Short answer: To lose weight, you need to eat fewer calories than your body burns in a day — usually 300–500 calories below your maintenance level. Maintenance is calculated in two steps: first your basal metabolic rate (BMR) using the Mifflin-St Jeor formula, then multiplying that by an activity factor. For a 154-lb (70 kg) person with moderate activity, a 500-calorie daily deficit produces roughly 0.8–1 lb of weight loss per week.

Weight loss comes down to one rule: your body burns energy on breathing, organ function, movement, and digesting food — the sum of all that is your total daily energy expenditure (TDEE). Eat less than that total, and your body starts pulling energy from fat stores.

The catch is that TDEE isn't a fixed number from an online calculator — it's an estimate you need to fine-tune against your own weight over 2–3 weeks. The formulas below give you a starting point, not a guaranteed result down to the calorie.

What a calorie deficit actually is, and why nothing works without it

A calorie deficit is the gap between how much energy your body burns in a day and how much you eat. As long as that gap is negative, your body makes up the difference by burning fat stores (and some muscle, if protein intake and strength training aren't enough to protect it).

No diet works without a deficit — keto, intermittent fasting, cutting sugar, whatever. These approaches only work because they indirectly reduce how many calories you eat, not because of some separate "metabolic effect."

How to calculate your basal metabolic rate (BMR)

Your basal metabolic rate (BMR) is how many calories your body burns at complete rest, just to stay alive — breathing, heart function, body temperature. The most accurate widely-available formula is Mifflin-St Jeor:

• For men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5.
• For women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161.

For example, a 30-year-old woman, 5'5" (165 cm), 154 lb (70 kg): BMR = 10×70 + 6.25×165 − 5×30 − 161 = 700 + 1031 − 150 − 161 = 1,420 calories a day. That's the floor — what her body burns even lying down all day.

Factoring in activity level: the TDEE multiplier

Multiply BMR by an activity factor to get TDEE — your actual daily energy expenditure, including movement and training.

• Sedentary, minimal movement — ×1.2.
• Lightly active, training 1–3 times a week — ×1.375.
• Moderately active, training 3–5 times a week — ×1.55.
• Very active, training 6–7 times a week — ×1.725.
• Extremely active, physical job plus training — ×1.9.

For the same woman at moderate activity: TDEE = 1,420 × 1.55 ≈ 2,201 calories a day. That's her maintenance level — what keeps her current weight stable.

How big a deficit is safe

The standard recommendation is a 300–500 calorie daily deficit below TDEE. That produces roughly 0.5–1 lb of weight loss per week — noticeable, but gentle on muscle mass, hormones, and energy levels.

A deficit larger than 750–1,000 calories a day speeds up muscle loss, raises the risk of binge-restrict cycles, and rarely holds up for more than a few weeks. Daily intake below 1,200 calories for women or 1,500 for men usually isn't recommended without medical supervision — it's hard to get enough protein, vitamins, and minerals at that level.

A full worked example, step by step

For a 30-year-old woman, 5'5" (165 cm), 154 lb (70 kg), moderately active: BMR ≈ 1,420 calories, TDEE ≈ 2,201 calories. A 500-calorie deficit gives a target intake of 2,201 − 500 ≈ 1,701 calories a day.

At that deficit, expected weight loss is roughly 0.8–1 lb a week, or about 3.5–4.5 lb a month. That's a guideline, not a guarantee — actual speed depends on how accurately food is tracked, sleep, stress, and hormones.

Why a calorie deficit sometimes doesn't seem to work

The most common reason is underestimating what's actually eaten — sauces, cooking oil, and mindless snacking easily add 200–400 calories a day without anyone noticing. Weighing portions for at least the first 2–3 weeks sharply improves accuracy.

The second reason is water retention, which can mask real fat loss on the scale for days or weeks, especially early in a diet or after a change in sodium or carb intake. Track a 7-day rolling average weight instead of any single reading.

The third reason is that TDEE drops as body weight drops — a smaller body burns less energy. Recalculate your numbers every 4–6 weeks, or sooner if progress stalls for more than two or three weeks in a row.

Frequently asked questions

Can you lose weight without counting calories?

Yes, by managing portion sizes and food quality intuitively, but progress is less predictable and it's harder to diagnose why the scale has stalled.

How often should you recalculate your calorie target?

Every 4–6 weeks, or right away if weight hasn't moved for 2–3 weeks despite sticking to the deficit.

Do you have to count calories every single day?

No — plenty of people succeed by tracking on weekdays only, or by tracking their largest meals, as long as their weekly total stays within the target range.

Why isn't the scale moving even though I'm in a deficit?

Usually inaccurate food tracking or water retention; less often an overestimated TDEE that needs to be recalculated after your body has adapted.

What's the best deficit size for preserving muscle?

300–500 calories a day combined with strength training and 0.7–1 gram of protein per pound of bodyweight is the standard guideline for losing fat without major muscle loss.

Should you eat under 1,200 calories a day to lose weight faster?

Usually not — intake below 1,200 calories for women or 1,500 for men raises the risk of nutrient deficiencies and binge-restrict cycles, with little long-term speed advantage.

If you're a coach and want to build calorie and deficit calculations into a structured nutrition program for your clients, create a course on FitSpace. See example programs on the authors page.