Calorie deficit calculator

How the calorie deficit calculator works

To lose body fat, you need to take in fewer calories than your body uses over time. That gap is your calorie deficit. This calculator estimates how many calories you use in a day, then subtracts a set amount for three different paces of weight loss.

It works in three steps.

1. BMR (basal metabolic rate). This is roughly what your body burns at complete rest, just to keep you alive. The calculator uses the Mifflin-St Jeor equation: 10 times your weight in kilograms, plus 6.25 times your height in centimeters, minus 5 times your age in years. Then it adds 5 for men or subtracts 161 for women.
2. Maintenance calories (TDEE). Your total daily energy expenditure is your BMR multiplied by an activity factor: 1.2 for sedentary, 1.375 for lightly active, 1.55 for moderately active, 1.725 for very active and 1.9 for extra active. This is about how much you can eat to keep your weight steady.
3. Daily targets. A common rule of thumb says a pound of body fat holds about 3,500 calories. So a daily deficit of 250 calories works out to about 0.5 lb a week, 500 calories to about 1 lb a week, and 750 calories to about 1.5 lb a week.

There is one safety rule on top of that. The calculator never suggests eating fewer than 1,200 calories a day for women or 1,500 for men. Below those levels it becomes hard to get enough protein, vitamins and minerals from food. When a plan would fall below the floor, it is held at the floor and the result says so, along with the slower loss you can expect.

How to read your result

The large number is your daily calorie target for the steady plan, about 1 lb a week. Underneath, you will see your BMR, your maintenance calories, and the minimum intake used for your sex. The table compares all three plans side by side, with the daily deficit and the estimated weekly loss for each.

The mild plan is the easiest to stick with and leaves the most room for food you enjoy. The faster plan can work for people with more weight to lose, but it is harder to keep up and can leave you hungry and tired. Many people do best starting with the steady plan and adjusting from there.

All of these numbers are estimates. The equation predicts the average person well, but your own needs can be a few hundred calories higher or lower. The best check is real life: follow a target for 2 to 4 weeks, weigh yourself under the same conditions a few times a week, and look at the trend rather than single days.

A worked example

The calculator starts with a 35 year old woman who is 5 ft 6 in tall, weighs 165 lb and is moderately active.

1. Convert units: 165 lb is about 74.8 kg, and 5 ft 6 in is 167.6 cm.
2. BMR: 10 x 74.8 = 748, plus 6.25 x 167.6 = 1,048, minus 5 x 35 = 175, minus 161. That gives about 1,460 calories.
3. Maintenance: 1,460 x 1.55 is about 2,263 calories a day.
4. Targets: mild is 2,263 minus 250, about 2,013 calories. Steady is 2,263 minus 500, about 1,763 calories. Faster is 2,263 minus 750, about 1,513 calories.

All three targets are above the 1,200 calorie floor for women, so none of them is adjusted. If she were sedentary instead, her maintenance would be about 1,752 calories, and the faster plan would be held at 1,200, a deficit of about 552 calories, or a bit over 1 lb a week.

Mistakes to avoid

• Overstating your activity. Pick the level that matches a normal week, not your best week. Choosing a level too high is one of the most common reasons weight loss stalls.
• Eating back all your exercise calories. The activity factor already counts your workouts. Adding fitness tracker calories on top can wipe out your deficit.
• Never updating the numbers. As you lose weight, your BMR and maintenance calories go down. Recalculate every 10 lb or so.
• Going too low, too fast. Very large deficits are hard to sustain and can cost you muscle along with fat. Enough protein and some strength training help you keep muscle.
• Judging by one weigh-in. Water, salt and digestion can move the scale by a few pounds from day to day. Watch the weekly average.

Common questions

How accurate is the Mifflin-St Jeor equation?

It is one of the most widely used and tested equations for estimating resting calorie needs in adults, and it is a good starting point for most people. It is still an estimate. It does not know your body composition, so very muscular people and people with certain health conditions may get a less accurate figure.

Is losing 1 lb a week realistic?

For many adults, a pace of about 0.5 to 1 lb a week is realistic and easier to keep up. The 3,500 calorie rule is a rough guide, and weight loss usually slows over time as your body gets smaller and uses fewer calories, so expect the pace to change.

Why won't the calculator go below 1,200 or 1,500 calories?

Those are widely used minimums for eating without medical supervision. Below them it is hard to meet your nutrient needs. Very low calorie diets do exist, but they should only be followed with a doctor involved.

Who should talk to a doctor first?

Talk to a doctor or registered dietitian before cutting calories if you are pregnant or breastfeeding, under 18, have diabetes, heart or kidney disease, take medication that affects appetite or blood sugar, or have a history of disordered eating. They can set a target that fits your health, not just the formula.