Calorie Deficit Calculator

What is a calorie deficit?

A calorie deficit means eating fewer calories than your body burns. When intake falls short of total daily energy expenditure (TDEE), the body makes up the gap from stored energy, mostly body fat, and weight goes down over time. The reverse, a calorie surplus, leads to weight gain, so the same calculator works as a weight gain calculator when your goal weight is above your current weight.

The size of the deficit controls the pace. A small deficit of 250 to 500 Calories a day produces slow, steady loss that is easier to keep up and spares more muscle. Large deficits look faster on paper but are harder to sustain and more likely to cause fatigue, hunger and loss of lean tissue.

How to use this calculator

Enter your age, sex, height, current weight and usual activity level, then your goal weight. Choose whether to plan by a weekly rate (for example 1 pound or 0.5 kg per week) or by a target date. In target date mode the calculator works out the weekly rate you would need and the deficit that goes with it.

The results show your maintenance calories, the daily calorie target, the size of the deficit, the weight you plan to lose as a percentage of your current weight and the estimated goal date. The weekly plan table recalculates maintenance at each projected weight, because a lighter body burns fewer calories and the target has to drift down to keep the same deficit.

How the calorie deficit is calculated

First the calculator estimates your basal metabolic rate with the Mifflin-St Jeor equation and multiplies it by your activity factor to get TDEE. It then converts the weekly rate to Calories using the common rule that a pound of body fat stores about 3,500 Calories, divides by seven for a daily deficit and subtracts that from TDEE.

Worked example with the default inputs: a 25-year-old man, 5 feet 10 inches and 165 pounds, exercising four to five times a week, has a BMR of about 1,740 Calories and a TDEE of about 2,549 Calories. Losing 1 pound a week needs a deficit of 3,500 ÷ 7 = 500 Calories a day, so his target is about 2,049 Calories. Reaching 150 pounds means losing 15 pounds, or 9.1% of his body weight, which takes 15 weeks. The weekly rate is 0.61% of his body weight, inside the usual 1% guideline.

The 3,500 rule is a simplification. Real weight loss slows over time as metabolism adapts and water and glycogen shift, which is why tools such as the NIH Body Weight Planner use dynamic models. Treat the goal date as an optimistic estimate.

Daily deficit = weekly loss × 3,500 ÷ 7; Target = TDEE − daily deficit

TDEE

Mifflin-St Jeor BMR (10W + 6.25H − 5A + 5 for men, − 161 for women) times the activity multiplier

3,500

approximate Calories stored in one pound of body fat (about 7,700 per kilogram)

Weekly loss

your chosen rate, or the weight to lose divided by the weeks until the target date

Safe rates and minimum calories

Health agencies generally describe a loss of 1 to 2 pounds (about 0.5 to 1 kg) a week as gradual and sustainable. A useful cross-check is percentage: losing more than about 1% of body weight per week is aggressive for most people. This weight loss percentage view matters more for smaller people, for whom 2 pounds a week can already exceed 1%.

The calculator also warns when the daily target falls below 1,200 Calories for women or 1,500 for men. Diets below these levels make it hard to meet protein, vitamin and mineral needs and are normally only used under medical supervision.

• Keep protein intake high and include strength training to protect muscle.
• Weigh yourself under the same conditions and judge the trend over two to four weeks, not single days.
• If your weight stalls for several weeks, recalculate with your new weight or adjust activity rather than cutting calories sharply.

Planning a weight gain

When the goal weight is higher than your current weight, the calculator adds a daily surplus instead of subtracting a deficit. For people trying to build muscle, a slower gain of around 0.25 to 0.5 pounds a week combined with resistance training tends to add more muscle relative to fat than a large surplus.

Frequently asked questions

Sources

• NIH NIDDK – Body Weight Planner
• NIH NHLBI – Aim for a Healthy Weight
• CDC – Healthy Weight and Growth
• Dietary Guidelines for Americans