How to calculate calorie deficit: a five-step method with a worked example

What you are actually calculating

A calorie deficit is the gap between the energy you take in from food and the energy you spend. Your total daily energy expenditure, or TDEE, is the spending side: resting metabolism plus everything you do on top of it. When intake sits below TDEE for long enough, the body covers the shortfall from stored tissue and weight comes down.

That makes the deficit itself a simple subtraction. The difficulty is that neither side of the subtraction can be measured directly at home. Intake is estimated from labels and food logs, and TDEE is estimated from an equation. So the number you calculate on day one is a starting estimate, not a fact about your body. The method on this page treats it that way: estimate, eat to the estimate, watch what happens, then correct.

It also helps to separate two questions that often get blended together. 'What is my calorie deficit?' asks for a size, usually in kcal per day. 'What should I eat?' asks for a daily intake target. The deficit is the difference between maintenance and the target; the target is the number you actually log. Both come out of the same few steps, and the worked example below produces both.

How to calculate calorie deficit in five steps

Do these in order. Each step depends on the one before it.

1. Estimate your maintenance (TDEE). Start with resting metabolism from the Mifflin-St Jeor equation, using weight in kilograms, height in centimetres and age in years. For men: 10 × weight + 6.25 × height − 5 × age + 5. For women: 10 × weight + 6.25 × height − 5 × age − 161. The equation was built from 498 healthy adults aged 19 to 78, and a 2005 systematic review by Frankenfield and colleagues found it came within 10% of measured resting metabolic rate in more people than the other three equations it compared. Then multiply by an activity multiplier. The calculator on this site uses five: 1.2 (Sedentary), 1.375 (Lightly active), 1.55 (Moderately active), 1.725 (Very active) and 1.9 (Extra active). Those five values are the site's convention rather than a measured property of you, and choosing the wrong one is the most common source of error. If you know your body fat percentage, the calculator can use Katch-McArdle instead: 370 + 21.6 × lean body mass in kg.
2. Choose a deficit size. The calculator offers three presets: a light cut at 10% below TDEE, a moderate cut at 20% and an aggressive cut at 25%. These percentages are site defaults designed to scale with your maintenance, not thresholds taken from a study. A percentage is useful because the same 500 kcal means something very different to a person who maintains on 1,700 kcal than to one who maintains on 3,200 kcal. A second way to size the deficit, by target rate of loss, is covered in its own section further down.
3. Subtract. Multiply TDEE by the percentage to get the deficit in kcal per day, then subtract it from TDEE to get your daily intake target. Round to whole kcal. Precision beyond that is false precision, because every input is itself an estimate.
4. Set protein before anything else. A 2018 meta-analysis by Morton and colleagues found that the benefit of extra protein for resistance-training gains levelled off at around 1.6 g per kg of body weight per day. The International Society of Sports Nutrition position stand on protein (2017) gives 1.4 to 2.0 g/kg/day for most exercising people and notes that 2.3 to 3.1 g/kg/day may help trained people in a calorie deficit hold on to lean mass. Fix the protein grams first, then fill the remaining calories with carbohydrate and fat in whatever split you can stick to.
5. Track the trend for two to four weeks, then recalibrate. Weigh under the same conditions on most mornings and compare weekly averages rather than single days. If the average is falling at a pace you planned for, carry on. If it is flat, your real maintenance is lower than the estimate or your intake is higher than logged; tighten tracking first, then lower the target. If it is falling much faster than planned, raise intake. Re-run step one after a meaningful change in weight, because a lighter body burns less.

Steps one to three are arithmetic. Steps four and five are where most of the result is decided.

Worked example: one hypothetical person, start to finish

The person below is invented for illustration. Every figure is computed from the formulas in the steps above and rounded to whole kcal at the end of each step.

Her answer to 'what is my calorie deficit?' is 418 kcal per day. Her answer to 'what should I eat?' is 1,671 kcal per day with about 128 g of protein. Neither number is final until two to four weeks of weigh-ins confirm it.

The same maintenance under each of the site's three presets

Same hypothetical woman, same maintenance of 2,089 kcal, three different deficit sizes. This is the choice step two asks you to make.

2,089 × 0.10 = 208.9 and 2,089 × 0.25 = 522.25, rounded to whole kcal. The last column describes the site's own presets; it is editorial guidance, not a study finding.

How big should my calorie deficit be?

There is no single correct size, which is why the calculator offers three. The percentage approach has one clear advantage: it scales. A 20% cut takes more calories away from a large, active person than from a small, sedentary one, which keeps the cut proportionate to what each body is actually spending. A fixed 500 kcal, by contrast, is a gentle trim for one person and close to a crash diet for another.

A second approach starts from the rate of loss you want. The best-known version comes from a 2014 review by Helms and colleagues on contest preparation for natural bodybuilders, which recommended losing roughly 0.5 to 1% of bodyweight per week. That guidance was written for lean physique athletes trying to keep muscle while getting very lean, and the same review flagged risks in that group, including disordered eating. It was not written for the general population, so on this site it serves only as a reference band. If your weekly average keeps falling faster than 1% of bodyweight per week after the first couple of weeks, your deficit is larger than even that aggressive group was advised to run.

Notice what this page does not do: it does not convert a target rate into a daily kcal figure using a fixed number of calories per kilogram. That conversion is the 3,500-calorie rule in disguise, and it overstates how quickly weight comes off (see the warning below). The practical way to use a target rate is to pick a percentage preset, eat to it, and let the measured rate tell you whether to move up or down a step.

Body composition matters as well. In the modelling work by Hall and colleagues published in The Lancet in 2011, people with more body fat lost more weight for the same reduction in intake than leaner people did. In practice, a leaner person usually sees slower progress from the same percentage cut, which is one reason the light preset tends to suit people who are already fairly lean, and why comparing your rate with someone else's tells you little.

Weeks two to four: reading the scale and recalibrating

The first week or two of any cut is noisy. Daily weight moves with water, salt, digestion and meal timing, so a single morning says very little. Weigh at the same time under the same conditions, average each week, and compare the averages. Two to four weeks is the minimum before drawing a conclusion, and four is more reliable than two.

Once the averages are in, there are three possible readings. Falling at a pace you can live with means the estimate was close, so leave it alone. Flat means the real gap between intake and expenditure is smaller than planned. Before cutting food, audit the log: cooking oils, drinks, sauces and weekend meals are the usual missing calories, and an activity multiplier set one level too high adds a phantom allowance every single day. Only once the log is honest should the target come down, and by a modest step rather than a large one. Falling faster than about 1% of bodyweight per week past the first fortnight means the deficit is bigger than it needs to be, and raising intake is reasonable.

Recalculate as your weight changes. Return to the hypothetical woman from the example. If her weekly average reaches 78 kg, her Mifflin-St Jeor estimate becomes 10 × 78 + 6.25 × 168 − 5 × 34 − 161 = 1,499 kcal. Her maintenance at 1.375 becomes 1,499 × 1.375 = 2,061 kcal, and a 20% cut now lands at 2,061 − 412 = 1,649 kcal instead of 1,671. The 22 kcal difference is small at first, but it widens with every kilogram lost, which is why the same intake eventually stops producing results.

The formula also misses part of the slowdown. A 2013 review by Müller and Bosy-Westphal estimated adaptive thermogenesis, the fall in expenditure beyond what the lost tissue explains, at about 120 kcal per day on average, with large differences between individuals, and noted that it takes more than two weeks of underfeeding to develop. No equation can predict your share of that, which is the main reason the scale outranks the formula. Once you have a few weeks of honest intake and weight data, the site's adaptive TDEE guide shows how to work out maintenance from your own numbers rather than from an equation.

Seven mistakes that throw the math off

• Rating activity by the gym schedule instead of the whole day. The multiplier covers every waking hour, not just training. A desk job with two or three gym sessions is not automatically Moderately active. When torn between two levels, start with the lower one and let the scale promote you.
• Using goal weight in the equation. Mifflin-St Jeor estimates what your current body burns. Entering a goal weight underestimates maintenance and quietly makes the deficit larger than you intended.
• Adding workout calories on top of the multiplier. The multiplier already includes your usual exercise. Eating back the calories a watch reports for each session counts the same activity twice and can erase the deficit.
• Treating the deficit as fixed. Maintenance falls as weight falls, so a deficit that was 418 kcal on day one is smaller several kilograms later unless the target is recalculated.
• Judging the plan after a few days. Water and glycogen shifts can hide or exaggerate change over short windows. Two to four weeks of weekly averages is the shortest fair test.
• Forecasting with 3,500 kcal per pound. Because the rule overestimates loss, expectations built on it end in apparent failure even when the plan is working as well as it can.
• Leaving protein until last. Calories decide how much weight changes; protein intake and resistance training influence how much of that change is lean tissue. When protein is the final item in the plan, it is the first thing squeezed out.