BodyCalStep 1: Work out your maintenance calories
A calorie deficit is simply eating less energy than your body uses, and the first thing you need is that 'uses' number — your Total Daily Energy Expenditure (TDEE). TDEE is your basal metabolic rate (the calories you burn at complete rest) multiplied by an activity factor that reflects how much you move.
The most reliable everyday formula is Mifflin–St Jeor. For women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161. For men, replace the −161 with +5. Multiply the result by 1.2 if you're sedentary, up to 1.9 if you train hard most days. That product is your maintenance calories.
Step 2: Pick a deficit you can actually live with
Once you know maintenance, your deficit is just a subtraction. A 500 kcal daily deficit produces roughly 0.45 kg (1 lb) of fat loss per week, because a kilogram of body fat stores about 7,700 kcal. A 1,000 kcal deficit roughly doubles that — but it's only sensible for people with higher maintenance calories.
A good rule of thumb is to keep your deficit between 15% and 25% of your TDEE. That's aggressive enough to see weekly progress, but gentle enough to protect muscle, energy, and your sanity. The fastest deficit on paper is almost never the one that wins over six months.
Step 3: Find your daily calorie target
Subtract your chosen deficit from your maintenance calories and you have your daily target. Worked example: a 30-year-old man, 180 cm, 90 kg, lightly active. His BMR is about 1,880 kcal; multiplied by 1.375 his TDEE is roughly 2,585 kcal. A 500 kcal deficit gives a target of about 2,085 kcal per day, for around 1 lb of loss per week.
Never drop below the safety floors — about 1,200 kcal/day for women and 1,500 kcal/day for men — without medical supervision, because below those levels it's very hard to hit your protein, vitamin, and mineral needs.
Step 4: Track, then adjust after three weeks
Your target is a starting estimate, not a contract. Weigh yourself a few mornings a week and compare weekly averages rather than reacting to daily swings, which are mostly water. After three to four weeks, check your real rate of loss against the prediction.
If you're losing slower than planned, trim 100–150 kcal. If you're losing much faster or feeling drained, add some back. And recalculate the whole thing every 5–7 kg lost, because a lighter body burns fewer calories — the deficit that worked at the start will quietly stall later.
The shortcut
If you'd rather skip the arithmetic, our calorie deficit calculator runs every step above instantly — your TDEE, a safe daily target, and a week-by-week forecast with your predicted goal date. Enter your details, pick a rate, and read off the number.