How to calculate a calorie deficit: the formula and the reality check

Calorie deficit = maintenance calories (TDEE) − calories you eat. If your body burns about 2,500 kcal a day and you eat 2,000, your deficit is 500 kcal a day, or 3,500 kcal a week. That is the whole formula. The work is in getting the two numbers right.

It takes two halves: estimate maintenance and pick a pace, then, after a few weeks of logging food and weighing in, replace the estimate with your real numbers. Most guides stop after the first half.

HeyCoach handles both halves in WhatsApp: it works out the estimate, then keeps your meals and weigh-ins in one chat so the numbers for the reality check are already there, and you can try it free for five days.

Step 1: Estimate your BMR

BMR, or basal metabolic rate, is the energy your body uses at rest. The Mifflin–St Jeor equation, built from measurements of 498 adults, is the standard starting point:

• Male: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
• Female: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161

Working in pounds and inches? Convert first: kg = lb ÷ 2.205, cm = inches × 2.54.

Step 2: Turn BMR into maintenance (TDEE)

TDEE, your total daily energy expenditure, is BMR times an activity factor:

The factor is the biggest source of error in the calculation, which is why TDEE calculators disagree. Torn between two? Pick the lower one.

Count exercise once. If your factor already includes your usual workouts, do not add their calories on top. Either choose 1.2 and add each session as it happens, or choose the factor that includes them and leave it there.

Step 3: Choose the size of your deficit

By pace. Use the conventional energy value of body weight, about 7,700 kcal per kilogram or 3,500 kcal per pound:

• Daily deficit = kg per week × 7,700 ÷ 7
• Daily deficit = lb per week × 3,500 ÷ 7

As a share of TDEE. About 10–25% of maintenance is a common planning range: the low end is gentle, the top end demanding. The 2013 US obesity guideline from the AHA, ACC and The Obesity Society lists a 500 or 750 kcal/day deficit among its standard options.

Read the last column. The CDC notes that people who lose about 1–2 lb a week are more likely to keep it off, but 2 lb a week is a 1,000-kcal daily gap, 40% of a typical maintenance. That only fits a large or very active body.

Step 4: Check the safe floors

Target intake = TDEE − daily deficit, but not below 1,200 kcal a day for women or 1,500 for men without medical supervision. Those are the bottom ends of the guideline's intake ranges: 1,200–1,500 kcal for women and 1,500–1,800 for men.

A floor is a guardrail, never a goal: if your math lands below it, eat at the floor and accept a slower pace. Teenagers, people who are pregnant or breastfeeding, anyone with a history of disordered eating and anyone managing a medical condition should get a plan from a clinician or registered dietitian, not a formula.

The calorie calculator runs steps 1–4 for you; how many calories to eat to lose weight covers choosing the target itself.

A worked example

Take a 42-year-old man, 180 cm (5 ft 11 in) and 95 kg (209 lb), with a desk job and three gym sessions a week.

BMR = 10 × 95 + 6.25 × 180 − 5 × 42 + 5 = 1,870 kcal

Mostly sitting plus some planned training puts him at 1.45:

TDEE = 1,870 × 1.45 = 2,711.5, call it 2,710 kcal/day

He wants to lose 0.5 kg (about 1.1 lb) a week:

Deficit = 0.5 × 7,700 ÷ 7 = 550 kcal/day

Target = 2,710 − 550 = 2,160 kcal/day

That is about 20% of his maintenance, well clear of the 1,500-kcal floor. His gym sessions are already inside the 1.45, so he does not add them again.

So far, this is a prediction. Step 5 tests it.

Step 5: Determine your actual deficit from real data

This is where calorie counting and the deficit meet. A formula guesses your maintenance; your food log and your scale measure it. After 2–4 weeks of logging everything and weighing under similar conditions:

• Actual daily deficit ≈ trend weight change (kg) × 7,700 ÷ days
• Actual maintenance ≈ average logged intake + actual daily deficit

In pounds, use 3,500 instead of 7,700.

"Trend weight" means a 7-day average, not a single morning: weight swings day to day with water, salt, carbohydrate and digestion. Compare one week's average with a later week's and count the days between them. If you can, start from the second week, because the first days of a new diet often shed extra water.

Back to our example. Four weeks in, his log averages 2,150 kcal a day, almost exactly on target. His 7-day average weight has gone from 95.0 kg to 93.4 kg: down 1.6 kg in 28 days.

Actual deficit = 1.6 × 7,700 ÷ 28 = 440 kcal/day

Actual maintenance = 2,150 + 440 = 2,590 kcal/day

In pounds: 3.5 lb × 3,500 ÷ 28 ≈ 440 kcal/day. Same answer.

So his real gap is 440, not 550, and he is losing about 0.4 kg a week. Either his maintenance is about 120 kcal lower than the formula said, or about 120 kcal a day is not reaching the log: a pour of oil, a latte. The data cannot tell those apart, and it does not need to. 2,590 is his working maintenance either way.

All three next moves are reasonable: keep going at 0.4 kg a week, tighten the log on oils and drinks, or eat about 2,040 kcal to restore a 550 gap. Reacting to a single weigh-in is not one of them. If a well-logged month shows a flat trend, work through not losing weight in a calorie deficit.

The TDEE calculator runs this check from your own numbers, and these calorie deficit apps can collect them. The same back-calculation works for a calorie surplus.

Daily deficit or weekly deficit?

Your body responds to the average, not to Tuesday. Our example's weekly deficit is 550 × 7 = 3,850 kcal, so he could eat 2,060 kcal Monday to Friday and 2,410 on the weekend: 15,120 kcal for the week, the same 2,160 average and the same deficit.

It cuts both ways. Five weekdays at 550 under bank 2,750 kcal; two weekend days at 1,400 over maintenance spend all of it. Log the weekend too.

Why the deficit shrinks over time

The 7,700 kcal/kg (3,500 kcal/lb) conversion works for short-window checks like Step 5. As a long-range forecast it overstates loss: a lighter body burns less, so the same intake produces a smaller deficit month after month. Repeat the reality check every month or so.

The full story is in the 3,500-calorie rule guide; the calorie deficit calculator shows flat and slowing timelines side by side.

Does exercise count toward your deficit?

Yes. Exercise is part of energy out, so an extra 30-minute walk each day widens the gap as surely as eating less; calories burned walking turns steps into numbers.

Two cautions. First, the double count from Step 2: if your activity factor already includes your workouts, logging them again and eating the calories back erases the deficit you planned. Second, burn numbers are estimates. A systematic review of 158 studies found consumer wearables measured steps well in the lab, but no brand measured energy expenditure accurately. Eat back part of a watch's "active calories" at most.

HeyCoach keeps that choice visible: log a workout by message or screenshot and the day shows both your base target and the burn-adjusted budget, so you decide which one to eat to.

Common mistakes

• Picking "very active" because the workouts feel hard. Three one-hour sessions are 3 of the week's 168 hours. Start low and let your data raise it.
• Forgetting calories that do not look like food. Cooking oil (about 120 kcal a tablespoon), dressings, milk in coffee, juice and alcohol. In a small 1992 study of people who reported dieting without success, participants underreported what they ate by an average of 47% and overreported their activity by 51%. That is a measurement problem, not a character flaw; a calorie chart and a few logging habits close most of the gap.
• Judging from one weigh-in. Normal water swings can hide a week of fat loss, or fake one. Compare weekly averages.
• A deficit too big to survive a week. A 1,000-kcal gap that collapses by Saturday loses to a 400-kcal gap you keep.

FAQ

Is a 500-calorie deficit enough to lose weight?

For most adults, yes. On the 3,500-kcal-per-pound conversion it works out to about 1 lb (0.45 kg) a week at first, and it is one of the standard options in the US obesity guideline. Whether you truly have 500 kcal depends on your real maintenance, which the four-week check measures.

How do I know if I'm in a calorie deficit?

Your 7-day average weight trends down over two to four weeks. If you log food as well, the size of that drop gives the size of the deficit: kilograms lost × 7,700 ÷ days. One low or high morning tells you nothing.

Can I calculate a calorie deficit without counting calories?

You can confirm some deficit exists from a falling weight trend. But without an intake number you cannot calculate its size or your real maintenance. Even a rough two-week log fills that gap.

What is the largest safe calorie deficit?

There is no single number. A practical ceiling for most people is about 25% of maintenance, with intake at or above 1,200 kcal (women) or 1,500 kcal (men). The US guideline reserves diets under 800 kcal a day for medically supervised settings.

Sources

• Mifflin et al., a new predictive equation for resting energy expenditure (1990)
• Jensen et al., 2013 AHA/ACC/TOS guideline for the management of overweight and obesity in adults
• Hall et al., quantification of the effect of energy imbalance on bodyweight (2011)
• Lichtman et al., self-reported versus actual calorie intake and exercise (1992)
• Fuller et al., systematic review of wearables for steps, energy expenditure and heart rate (2020)
• CDC, steps for losing weight

The easy way to do this

The formula takes five minutes; the four-week reality check is the part HeyCoach makes easy. It sets your target in about 30 seconds with the floors built in, then logs each meal from a photo, a sentence or a voice note in WhatsApp, with nothing to download. Text your weight into the same chat and ask "how was my month?" to see your average intake next to your weight change, the two numbers Step 5 needs. Message HeyCoach and try it free: five days of Pro, no card needed.