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The calorie deficit formula is simple: calorie deficit = calories your body burns in a day − calories you eat. To find the first number, work out your basal metabolic rate (BMR) with the Mifflin–St Jeor equation. For women, BMR = 10 × weight in kg + 6.25 × height in cm − 5 × age − 161. For men, end with + 5 instead of − 161. Multiply BMR by an activity factor to get your total daily energy expenditure, or TDEE. Subtract the deficit you choose, and what is left is your daily calorie target.
Most plans aim for a daily gap of 500–750 calories, the range in the 2013 American obesity guideline. Smaller people who sit most of the day belong at the low end. Whatever number the formula gives, it is a first guess; three weeks of morning weighings will tell you how far off it is.
What is the calorie deficit formula?
Most calorie deficit calculators you will find online run some version of three lines. TDEE = BMR × activity factor. Deficit = TDEE − calories eaten. Daily target = TDEE − the deficit you choose. Your phone calculator will do, and so will the back of an envelope.
BMR is the energy your body spends lying still: keeping the heart beating, the brain working, the liver and kidneys busy. Strictly, the equation below predicts resting energy expenditure, which runs a little above true basal rate, but calculators label it BMR and the gap is small. The activity factor scales it up for everything else, from walking to the bus stop to an hour of badminton.
For the first line, most calculators use the Mifflin–St Jeor equation. Mifflin and St Jeor built it in 1990 from measured resting energy in 498 adults aged 19–78, about half of them living with obesity. In 2005 David Frankenfield's systematic review compared it with the other common equations. It came within 10% of the measured value in more people than any other, with the narrowest spread of errors.
How do you calculate BMR with the Mifflin–St Jeor equation?
Take Meera, who is 38, works in HR in Pune, weighs 72 kg and is 160 cm tall. Ten times her weight is 720. Her height times 6.25 is 1,000. Five times her age is 190. So 720 + 1,000 − 190 − 161 gives a BMR of 1,369 calories a day.
Her husband Arjun is 45, weighs 92 kg and stands 175 cm. For him it is 920 + 1,093.75 − 225 + 5, which comes to about 1,794. Meera and Arjun are an illustration, not patients; swap in your own numbers as you read.
One caution for Indian readers. Mifflin–St Jeor was fitted to American adults, and the sources here do not test it on a large Indian group. What they do show is that international equations tend to run high in India. When Sinha, Kurpad and Kuriyan measured young men in Bengaluru, the FAO/WHO/UNU equation put BMR 6.5% above what the machine recorded. Read any calculator's BMR as a starting estimate, not a measurement of you.
Which activity factor should you use?
Calculators multiply BMR by 1.2 if you sit most of the day, 1.375 for light activity, 1.55 for moderate, 1.725 for very active and 1.9 for hard physical work. These steps are a convention rather than a measurement. Choose by your whole day rather than your workouts. An office job with under 5,000 steps is sedentary, a day of 7,000–10,000 steps is light, and a job spent on your feet is moderate.
The Bengaluru study measured the men instead of asking them. Over 14 days, using doubly labelled water, the men's real ratio of total burn to BMR was about 1.35. A questionnaire gave 1.53, the figure Indian guidance then assumed. People thought they moved more than they did, and their measured daily burn came out 15% below the recommended intake. If you sit at a desk most of the day, pick the lower factor, not the one that flatters you.
Meera drives to an office and does about 4,000 steps on a workday. At 1.2 her TDEE is 1,643; at 1.35 it is 1,848. Call it 1,650–1,850. Arjun spends his day visiting shops on foot, so 1.55 fits him, giving about 2,780.
Age is already inside the formula, through the minus five per year. Herman Pontzer's team pooled measured energy use from thousands of adults and children. After allowing for body size, daily burn held fairly steady between about 20 and 60 and fell after that. In your thirties and forties, a change in daily steps moves the answer far more than another birthday.
How big should the deficit be, and what is the percentage formula?
The percentage formula is: deficit % = deficit ÷ TDEE × 100. Run it the other way and daily target = TDEE × (1 − deficit % ÷ 100). It matters because a flat 500 calories is a very different cut for different bodies.
For Arjun, 500 out of 2,780 is an 18% cut. For Meera, 500 out of roughly 1,750 is 29%. A 750-calorie cut would leave her on about 1,000 a day, a 43% cut, which is a very steep drop for a body her size.
The 2013 obesity guideline from the American Heart Association, the American College of Cardiology and The Obesity Society gives the usual range. It tells doctors to prescribe a deficit of 500 or 750 calories a day, or a 30% deficit. It adds that this usually means eating 1,200–1,500 calories a day for women and 1,500–1,800 for men.
So Meera starts with a 20–25% cut, which means about 1,350–1,400 calories, inside the range for women. She keeps a protein food at every meal: a katori of dal, a bowl of curd, some paneer or two eggs. Arjun can take 500–600 off his 2,780 and still eat three full meals. Both targets leave room to tighten a little later if the scale stalls.
Sources: [12]
How much weight will a 500-calorie deficit lose?
The rule most people know says 7,700 calories make one kilo of fat, the metric version of 3,500 calories a pound. On that arithmetic, 500 calories a day is 3,500 a week, about half a kilo a week and roughly 24 kg in a year.
Diana Thomas and colleagues tested the rule against seven tightly supervised experiments, where intake was measured rather than reported. It overestimated weight loss badly. Kevin Hall at the US National Institutes of Health had already shown why. The energy in each lost kilo depends on how much fat a person starts with, and the rule fits only people with a lot of it.
Hall's 2011 model in The Lancet gives a better rule of thumb. It works from a lasting change in what you eat, measured against the intake that held your weight steady. Each 100 kJ a day of that change, about 24 calories, eventually moves weight by about 1 kg. Roughly half arrives in the first year and nearly all of it by the third.
Put Meera's 400-calorie cut through that rule and the model's answer is a final change of roughly 16–17 kg, about half of it in the first year. Treat that as what the arithmetic says for an average adult who never drifts back to old portions, not as a forecast for Meera. In practice portions creep back, and the body adapts, as the next section explains.
Why does the same deficit stop working after a few months?
A lighter body burns less. Run the formula again for Meera at 66 kg and her BMR falls by 60 calories; at a 1.35 factor her TDEE drops by about 80. Her 400-calorie gap is now nearer 320, without a single change to her plate.
There is also a defence the formula does not include. Michael Rosenbaum and Rudolph Leibel describe adaptive thermogenesis. After weight loss, metabolic, hormonal and nervous-system changes push energy use below what the new size predicts and turn hunger up. The fat hormone leptin carries much of that signal, and their review links it to regain after more than 80% of successful diets.
So recalculate every four or five kilos. Before cutting further, check the food record and the step count, because drift there is common. Hall's group tested their dynamic model against two years of measured data: it tracked the average person to within half a kilo, while individual predictions spread widely.
This is the stage at which a lot of people give up. Meera, months in, eating 1,250 and stuck at 66 kg, has no safe room left to cut. What helps then is finding out why the body is holding on. At Metaboliq that starts with a home blood test and a doctor going through the thyroid, sugar and insulin results with her. The plan built from them aims to lose up to 20% of body weight while she keeps her muscle.
How do you keep a calorie deficit from costing muscle?
The formula tells you how much energy to remove, but not where the body takes it from. Weinheimer's review of 52 studies in adults over 50 makes the risk plain. In 81% of diet-only groups, at least 15% of the weight lost was fat-free mass, which includes muscle. When exercise was added, that fell to 39% of groups.
Protein matters as much as training. In Thomas Longland's trial at McMaster University, young men ate 40% below their needs for four weeks while training six days a week. Those on the higher-protein diet gained 1.2 kg of lean mass and lost 4.8 kg of fat. The lower-protein group held lean mass level and lost 3.5 kg of fat.
The volunteers were fit young men training under supervision, which is a long way from Meera's life. For Meera the principle looks like a protein food at every meal and two strength sessions a week, squats and rows at home if a gym is out of reach. Muscle is also the tissue that keeps BMR up, so protecting it is what stops the formula's answer shrinking faster than it has to.
How do you check whether your numbers are right?
Eat at your target for three weeks and weigh yourself at the same time each morning, after the toilet and before tea. Compare weekly averages, not single days. Salt, a late dinner or a period can swing one morning's reading by a kilo.
If the weekly average falls by roughly a quarter to half a kilo, your estimate is close. If nothing moves, your true burn is lower than the formula said. Trim 150 calories or add a 30-minute walk after dinner. A flat fortnight can also be water: salt, constipation or the days before a period hide fat loss for a while. The first week often drops faster than the maths, mostly water and stored carbohydrate. If the pace stays above a kilo a week after that, eat a little more, since steep cuts cost more lean tissue.
Most errors are on the intake side. In Hall's two-year analysis, mistakes in measuring what people ate drove much of the gap between prediction and result, and that was in a research study. At home the usual culprits are cooking oil, chai sugar and the second helping of rice. Measure the oil for a week, by the spoon, before deciding the formula is wrong.
Sources: [9]