Calculators and numbers: questions and answers

BMI, or body mass index, is your weight relative to your height. The formula is 703 times your weight in pounds divided by your height in inches squared, or weight in kilograms divided by height in meters squared.

For adults, under 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight and 30 or higher falls in the obesity range. At 5 ft 6 in and 180 pounds, BMI is 29.1. Check yours with the BMI calculator.

BMI is a quick screening number, not a diagnosis. It's useful across large groups, but it doesn't measure body fat directly and can mislead for individuals.

It can overstate fat in very muscular people and understate it in older adults who have lost muscle. Risk at a given BMI also differs across ethnic groups. Clinicians usually look at BMI alongside waist size, blood pressure, blood sugar, cholesterol and your history. Adding a body fat estimate gives more context.

Because BMI only uses height and weight, it can't tell muscle from fat. Athletes and people who lift seriously can land in the overweight range with healthy body fat levels.

If that sounds like you, a waist measurement or body fat estimate is more informative. A waist under half your height is a common healthy screen. For calorie needs, entering your body fat percentage in our TDEE and BMR calculators switches them to the Katch-McArdle formula, which uses lean mass and is often closer for muscular people.

For some purposes, yes. Many people of Asian descent face higher risk of type 2 diabetes at lower BMIs, so some guidelines use lower cutoffs.

For example, the US Preventive Services Task Force recommends screening adults 35 to 70 with overweight or obesity for prediabetes and type 2 diabetes, and suggests a lower BMI cutoff of 23 for Asian Americans. The standard adult BMI categories on our calculator are the CDC's, so ask your clinician how they apply to you.

TDEE, or total daily energy expenditure, is the number of calories you burn in a typical day. It includes your resting burn, the energy used to digest food, exercise and all your everyday movement.

Eating at your TDEE should keep your weight roughly stable, so it's the starting point for any calorie target. Eating about 300 to 500 calories below it leads to gradual weight loss for most people. Find yours with the TDEE calculator, which shows the math with your numbers.

BMR, or basal metabolic rate, is the energy your body uses at complete rest to keep you alive: breathing, circulation, organ function and body temperature. For most people it's the largest part of daily calorie burn.

BMR depends mainly on body size, muscle, age and sex. It isn't a calorie target, since your real daily burn is higher once digestion and movement are added. See yours with three formulas side by side in the BMR calculator.

BMR is what you burn at complete rest. TDEE is what you burn in a full day, which is BMR multiplied by an activity factor that accounts for movement, exercise and digestion.

For example, a 35-year-old woman at 5 ft 6 in and 180 pounds has a BMR of about 1,528 calories. If she's lightly active, her TDEE is about 2,101. Most people lose weight well by eating between their BMR and TDEE, not below BMR. Our guide TDEE explained covers the details.

Our TDEE, BMR and calorie deficit calculators use the Mifflin-St Jeor equation, published in 1990. For men it's 10 times weight in kg, plus 6.25 times height in cm, minus 5 times age, plus 5. For women, the last step is minus 161.

If you enter a body fat percentage, the TDEE tool switches to Katch-McArdle, which uses lean mass. A 2005 systematic review found Mifflin-St Jeor was the most reliable of the common equations for adults. Every calculator page names its formula and shows your math.

They're good starting estimates, not exact measurements. A 2005 systematic review found Mifflin-St Jeor predicted resting burn within 10% of measured values for more people than other common equations, but some individuals were further off.

The activity level you choose adds more uncertainty than the formula itself. Treat your number as a first guess: eat at the target for 2 to 3 weeks, watch your weekly average weight and adjust by 100 to 200 calories. Your real-world trend is the best calibration.

Most use similar formulas but different activity multipliers, rounding and assumptions. Some use the older Harris-Benedict equation, which tends to give slightly higher results than Mifflin-St Jeor.

The activity level you pick usually changes the result far more than the formula does. A gap of 100 to 300 calories between sites is common. Pick one tool, use it consistently and adjust based on your actual weekly trend rather than chasing the most appealing number.

Choose the level that matches a typical week, not your best week. Most people with desk jobs who exercise a few times a week fit "lightly active" or "moderately active."

Our multipliers range from 1.2 for sedentary to 1.9 for a physical job plus training. When in doubt, pick the lower option. It's easier to add calories if you lose faster than planned than to find a deficit that isn't there. Your weight trend over 2 to 3 weeks will tell you if the choice was right.

The simplest options are a tape measure or a smart scale. The US Navy tape method uses your height, neck and waist, plus hips for women, and is often within about 3 to 4 percentage points of lab methods.

Smart scales use a small electrical current and can swing with hydration, so use them for trends rather than single readings. Measure at the same time of day each time. DEXA scans are more precise but cost more. Our guide on how to measure body fat compares the methods.

There's no single healthy number, but many references, including the American Council on Exercise chart, place about 21% to 31% for women and 14% to 24% for men in the fitness-to-average ranges.

Women naturally carry more essential fat than men. Very low body fat isn't healthier and can affect hormones and energy. Rather than chasing a precise number, track your own trend over time, along with waist size and how you feel. Estimate yours with the body fat calculator.

Smart scales are good at weight, but their body fat readings are rough estimates. They send a small electrical current through your body and use equations to guess fat, and the result can shift with hydration, meals, exercise and skin temperature.

They're most useful for spotting trends over weeks, if you weigh under the same conditions each time, such as first thing in the morning after using the bathroom. Don't read much into a single day's body fat number. Smart scales may not be suitable for people with implanted medical devices, so check the manual.

Ideal weight formulas add a set amount of weight for each inch of height over 5 feet. The best known is Devine (1974): 50 kg for men or 45.5 kg for women, plus 2.3 kg per inch over 5 feet. Robinson, Miller and Hamwi use slightly different numbers.

Devine was created to help dose medicines, not to set personal weight goals. Our ideal weight calculator shows all four formulas alongside the healthy BMI range for your height.

Usually not as a first goal. Ideal weight formulas ignore body frame, muscle, age and health history, and the results can be far from what's realistic or necessary for you.

For many people, losing 5% to 10% of body weight brings meaningful health benefits, and that's a more useful first target. Think of the ideal weight range as background information. Your clinician can help you set a goal that fits your health, and you can revisit it once you reach a first milestone.

NIH guidance has long flagged higher health risk at a waist over 40 inches for men and over 35 inches for women who aren't pregnant. Fat around the abdomen is more closely linked to heart disease and type 2 diabetes than fat elsewhere.

These cutoffs may be lower for some groups, including many people of Asian descent. Waist size is a helpful number to track because it can shrink even when the scale stalls. See our guide on measuring body fat and waist.

For a health check, stand up, breathe out normally and wrap the tape around your bare stomach just above your hip bones. Keep the tape level and snug without pressing into the skin.

The body fat calculator uses its own measuring points for the Navy formula, so follow the instructions on that page when you use it. Whatever method you pick, measure the same way, at the same time of day, every few weeks. Small changes of a quarter inch are within normal error.

It's your waist measurement divided by your height, in the same units. A ratio under 0.5, meaning a waist less than half your height, is a common screen for lower health risk.

For someone who is 5 ft 6 in (66 inches), that means a waist under 33 inches. The ratio works across heights and is simple to track. Like BMI, it's a screening tool rather than a diagnosis. For a fuller picture, add a body fat estimate.

Daily weight can swing 2 to 4 pounds from water alone. Salt, carbohydrate, alcohol, hard workouts, poor sleep, constipation, the timing of meals and hormones around a period all shift how much fluid your body holds.

None of that reflects fat gained or lost overnight. Gaining a pound of fat would take thousands of extra calories. Look at your weekly average and the trend over 3 to 4 weeks instead. The weigh-in tracker plots your trend line.

First thing in the morning, after using the bathroom and before eating or drinking, gives the most consistent readings. Wear the same thing, or nothing, each time.

Use the same scale on a hard, flat floor. Consistency matters more than the exact time. Expect normal ups and downs, especially after salty meals or around your period. If you weigh daily, look at the weekly average rather than reacting to any single day.

Once a week at the same time works well for most people. Daily weighing is also fine if you look at the weekly average and don't react to single days.

Regular self-weighing is a common habit among people who keep weight off, because it catches small regains early. If weighing makes you anxious or triggers unhealthy behavior, it's okay to weigh less often and track other markers such as waist size, energy or how clothes fit. Talk to a clinician if weighing feels distressing.

Our medicine projections apply a trial's average result to your starting weight, using a smooth curve shaped to hit the published endpoint. They show what average looked like in that trial, not what will happen to you.

In every trial, some people lost far more than average and some lost little. Your dose, time on treatment, diet, activity, sleep and other medicines all matter. The placebo result is shown too, for context. Use the weight-loss projection as a planning aid and discuss realistic goals with your prescriber.

Because it assumes your deficit stays the same, and in real life it shrinks. As you lose weight, your daily burn falls, so the same intake produces a smaller deficit and slower loss.

Research models like the NIH Body Weight Planner show this slowing clearly over months. Real life adds holidays, illness and busy weeks too. Treat the date as a best case, recalculate every 10 pounds or so and judge progress by your monthly trend. Try it in the calorie deficit calculator.

Our calorie tools hold targets at 1,200 calories a day for women and 1,500 for men, a common minimum without medical supervision. If your chosen pace would go lower, the tool keeps the floor and stretches the timeline instead.

Very low intakes make it hard to get enough protein and nutrients and raise the risk of muscle loss and gallstones. If a clinician has prescribed a lower-calorie plan, follow their guidance, since they can monitor you. The floor is a safety default, not a recommended target.

Every 10 to 15 pounds lost, or whenever your activity level changes a lot, such as a new job or training plan. A smaller body burns fewer calories, so your target drifts down as you lose.

It's also worth rechecking if your weekly average hasn't moved for 3 to 4 weeks despite steady habits. Keep the same calculator and settings each time so the comparison is fair. Update your numbers in the TDEE calculator.

Our heart rate tool uses the Tanaka formula: 208 minus 0.7 times your age. At 40 that's 180 beats per minute, the same as the older 220-minus-age formula, but at 60 it's 166 versus 160.

Tanaka fits large datasets better, especially for older adults. Any formula predicts an average, though, and your true maximum can be 10 or more beats higher or lower. If you take a medicine that slows your heart rate, use perceived effort instead. See your zones in the heart rate zone calculator.

We estimate stride length from height: about 0.415 times height for men and 0.413 times height for women. Miles equal steps times stride length in inches, divided by 63,360.

For a woman who is 5 ft 6 in, that's a stride of about 27.3 inches, so 10,000 steps is roughly 4.3 miles. Real stride changes with pace and terrain, so a tracker that measures your stride will be more precise. Try the steps to miles calculator.

The National Academies set adequate total water intake at about 3.7 liters a day for men and 2.7 liters for women, from all drinks and food. Food usually provides about 20% of that.

Needs rise with heat, exercise, pregnancy and breastfeeding. Pale yellow urine is a simple sign you're drinking enough. People with heart failure or kidney disease may be told to limit fluids, so follow your clinician's advice. Estimate your daily target with the water intake calculator.

Our calculators are built for adults who aren't pregnant. Children and teens use BMI percentiles based on age and sex, not the adult categories, and their calorie needs include growth.

During pregnancy and breastfeeding, calorie, protein and fluid needs change, and weight loss is usually not advised during pregnancy. If you're pregnant, planning pregnancy, breastfeeding or under 18, please use these tools only for general interest and talk to your clinician about your numbers.