What is a calorie deficit?
A calorie deficit occurs when you consume fewer calories than your body burns in a day. Your body requires energy for everything it does — breathing, moving, digesting food, maintaining body temperature, and repairing cells. When calorie intake falls below this requirement, your body draws on its stored energy reserves — primarily body fat — to make up the shortfall. Over time, this leads to measurable fat loss and weight reduction.
One pound of stored body fat contains approximately 3,500 calories of energy (about 7,700 calories per kilogram). A consistent daily deficit of 500 calories therefore produces roughly one pound of fat loss per week. This relationship is not perfectly linear in practice — water retention, metabolic adaptation, and variations in food composition all cause day-to-day fluctuations — but over weeks and months, the math holds up reliably.
The key word is "consistent." A 500-calorie deficit for one day produces negligible fat loss; a 500-calorie deficit maintained over six months produces approximately 26 pounds of fat loss. Sustainability — choosing a deficit size and dietary approach you can actually maintain — is more important than maximising your deficit in the short term.
How your TDEE is calculated
Your Total Daily Energy Expenditure (TDEE) is the number of calories your body burns in an average day. It is made up of four components. Basal Metabolic Rate (BMR) — the calories burned at complete rest, accounting for roughly 60–70% of TDEE — is calculated using the Mifflin–St Jeor equation, which uses your weight, height, age, and sex. The thermic effect of food (TEF) — energy used to digest and process food — adds roughly 10%. Non-exercise activity thermogenesis (NEAT) covers all movement that is not formal exercise. Exercise activity adds the rest.
Because direct measurement of TDEE requires laboratory equipment, this calculator uses an activity multiplier to estimate the contribution of activity to your total energy expenditure. These multipliers range from 1.2 (sedentary — desk job, little exercise) to 1.9 (extremely active — very hard daily exercise or a physically demanding job). Choosing the right activity level is one of the most important inputs in the calculation.
A very common mistake is overestimating activity level. Someone who works at a desk and exercises three times per week is likely moderately active at best — not very or extremely active. When in doubt, choose a lower activity level and increase it only if your real-world results suggest your deficit is larger than expected.
Choosing the right deficit size for your goals
The right deficit size depends on how much weight you have to lose, your lifestyle, and your tolerance for restriction. A deficit of 250 calories per day (about half a pound per week) is ideal for people who are close to their goal weight, who have struggled with larger deficits, or who want to minimise hunger and preserve as much muscle as possible. The pace is slow but the approach is very sustainable.
A 500-calorie deficit (about one pound per week) is the most commonly recommended target for general weight loss. It strikes a reasonable balance between speed of results and sustainability for most people. A 750-calorie deficit (about 1.5 pounds per week) produces faster results but requires more discipline and increases the risk of muscle loss if protein intake is not kept high.
Most clinical guidelines recommend a maximum deficit of 1,000 calories per day for otherwise healthy adults, equating to roughly two pounds per week. Larger deficits increase the proportion of weight lost as muscle rather than fat, cause more severe hunger and fatigue, and are associated with stronger rebound weight gain after the diet ends. The minimum recommended intake is 1,200 calories per day for women and 1,500 for men.
Frequently asked questions
How many calories should I eat to lose weight?
This depends on your TDEE. Most people lose weight eating 1,400–2,000 calories per day, but the right number is specific to you. Enter your details in the calculator above to get a personalised target. A deficit of 500 calories below your TDEE is a good starting point for most people.
Can I create a calorie deficit through exercise alone?
Technically yes, but it is difficult in practice. A one-hour run burns roughly 400–600 calories — equivalent to a moderate meal. Research consistently shows that creating a deficit primarily through diet is more efficient and sustainable than exercise alone. The most effective approach combines moderate dietary reduction with regular physical activity.
Why am I not losing weight in a calorie deficit?
The most common reasons are: (1) the deficit is smaller than you think — people typically underestimate food intake by 20–30%; (2) your TDEE is lower than estimated due to a sedentary lifestyle or metabolic adaptation; (3) water retention is masking fat loss — this often resolves after 2–3 weeks. If you have tracked accurately for 2–4 weeks with no weight change, reduce your calorie target by 100–150 and reassess.
Does it matter what I eat, or just how many calories?
For weight loss, total calories are the primary determinant. However, food quality significantly affects how easy it is to maintain a deficit. High-protein foods and fibre-rich vegetables increase satiety per calorie, making it easier to feel full on fewer calories. Ultra-processed foods tend to be highly palatable and low in satiety, making it easier to overconsume. You can lose weight on any type of diet as long as the calorie deficit is maintained, but some approaches are much easier to sustain than others.
How often should I recalculate my calorie target?
Recalculate every 4–6 weeks or after losing 5–10 pounds. As you lose weight, your TDEE decreases because a lighter body burns fewer calories. Your original calorie target therefore produces a progressively smaller deficit over time. Updating your target periodically ensures you maintain the intended rate of loss throughout your journey.