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Calorie Deficit Calculator

Get a rough idea of how long it might take to reach a goal weight if you eat the same amount below your needs every day.

Level: Grades 6–7

Printable worksheet

Calorie Deficit Calculator

Know the answer you need? Pick an input to work backward.

Inputs
Units
Sex

Used for the minimum-intake check and, if estimating, the BMR equation.

How many calories below maintenance you plan to eat each day.

Maintenance calories

What you'd eat each day to keep your weight steady (your TDEE).

Result

Estimated time to goal

15 weeks

Losing 15 lb at 500 kcal a day below maintenance would take about 15 weeks by the simple rule. Real progress is usually slower.

Estimated days
105 days
Weight to lose
15 lb
Total deficit needed
52,500 kcal
Estimated weekly loss
1 lb/week
Planned daily intake
1,900 kcal/day

See the step-by-step solution ↓

Step-by-step solution

  1. Find how much weight you want to lose: your current weight minus your goal

    180−165=15 lb

  2. Turn that into calories, using the rough rule of 3,500 kcal per lb

    15×3,500=52,500 kcal

  3. Divide by your daily deficit to see how many days that takes

    52,500500=105 days

  4. Divide the days by 7 to get weeks

    1057=15 weeks

  5. Find what you'd eat each day: your maintenance calories minus the deficit

    2,400−500=1,900 kcal/day

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

Please note: This calculator gives a general screening estimate, not a diagnosis. Talk to a qualified healthcare professional about your individual health.

What this calculator does

Get a rough idea of how long it might take to reach your goal weight. A calorie deficit means eating fewer calories than your body uses. An old rule of thumb says about 3,500 kcal (calories) equals a pound of body weight. For a kilogram, it's roughly 7,700 kcal. Divide the total deficit you need by the deficit you plan each day, and you get a simple time estimate.

The calculator uses that rule and shows how much you'd eat each day on your plan. It adds a note if that falls below commonly advised minimums, or if the pace is faster than usually suggested. The result is a rough estimate, not a prediction or medical advice. Real weight loss nearly always slows over time, for reasons explained below. Talk to a doctor or registered dietitian before starting a weight-loss plan. That matters most if you have a health condition or take medication.

Where it's used

Why this math matters outside the classroom:

  • Rate reasoning. At 500 kcal a day, a 3,500 kcal gap takes 7 days, so a 10 lb goal works out to about 70 days.
  • Metric version. Losing 5 kg at about 7,700 kcal per kg is 38,500 kcal, which is 77 days at a 500 kcal daily deficit.
  • Comparing paces. Comparing a 250 kcal daily deficit with a 500 kcal one shows the same goal taking twice as many days at the smaller one.
  • Planning a timeline. People sketch a rough target date for a goal weight by dividing the total energy gap by a steady daily deficit.
  • Models and reality. Health classes use the 3,500 kcal rule to show how a static model differs from real bodies, which adapt as weight changes.

Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.RP.A.3.b, 6.RP.A.3.d. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Choose your units. Then enter your sex, current weight and goal weight.
  2. Enter the daily deficit you plan to keep, for example 500 kcal below maintenance (what you'd eat to keep your weight steady).
  3. Enter your maintenance calories if you know them. Or choose Estimate them and add your age, height and activity level.
  4. Read the estimated weeks to your goal and the daily intake your plan means. Check any notes about very low intakes or a fast pace.

What the inputs mean

Sex
Your sex sets which minimum-intake level the warning uses. When estimating, it also sets one number in the BMR equation (BMR is the energy your body burns at rest).
Daily calorie deficit
About 250 to 500 kcal a day is commonly suggested as a moderate place to start. Bigger deficits shorten the estimate on paper, but they're harder to keep up.
Maintenance calories
Maintenance calories are what your body burns in a full day, also called your total daily energy expenditure (TDEE). To estimate them, the calculator uses the Mifflin-St Jeor equation, a standard formula based on your current weight. It then multiplies by a standard activity factor.
Age
These equations were developed for adults.

Formula and method

Weight to lose = current weight − goal weight
Total deficit = weight to lose × 3,500 kcal/lb   (or × 7,700 kcal/kg)
Days = total deficit ÷ daily deficit;   weeks = days ÷ 7
Weekly loss = daily deficit × 7 ÷ 3,500   (or ÷ 7,700)
Planned intake = maintenance calories − daily deficit

The 3,500 kcal per pound and 7,700 kcal per kilogram figures are two separate rounded rules. So pounds and kilograms can give slightly different answers for the same goal. Both are rough guesses at the energy stored in body fat.

When it estimates maintenance calories, the calculator first finds your BMR with the Mifflin-St Jeor equation. That's 10 × your weight in kg, plus 6.25 × your height in cm, minus 5 × your age. Then it adds 5 for males or subtracts 161 for females. It multiplies the result by the activity factor you choose.

The calculation assumes your daily deficit stays the same every day until you reach your goal.

Worked example

Someone weighing 180 lb wants to reach 165 lb. They keep their weight steady on about 2,400 kcal a day, and they plan a 500 kcal daily deficit.

  1. Weight to lose: 180 − 165 = 15 lb.
  2. Total deficit: 15 × 3,500 = 52,500 kcal.
  3. Days: 52,500 ÷ 500 = 105 days, which is 105 ÷ 7 = 15 weeks.
  4. Planned intake: 2,400 − 500 = 1,900 kcal a day, a pace of about 1 lb per week.

Result: By the simple rule, the goal would take about 15 weeks.

Understanding the result

Think of the result as a best case. The simple rule treats every pound lost the same way. But early loss includes water and stored carbohydrate, and later loss usually comes more slowly.

Weight loss slows for several reasons. A lighter body needs less energy, so the same intake gives a smaller deficit as you lose weight. The body also tends to adapt, using a little less energy at rest and moving a little less. Some lost weight is lean tissue, like muscle, which holds less energy per pound than fat. And day-to-day water changes can hide progress for a week or more, so don't be discouraged by a slow week.

A note appears if your planned intake falls below about 1,200 kcal a day for women or 1,500 kcal a day for men. Many health organizations advise against going that low without medical supervision. Another note appears if the pace is faster than about 1 to 2 lb (0.5 to 1 kg) per week.

Assumptions and limitations

  • Your daily deficit stays the same, measured against maintenance calories that also stay the same.
  • About 3,500 kcal per pound or 7,700 kcal per kilogram of weight lost.
  • You're an adult who isn't pregnant. Estimated maintenance uses your current weight and an average week's activity.
  • The energy-per-pound rule is a fixed shortcut, and it tends to overstate long-term loss. More detailed models that allow for the body adapting predict slower progress.
  • Maintenance calories, whether you enter them or the calculator estimates them, are estimates too. They can be off by 10% or more.
  • It doesn't consider body makeup (how much is fat or muscle), medical conditions, medication or pregnancy. It's not for children or teens.

Frequently asked questions

Why am I losing weight more slowly than the estimate?

As you lose weight, your maintenance calories fall. So eating the same amount gives a smaller and smaller deficit. The body also adapts somewhat, and water changes can hide fat loss. Recalculating every few weeks with your new weight gives a more realistic picture.

Is a bigger deficit better?

A bigger deficit shortens the estimate, but very low intakes are hard to keep up. They can make it hard to get enough nutrients, and you may lose more muscle. A healthcare professional can help you choose a pace that suits you.

Where do 3,500 and 7,700 come from?

They're long-standing rough estimates of the energy stored in a pound or a kilogram of body fat. They're useful for rough planning. But they were never meant to predict long-term weight change exactly.

Can I use this to plan weight gain?

No. This calculator only handles a goal below your current weight. For gaining weight, the TDEE calculator shows your maintenance calories and surplus levels, meaning amounts above maintenance.

How this calculator is checked

Method: Static energy-balance approximation: about 3,500 kcal per lb or 7,700 kcal per kg of weight change. Optional maintenance estimate from Mifflin-St Jeor (1990) × conventional activity factors. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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