Weight Loss Calculator

What the calculator does

Most weight loss advice stops at "eat less". This tool turns a concrete goal, for example lose 10 kg in 10 weeks, into a daily calorie number and shows what that plan would do to your fat and lean mass. It reports four things: your current daily energy use, the intake needed to reach the goal, an alternative intake if you also raise your activity by one level, and the intake that maintains your new weight afterwards.

Unlike the familiar "3,500 calories per pound" shortcut, the size of the deficit is not a fixed number. It depends on how much of the weight you lose will be fat and how much will be lean tissue, and that depends on how much body fat you carry now.

These are estimates from population equations, not a prescription. People with medical conditions, adolescents, and women who are pregnant or breastfeeding should not diet without professional guidance. Read the disclaimer and speak to a doctor or dietitian before starting a large calorie cut.

How to use it

1. Choose US or metric units, then enter age, gender, weight and height.
2. Measure your neck and waist (and hip if you are female) and enter them in the same unit system. They are required, because they are used to estimate body fat.
3. Pick your current activity level. The future activity level starts at the same value; change it only if you plan to exercise more or less.
4. Enter the target weight and how long you want to take, in days, weeks, months or years.
5. Press Calculate and read the results from the top down.

Keep the weight unit and the target unit consistent, and make sure the unit selector beside each weight matches what you typed.

Reading the results

• Current caloric intake is your estimated daily energy expenditure: the calories that would hold your weight steady at your current activity.
• You should consume is the daily intake that reaches the target on time, using the activity level you chose for the future.
• Alternatively, consume appears when your future activity equals your current one. It shows how much more you could eat if you moved up one activity level, a useful way to see what exercise is worth.
• After the period, consume is the intake that holds your new weight. It is a little lower than a plain weight-based estimate would suggest, because the body tends to burn slightly less after weight loss.
• BMI, body fat and lean body mass are shown before and after, so you can see whether the plan sacrifices more muscle than you expect.

If the plan would push intake below 1,000 calories a day, the calculator does not give a number. It warns you instead and suggests a longer period or more activity, since eating that little is unsafe for most people.

The method behind the numbers

The calculator chains four steps.

1. Energy expenditure. Resting energy comes from the Mifflin-St Jeor equation and is multiplied by an activity factor of 1.2, 1.375, 1.55, 1.725 or 1.9. The same approach is explained in the BMR calculator and the TDEE calculator.
2. Starting body composition. Body fat percentage is estimated from neck, waist (and hip) and height with the US Navy circumference method. Compare it with the body fat calculator.
3. Fat versus lean loss. The weight change is divided into fat mass and fat-free mass with Forbes' relationship as refined by Hall, which uses the Lambert W function. People with more fat tend to lose a larger proportion of fat.
4. Energy cost of the change. The calculator prices the change at about 9,500 kcal per kg of fat and 1,100 kcal per kg of lean tissue, and spreads that total over the days available.

daily deficit = (9,500 × fat lost + 1,100 × lean lost) ÷ daysMasses in kg, with losses counted as positive. If the target is above your current weight, the same logic produces a surplus instead.

intake = energy expenditure at the future activity level − daily deficit

Because fat holds more than eight times as much energy per kilogram as lean tissue, losing the same weight with more fat in the mix needs a larger total deficit. That is why one fixed rule cannot fit everybody.

Worked example

A 30-year-old man is 175 cm tall, weighs 90 kg, has a 38 cm neck and a 90 cm waist, and exercises one to three times a week. He wants to weigh 80 kg in 10 weeks (70 days).

• Resting energy is 10 × 90 + 6.25 × 175 − 5 × 30 + 5 = 1,848.75 kcal; × 1.375 gives 2,542 kcal a day.
• The Navy method puts his body fat at about 20.7%, or 18.6 kg, leaving 71.4 kg of lean mass.
• Losing 10 kg splits into about 5.971 kg of fat and 4.029 kg of lean tissue, ending at 12.6 kg of fat (15.8%).
• Energy to remove: 9,500 × 5.971 + 1,100 × 4.029 ≈ 61,157 kcal, or about 874 kcal a day over 70 days.
• Target intake: 2,542 − 874 ≈ 1,668 kcal a day. Raising activity one level would allow roughly 1,992 kcal.
• Afterwards, maintaining 80 kg takes about 2,367 kcal a day.

The "3,500 per pound" shortcut would call for 22.05 lb × 3,500 = 77,162 kcal, or 1,102 kcal a day, noticeably more than this model for this man.

Why there is no single deficit that works for everyone

The table shows what this calculator returns for three 30-year-old men, all 175 cm tall with light activity, who each take 10 weeks to lose weight. The last column is the 3,500-per-pound figure for the same weight change.

The leaner the person, the smaller the deficit needed per kilogram, because more of the loss is lean tissue, which stores less energy. The rule is close only for someone with a lot of excess fat. It also ignores the fact that the body adapts as you lose weight, which is why the calculator reports a separate maintenance figure.

How to take your measurements

• Neck: measure just below the larynx, with the tape sloping slightly down towards the front. Do not flare the shoulders.
• Waist: for men, at the level of the navel; for women, at the narrowest point of the torso. Measure after a normal exhale, without pulling the tape tight into the skin.
• Hip (women): around the widest part of the buttocks, with feet together.
• Measure to the nearest half centimetre or quarter inch, take each measurement twice and use the average.

Small errors matter: a 1 cm change in waist can move the body fat estimate by roughly a percentage point, which nudges the calorie figure. If the waist is smaller than the neck, or the numbers imply a negative body fat, the calculator reports the measurements as unrealistic.

Using the result sensibly

• Treat the number as a starting point and adjust after two to three weeks based on how your weight and energy actually respond.
• A slower timeline protects muscle and is easier to keep up. Aim to lose weight over months, not days.
• Pair the calorie target with enough protein; see the macro calculator and the calorie calculator.
• To judge whether your target is reasonable, compare it with the healthy weight calculator and the BMI calculator.