthy adult who wants to lose fat and keep muscle. Treat every number as a starting point rather than a verdict; three weeks of scale data will tell you far more than any formula.

Step 1: estimate what you burn

The quick method: multiply your body weight in kilograms by 30 to 33 if you train three or four times a week and are not sat down all day. An 85 kg person lands somewhere between 2,550 and 2,800 kcal. Desk job and no training, use 26 to 28 per kilogram instead.

The longer method has two stages. First, resting expenditure — what your body spends doing nothing:

Men: 10 × kg + 6.25 × height in cm − 5 × age + 5
Women: 10 × kg + 6.25 × height in cm − 5 × age − 161

Then multiply by an activity factor: 1.2 for sedentary and no training, 1.375 for two or three sessions a week, 1.55 for four or five, 1.7 if you also do physical work. Most people with an office job and a regular lifting habit land between 1.35 and 1.5.

Whatever number comes out carries roughly ten per cent of error in either direction. The formula gives you somewhere to start. The scale calibrates it.

Step 2: choose the size of the deficit

Aim to lose 0.5 to 1 per cent of body weight per week. At 85 kg that is 0.4 to 0.85 kg; at 70 kg it is 0.35 to 0.7 kg. The leaner you already are, and the more you care about your lifts, the closer you sit to the bottom of that range.

A kilogram of fat carries roughly seven thousand calories. So 500 kcal a day is 3,500 a week, which is about half a kilo. A 300 kcal deficit gets you around 0.3 kg a week — slower, but noticeably less hunger and less damage to your training.

For most people the working range is 300 to 500 kcal below maintenance. A bigger cut of 700 to 1,000 kcal makes sense only if you are carrying a lot of excess fat, only for a limited stretch, and only alongside high protein and resistance training.

There is a floor. Below roughly 1,500 kcal for men and 1,200 for women it becomes genuinely hard to cover protein, fat and micronutrients from ordinary food. Don't go under that without a doctor or dietitian involved.

Step 3: divide that number across the plate

Protein comes first in a deficit, because it protects muscle and keeps you fuller than anything else per calorie. Set 1.6 to 2.2 g per kilogram of body weight. If you are carrying a lot of fat, base it on your target weight rather than your current one.

Don't push fat below 0.6 to 0.8 g per kilogram — satiety and hormonal function both suffer under that. Whatever calories are left go to carbohydrate, and that is what fuels your sessions. Keep fibre at 25 to 35 g a day, mostly from vegetables, fruit and pulses.

Example one — a man, 85 kg, 180 cm, 35 years old, desk job, four sessions a week. Resting expenditure comes out near 1,805 kcal; times 1.4 gives about 2,530, so call maintenance 2,500. A 400 kcal cut means 2,100 kcal a day: 150 g protein (600 kcal), 70 g fat (630 kcal), 215 g carbohydrate (860 kcal).

Example two — a woman, 70 kg, 165 cm, 30 years old, three sessions a week. Resting expenditure about 1,420 kcal; times 1.4 gives roughly 1,990, so maintenance is 2,000. A 300 kcal cut means 1,700 kcal a day: 125 g protein (500 kcal), 55 g fat (495 kcal), 175 g carbohydrate (705 kcal).

Meal frequency matters far less than the daily total. Three to five meals, each carrying 30 to 40 g of protein, suits most people.

Step 4: checking whether it is working

Weigh yourself every morning, after the loo and before breakfast, and read the weekly average rather than any single day. Swings of 1 to 2 kg come from salt, water, glycogen, bowel contents and cycle phase. None of that is fat.

Alongside that, measure your waist once a week at the same spot and time, and take photos every fortnight in the same light. It is common for the scale to sit still while your waist drops two centimetres.

If the weekly average hasn't moved for three consecutive weeks, take 150 to 200 kcal off or add 1,500 to 2,000 steps a day. One change at a time — do both and you learn nothing.

Step 5: training and movement while cutting

Lifting two to four times a week is what gives your body a reason to hold onto muscle. Don't rewrite your programme the day the diet starts, and don't turn your sets into cardio. Keep the loads and the set count as long as you can, and trim total volume by 10 to 20 per cent only if recovery falls apart. When the weights do start to feel heavier, that is a normal part of dieting with understandable causes.

Cardio is a tool for spending energy, not a punishment for eating. Two to four sessions of 20 to 40 minutes a week is plenty, and the choice between intervals and steady-state depends more on your recovery than on calories burned per minute. The worry that cardio eats muscle is mostly overstated, though there is a point where it becomes real.

The most underrated variable is daily movement. In a deficit the body quietly reduces spontaneous activity — you walk less, fidget less, sit more. Set a target of 8,000 to 10,000 steps and track it as seriously as you track food.

When the scale stops

A genuine plateau is three weeks of an unchanged weekly average while you have been eating consistently. Nine times out of ten the cause is intake creeping up: portions growing, oil going unmeasured, weekends never making it into the log.

The second cause is adaptation. You are lighter, so you burn less, and you move less without noticing. The fix is a small adjustment to food or steps, not another dramatic cut.

If you have been in a deficit for eight to twelve weeks, spend one or two weeks eating at maintenance. Hunger settles, strength comes back, and weight loss resumes afterwards. And once you reach the last few kilos, the rules shift again and patience becomes the main tool.

Common mistakes

Not measuring the things that count most. A tablespoon of oil is 90 kcal, a handful of walnuts 200, a spoon of peanut butter 95. Three of those slips is your entire deficit.
Weekends that don't exist in the log. Five days at −400 and two days at +800 nets zero. Count all seven.
Liquid calories. Juice, fizzy drinks, milky syrupy coffee, beer — they pass through without filling you and can add 400 kcal a day.
Cutting too hard at the start. Jumping from 2,600 to 1,500 kcal gives you a fast fortnight and a near-certain collapse in the third week.
Changing the plan every five days. No intake level can be judged on fewer than two or three weeks of data.
Too little protein. Under 1.2 g/kg in a deficit you lose more muscle than you need to, and you stay hungry doing it.

When to see a doctor

Speak to a doctor before starting if you have diabetes, thyroid, kidney or liver conditions, if you are pregnant or breastfeeding, or if you take medication that is adjusted around food intake.

During the diet, reasons to get checked include periods stopping, fainting, a racing or irregular heartbeat, weight dropping without you trying, persistent exhaustion that doesn't lift after a few days of normal eating, or food starting to occupy most of your thinking.

Common questions

Do I have to count calories forever?

No. But the first six to eight weeks of weighing food teach you what things actually cost, and that knowledge stays with you. After that, most people manage on protein targets, portion habits and the weekly scale average.

Does meal timing matter?

Not for fat loss itself — the daily and weekly total is what moves the needle. Timing matters only through adherence: if evening hunger wrecks your plan, shift more of your calories into the second half of the day.

Why did I lose three kilos in week one and nothing in week two?

Week one is mostly water and glycogen, especially if you dropped carbohydrates. Any honest estimate of your rate of fat loss starts from week three onwards.

What happens once I hit my target?

Calories come back gradually, roughly 100 to 150 kcal a week, until your weight settles. That transition is its own phase and has rules of its own that largely decide whether the result sticks.

The short version

Estimate maintenance: body weight in kg × 30–33, or the formula plus an activity factor of 1.35–1.5.
Subtract 300 to 500 kcal. Target 0.5 to 1 per cent of body weight per week.
Set protein at 1.6–2.2 g/kg, fat no lower than 0.6–0.8 g/kg, carbohydrate takes the rest.
Weigh daily, read the weekly average, measure your waist once a week.
Lift two to four times a week and hold 8,000–10,000 steps a day.
Three weeks with no change: cut 150–200 kcal or add 1,500–2,000 steps. One variable at a time.