How BMI is calculated

Body mass index compares your weight to your height. It was designed in metric units as kilograms divided by meters squared. With pounds and inches you multiply by 703 to convert.

Worked example

Someone who is 5 ft 10 in (70 in) and weighs 185 lb has a BMI of 703 × 185 ÷ 4,900 = 26.5. That falls in the overweight range. For that height, a BMI of 18.5 to 24.9 corresponds to about 129 to 174 lb.

BMI categories for adults

These cutoffs apply to adults 20 and older, regardless of sex. Children and teens are assessed with BMI-for-age percentiles instead, because body fat changes as they grow.

What BMI can and cannot tell you

BMI is a quick screening tool. It is cheap, needs no equipment and tracks well with body fat across large groups of people. It does not measure body fat directly, so it has known blind spots:

• Muscular people such as athletes and lifters can have a high BMI with low body fat.
• Older adults lose muscle with age and can carry more fat at the same BMI.
• Fat distribution matters. Fat around the waist carries more health risk than fat on the hips and thighs. A waist over 40 in for men or 35 in for women is a risk sign at any BMI.

For a fuller picture, pair BMI with a waist measurement or a body fat estimate, and talk with your doctor about your results.

Sources: Centers for Disease Control and Prevention (CDC), Adult BMI Categories; National Heart, Lung, and Blood Institute (NHLBI).

Frequently asked questions

How your daily calories are calculated

Your body burns calories in two ways. Your basal metabolic rate (BMR) is what it takes to keep you alive at rest: breathing, circulation, keeping warm. On top of that you burn calories moving, exercising and digesting food. Together they make your total daily energy expenditure (TDEE), the calories you need to keep your weight steady.

This calculator uses the Mifflin-St Jeor equation, which research has found to be the most accurate everyday formula for estimating BMR in healthy adults.

Worked example

A 35-year-old man who is 5 ft 10 in (177.8 cm) and 185 lb (83.9 kg) has a BMR of about 1,780 calories. Moderately active, his maintenance is 1,780 × 1.55 ≈ 2,760 calories. To lose 1 lb a week he would eat about 2,260 calories a day.

Activity levels

Most people overestimate how active they are. If you are unsure between two levels, pick the lower one, then adjust after two or three weeks based on what the scale does.

How fast should you lose weight?

A pound of body fat stores roughly 3,500 calories, so eating 500 fewer calories a day leads to about 1 lb of loss per week. Losing 0.5 to 2 lb a week is the range most health authorities consider safe. Faster loss costs more muscle and is harder to sustain. Women should not go below about 1,200 calories a day and men below about 1,500 without medical supervision.

How the macros are set

• Protein: 0.8 g per pound of body weight, which helps keep muscle while you lose fat.
• Fat: 25% of your calories, within the 20–35% range in the Dietary Guidelines for Americans.
• Carbohydrates: the calories that are left.

Sources: Mifflin MD, St Jeor ST et al., American Journal of Clinical Nutrition (1990); Dietary Guidelines for Americans 2020–2025; NIH Body Weight Planner.

Frequently asked questions

How the Navy body fat formula works

The U.S. Navy method estimates body fat from a few circumference measurements and your height. The waist (and hips, for women) reflect where the body stores fat, while the neck adjusts for frame size. Military services use it because it only needs a tape measure and gives consistent results.

Worked example

A man who is 70 in tall with a 36 in waist and a 15.5 in neck scores 86.010 × log10(20.5) − 70.041 × log10(70) + 36.76 ≈ 20.4% body fat. At 185 lb that is about 37.7 lb of fat and 147.3 lb of lean mass.

How to measure

• Use a flexible, non-stretch tape. Keep it level and snug without pressing into the skin.
• Neck: just below the larynx (Adam's apple), with the tape angled slightly down toward the front.
• Waist: men at the navel, women at the narrowest point of the torso. Measure at the end of a normal breath out.
• Hips (women): around the widest part of the buttocks, feet together.
• Measure at the same time of day, ideally in the morning, and average two readings.

Body fat categories

How accurate is it?

Studies comparing the Navy formula with DEXA scans find it usually lands within 3 to 4 percentage points. It is less accurate for very lean or very muscular people and for people who carry most of their fat outside the waist. Its real strength is tracking change: if you measure the same way each time, a falling number means you are losing fat.

Sources: Hodgdon JA, Beckett MB, Naval Health Research Center (1984); American Council on Exercise (ACE) body fat categories.

Frequently asked questions

How much water do you need?

There is no single number that fits everyone, but a common and practical rule is to drink half your body weight in ounces each day, then add more when you sweat. The calculator adds 12 oz for every 30 minutes of exercise and 16 oz on hot or humid days.

Worked example

A 185 lb person who exercises for 30 minutes needs about 92.5 + 12 = 104.5 fl oz a day. That is about 13 cups, 3.1 liters or seven 16.9 oz bottles. Spread over 16 waking hours, it works out to about 6.5 oz an hour.

What the official guidelines say

The National Academies of Sciences, Engineering, and Medicine set an adequate daily intake of about 125 oz (3.7 L) of total water for men and 91 oz (2.7 L) for women. That counts water from all drinks and food. Food usually supplies about 20% of it, which leaves roughly 100 oz from drinks for men and 73 oz for women.

Signs you are drinking enough

• Your urine is pale yellow, like lemonade. Dark yellow means you need more.
• You rarely feel thirsty and do not get afternoon headaches.
• After a workout, you drink about 16–24 oz for every pound lost in sweat.

You can drink too much. Drinking far more than your kidneys can handle, over 27–33 oz an hour for hours, can dilute blood sodium. People with heart or kidney conditions should follow their doctor's fluid limits.

Sources: National Academies of Sciences, Engineering, and Medicine, Dietary Reference Intakes for Water (2004); CDC, Water and Healthier Drinks.

Frequently asked questions