What is a calorie deficit?

It is eating fewer calories than your body uses in a day. The body makes up the difference from stored energy, mainly body fat, so a steady deficit leads to weight loss over time.

How big should my calorie deficit be?

A deficit of about 250 to 1,000 calories a day matches a loss of about half a pound to two pounds a week. Many health agencies describe 1 to 2 pounds a week as a steady pace. Smaller deficits are easier to keep and carry less risk of losing muscle.

Why does weight loss slow down?

As you lose weight your body needs fewer calories, so the same deficit shrinks over time. You may also move less without noticing. Recalculate your maintenance calories every 10 to 15 pounds and adjust your intake or activity.

Is the 3,500 calories per pound rule accurate?

It is a useful average, not an exact rule. Actual results depend on your starting weight, how much muscle you lose, hormones, sleep and how well you follow the plan. Use it for planning and check it against your own scale.

What is the lowest number of calories I should eat?

A commonly cited floor without medical supervision is about 1,200 calories a day for women and 1,500 for men. If your target is lower, choose a slower pace, add activity or talk to a doctor or registered dietitian.

Should I use this if I have a health condition?

Ask a doctor first if you have diabetes, heart disease, an eating disorder, are pregnant or breastfeeding, take medication or are under 18. A calculator cannot account for these situations.

💪 Health & Fitness

Calorie Deficit Calculator · How Long to Reach Your Goal Weight

Losing weight comes down to eating fewer calories than your body uses, and the size of that gap decides how fast it happens. This calculator turns your goal into numbers: the daily calorie deficit you need, the calories to eat each day, and how many weeks it takes to reach your goal weight at the pace you choose.

It starts by estimating your maintenance calories from your sex, age, height, weight and activity with the Mifflin-St Jeor equation. It then subtracts the deficit that matches your pace, using the common rule of thumb that about 3,500 calories equal a pound, so 500 calories a day is about a pound a week. You also see your weight after 4 and 12 weeks, the maintenance calories at your goal weight and the BMI of that goal.

The tool is built to warn you. It flags a calorie target below the usual minimum of 1,200 for women or 1,500 for men, a pace above about 1% of your body weight a week, and a goal weight that would be underweight for your height. A steady loss of about 1 to 2 pounds a week is the pace that health agencies describe as most likely to last. This calculator is educational and is not medical advice.

Detailed Breakdown

10 metrics

PRIMARY RESULT

20 weeks (about 4.6 months)

Time to Reach Your Goal

Pounds to Lose

20 lb

Calories to Eat Each Day

1,658 calories a day

Daily Calorie Deficit

500 calories

Maintenance Calories Now

2,158 calories a day

Maintenance Calories at Your Goal Weight

2,017 calories a day

Weight After 4 Weeks

146 lb

Weight After 12 Weeks

138 lb

Your Pace as a Share of Body Weight

0.67% a week

BMI at Your Goal Weight

21.0

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When to Use This Calculator

Everyday financial situations where this tool gives you fast, accurate clarity:

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Set a Goal Date

See how many weeks a goal weight takes at different paces.

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Find Your Daily Calories

Get the calories to eat each day for the deficit you choose.

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Check If a Plan Is Safe

Spot targets below the usual minimum, paces that are too fast and goal weights that are too low.

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Plan Beyond the Goal

See your maintenance calories at the goal weight so you can keep the weight off.

Calculation Formula

Standard Method

Daily deficit = pounds per week × 3,500 ÷ 7 Calories to eat = maintenance calories (TDEE) − daily deficit Weeks to goal = (current weight − goal weight) ÷ pounds per week, rounded up Maintenance calories use the Mifflin-St Jeor equation: BMR (men) = 10 × kg + 6.25 × cm − 5 × age + 5; BMR (women) = 10 × kg + 6.25 × cm − 5 × age − 161; TDEE = BMR × activity factor (1.2 to 1.9). Maintenance at your goal weight is calculated with the same equation at the goal weight, because your needs fall as you lose weight. The plan keeps the deficit constant. In practice the pace slows as your maintenance calories drop, and the 3,500 calories-per-pound figure is an average, not an exact law. Warnings: calories below 1,200 (women) or 1,500 (men), a pace above 1% of body weight a week and a goal BMI below 18.5.

Benchmark Example:

A woman, 35, 5 ft 6 in, 150 lb, moderately active, who wants to reach 130 lb at 1 lb a week: • Maintenance calories: 2,158 a day • Deficit: 1 × 3,500 ÷ 7 = 500 calories a day, so she eats about 1,658 calories a day • Time: 20 lb ÷ 1 lb a week = 20 weeks, about 4.6 months • Weight after 4 weeks: 146 lb; after 12 weeks: 138 lb • Maintenance at 130 lb: about 2,017 calories a day • BMI at 130 lb: 21.0, healthy At 2 lb a week the target would be 1,158 calories a day, below the 1,200 minimum for women, and the pace would be 1.3% of her weight, so the tool shows warnings.

Frequently Asked Questions

6 answers

Applicable Laws & Regulatory Standards

Centers for Disease Control and Prevention, NIH & Federal Trade Commission

Official Legal Reference

CDC — Losing Weight: steady weight loss of about 1 to 2 pounds a week

The CDC describes a loss of about 1 to 2 pounds a week as steady and more likely to be kept off, which is the pace this calculator uses as a reference.

NIH / NIDDK — Choosing a Safe and Successful Weight-loss Program

Advises that a program should be safe, include healthy eating and physical activity, and be supervised when a very-low-calorie diet is used, which is why the calculator flags targets below the usual calorie minimum.

FTC — Health Products Compliance Guidance (weight-loss claims)

Sets out that claims about weight loss in advertising must be backed by competent and reliable scientific evidence, a useful check when a program promises fast or effortless results.

Legal citations and statutes are provided for consumer transparency, analytical validation, and educational reference under U.S. consumer financial regulations. This tool is not a substitute for formal legal, accounting, or tax counsel.

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