Calorie Deficit Calculator

What is a calorie deficit and why does it cause fat loss?

A calorie deficit exists when the energy you consume from food and drink is less than the total energy your body expends in a day. Energy balance is the fundamental driver of body-weight change: eat more than you burn and your body stores the surplus mostly as fat; eat less than you burn and your body draws on stored energy — primarily body fat — to make up the shortfall.

Body fat is essentially stored chemical energy. One pound of adipose tissue holds roughly 3,500 kilocalories (kcal); one kilogram holds approximately 7,700 kcal. When you sustain a consistent calorie deficit over days and weeks, your body gradually oxidises that stored fat to fuel its normal functions — heart rate, organ activity, movement, and thermoregulation — producing the gradual, measurable weight loss that you see on the scale.

Importantly, fat loss is a physiological process driven by energy balance, not by any single food, nutrient timing, or dietary pattern. The specific foods you choose matter for health, satiety, and muscle preservation — but the arithmetic of calories in versus calories out is what ultimately determines whether body fat increases, stays the same, or decreases.

How a calorie deficit works: the key numbers

Your Total Daily Energy Expenditure (TDEE) is the total number of calories your body burns each day. It is the sum of your Basal Metabolic Rate (BMR — calories burned at complete rest to keep you alive) plus the energy used for all physical activity, digestion, and non-exercise movement such as walking, fidgeting, and standing.

The formula is straightforward:

Because roughly 3,500 kcal equals about 1 lb (0.45 kg) of stored body fat, a sustained daily deficit of 500 kcal produces a weekly deficit of 3,500 kcal — enough to lose approximately 1 lb per week. Doubling the daily deficit to 1,000 kcal theoretically doubles the rate to about 2 lbs per week, which is generally considered the upper limit of sustainable fat loss for most people.

Keep in mind that this 3,500 kcal-per-pound figure is a useful approximation rather than a biological constant. In practice, early weight loss often includes water and glycogen, and the body adapts over time by slightly reducing TDEE — meaning actual results may differ from the linear prediction. You can use our TDEE calculator to find a precise maintenance figure before setting your deficit.

Worked example: from TDEE to weekly fat loss

Here is a concrete example that shows each step of the calculation:

In this scenario the person eats 500 kcal below their maintenance level every day. Over a week that adds up to a 3,500 kcal deficit, which corresponds to roughly 1 lb of fat. If they sustain this for ten weeks — adjusting their intake as their TDEE drops slightly with lower body weight — they can expect to lose around 10 lbs. To break down your target intake further into protein, carbohydrate, and fat targets, try our macro calculator.

Safe and sustainable calorie deficits

Disclaimer: the information below is general educational content and is not a substitute for personalised medical advice. If you are considering a very-low-calorie diet or have any underlying health condition, consult a qualified healthcare professional before making significant changes to your eating habits.

A broadly accepted guideline is to target a weight-loss rate of roughly 0.5–1% of bodyweight per week. For a person weighing 185 lbs that translates to about 0.9–1.85 lbs per week. Staying within this band generally preserves lean muscle mass, avoids sharp drops in energy, and reduces the likelihood of nutrient deficiencies.

Most nutrition guidelines recommend against dropping below approximately 1,200 kcal per day for women or 1,500 kcal per day for men without medical supervision. Eating below these thresholds can make it very difficult to consume adequate protein, vitamins, and minerals, and may trigger adaptive thermogenesis — a metabolic slowdown that makes further loss harder. Very-low-calorie diets (typically defined as below 800 kcal/day) should only be undertaken with close clinical oversight.

Two factors significantly protect against muscle loss during a deficit:

• Adequate protein intake — aim for roughly 0.7–1 g of protein per pound of bodyweight (1.6–2.2 g/kg). High protein intakes also promote satiety, making the deficit easier to sustain.
• Resistance training — lifting weights or performing bodyweight exercises sends a signal for the body to retain muscle tissue even in a calorie-reduced state. Without a training stimulus, some of the weight lost will inevitably come from lean mass.

Knowing your current body composition — how much of your weight is fat versus lean mass — can help you set a more precise fat-loss target. Our body fat calculator estimates body-fat percentage using standard circumference measurements.

Common mistakes that stall weight loss

• 1.
  Setting an overly aggressive deficit. Cutting calories too sharply often backfires — hunger surges, energy crashes, training performance drops, and adherence collapses within days to weeks. A moderate 300–500 kcal daily deficit is sustainable for most people; a 700–1,000 kcal deficit is viable short-term for those with significant excess body fat but harder to maintain.
• 2.
  Underestimating calorie intake. Research consistently shows that people underreport food intake by 20–50%, even when they believe they are tracking carefully. Common culprits are cooking oils, sauces, drinks (including alcohol), and restaurant portions. Using a food scale — at least initially — is the single most effective habit for accurate tracking.
• 3.
  Not adjusting intake as weight drops. As you lose weight, your TDEE decreases because a lighter body requires fewer calories to function. A target of 1,700 kcal that produced a 500 kcal deficit at 185 lbs may only create a 350 kcal deficit once you reach 170 lbs. Recalculate your TDEE every 10–15 lbs of loss.
• 4.
  Judging progress by a single day's scale weight. Scale weight fluctuates by 1–4 lbs day to day due to water retention, sodium intake, hormonal cycles, food volume in the gut, and bowel regularity — none of which reflect actual fat change. Track a 7-day or 14-day rolling average to see the real trend.
• 5.
  Overestimating calories burned through exercise. Calorie-burn figures from cardio machines and fitness trackers are notoriously inaccurate, often overstating expenditure by 20–40%. Logging exercise and then "eating back" all those estimated calories can easily erase the intended deficit. Treat exercise as a small bonus rather than a license to eat significantly more.