Caloric Deficit Calculator

First the tool estimates the calories you burn on a normal day, called maintenance. It starts from your resting burn (BMR) on the Mifflin-St Jeor equation, then multiplies by an activity factor from 1.2 for a desk job up to 1.9 for a physical job plus training.

Then it turns your weekly pace into a daily number. The common rule of thumb says a kilogram of body weight holds about 7,700 kcal, which is 3,500 kcal per pound:

Your target is maintenance minus that deficit (or plus it, if you want to gain). Weeks to goal is the weight to lose divided by the weekly pace. In the other mode you type the calories you plan to eat, and the tool works backward to the pace and the weeks.

Every target is checked against published floors: 1,200 kcal a day for women and 1,500 for men, the bottom of the ranges in the 2013 AHA/ACC/TOS obesity guideline. Below 800 kcal is a very-low-calorie diet, which is only used under medical care.

Emma: 70 kg to 64 kg at half a kilo a week

Emma is 30, 165 cm tall and exercises 3 to 5 days a week. Her maintenance is about 2,201 kcal. Half a kilo a week needs a deficit of 550 kcal a day, so her target is 1,651 kcal, and the plan takes 12 weeks. By the time she reaches 64 kg her maintenance will be about 2,108 kcal, so the last weeks will go slower.

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Marcus: 200 lb to 180 lb at 1 lb a week

Marcus is 40, 5 ft 10 in and lightly active. His maintenance is about 2,507 kcal, so 1 lb a week means about 2,008 kcal a day for 20 weeks. Doubling the pace to 2 lb would cut him to 1,509 kcal, which is still above the 1,500 kcal floor for men, but it sits at the top of the CDC's 1 to 2 lb range.

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Ruth: when the numbers say no

Ruth is 50, 155 cm and 55 kg, with a desk job. Her maintenance is only about 1,329 kcal. Losing 1 kg a week would need a target of 229 kcal, far below the 1,200 kcal floor and even below 800. The tool flags this plainly. The fastest pace that keeps her at 1,200 kcal is 0.12 kg a week, about 43 weeks for 5 kg. More activity, not less food, is the better lever for her.

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The target is a starting point. The 2013 guideline suggests either a fixed intake (1,200 to 1,500 kcal for women, 1,500 to 1,800 for men) or a deficit of 500 to 750 kcal a day. The CDC notes that people who lose about 1 to 2 lb a week are more likely to keep it off.

The weeks figure is the most hopeful one you will see. The 7,700 kcal rule treats your body like a fixed bank account. A 2011 study in The Lancet by Kevin Hall and colleagues showed that real bodies adjust: as you lose weight you burn less. For an average adult with overweight, a lasting change of 24 kcal a day moves weight by about 1 kg in the end, with only half of that in the first year.

Planning from an intake you can live with is often easier. Dana, 45, 168 cm and 85 kg, can stick to 1,800 kcal. With light activity her maintenance is about 2,082 kcal, so that is a 282 kcal deficit: about 0.26 kg a week and 39 weeks to lose 10 kg.

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• Picking your best week as your activity level. One extra step up the scale adds 10 to 15% to maintenance, and the deficit you planned disappears.
• Never updating the plan. Maintenance falls as you lose. Recheck with your new weight every 4 to 6 weeks.
• Eating below the floor to go faster. Under 1,200 kcal (women) or 1,500 kcal (men) it gets hard to meet protein, vitamin and mineral needs without a supervised plan.
• Reading a scale jump as fat. Day-to-day weight moves with water, salt and what is still in your gut. Judge the trend over two or more weeks, not one morning.
• Using it when it does not apply. The equations are for healthy adults. If you are pregnant, breastfeeding, under 18 or have had an eating disorder, plan with a doctor or dietitian.