Do you need to lose weight? Our information & simple tools can help you figure out where you should be, then keep you on track as you slim down. Unless you drop serious cash on futuristic weight loss equipment, don’t expect your bathroom scale to give you anything besides a daunting or delightful number. Interpretation is up to you. Luckily, body weight and calorie calculators can add context to those digits, taking into account factors like your age, gender, height, body fat positioning, and activity level to help you determine if your weight is a risk to your overall health.
While crunching numbers at your desk doesn’t replace a visit to the doctor, it can help you determine if a slim-down strategy is in order, and help you chart your progress along the way. (Search: Why do I need to track my weight and body fat percentage?) “Although they aren’t always perfect, weight loss calculators can be fun tools to engage people who are interested in changing their weight,” says Alice Burron, exercise physiologist and spokesperson for the American Council on Exercise. “They can be useful for tracking over time, and they are slightly more interactive than the bathroom scale.”
Still, with similar-sounding names—BMI, BMR, BAI—it’s tough to decide which at-home test you can trust to tell you if you’re fit—or not as fit as you had hoped. Here, we deliver the lowdown on the most popular weight calculators to determine what’s in, what’s out, and what can actually help you lose weight.
To determine if your weight is “normal”
The body mass index (BMI) is a tried-and-true weight metric. Invented in the late 1800s, the formula uses weight and height to calculate a number that indicates if a person is underweight, normal, overweight, or obese. While this formula is a fairly reliable indicator of obesity and health risks, it doesn’t measure your percentage of body fat.
Going one step further, an “ideal weight” or “healthy weight” calculator, many of which can be found online, can tell you what a healthy weight range is for your height.
How it works: Plug your weight and height into an online calculator (like this one) and you’ll get a number indicating whether you’re underweight (below 18.5), normal (18.5 to 24.9), overweight (25 to 29.9), or obese (30 and above). These standards were set in 1985 by the National Institutes of Health Consensus Panel, which concluded that the BMI was an appropriate measurement of obesity. For all you math geeks, the formula is Weight (in pounds) / [Height (in inches)]2 x 703.
How it can help you manage your weight: “For average to overweight individuals who want to lose weight, BMI can be used as a motivation tracking tool to see changes over time,” says Burron. For example, BMI can be used to set a weight loss goal like, “My goal is to be in the ‘normal’ weight category by [a certain date].” (Related: Learn how you can lose up to 15 pounds in 32 days while cracking down on stubborn belly fat)
How accurate is it? “Since BMI uses total body weight, rather than estimates of fat and lean body mass separately, it does not discriminate between the overweight and the athletic or more muscular body type,” says Burron. “It also does not take into account age, gender, or muscle mass. It would not be a useful tool for heavy-muscled athletes, the elderly, pregnant women, or teenagers.” According to a recent study published in the Journal of the American College of Cardiology, researchers found that BMI also fails to predict health risks for individuals who fall in the “skinny fat” category—those who have a normal, healthy BMI and likely appear thin but have a high percentage of body fat.
To determine if your weight is a health risk
The waist-to-hip ratio measures where your body stores fat. The ratio compares the circumference of your waist with that of your hips. (Related: 20 Tips for a Smaller Waistline)
How it works: You can calculate at home by measuring your waist and hip circumferences and dividing the numbers. Wrap a tape measure around the smallest section of your natural waist, usually just above the belly button, and record the number. Then measure your hips around the widest part of your bottom. Finally, divide your waist measurement by your hip measurement. For example, if your waist is 30 inches and your hips are 29, the ratio is 1.03. According to the National Institute of Diabetes, Digestive and Kidney Diseases, women with waist-to-hip ratios of more than 0.8 are at increased risk for cardiovascular disease, diabetes, and certain cancers because of their fat distribution. And men with waist-to-hip ratios of more than 1.0 are at increased health risk for the same reason.
How it can help you manage your weight: This test can help you determine if you’re an apple or a pear shape. A ratio of 0.8 or above indicates that you are an apple shape. A ratio of under 0.8 means you are a pear shape. Previous research suggested that those with central obesity (apple shape) were 3 times more likely to suffer from heart disease than those with more generally distributed fat.
How accurate is it? Current research suggests that you’re better off sticking with BMI. A new study published in the British medical journal The Lancet studied 220,000 people over almost 10 years and found that your risk for cardiovascular disease is not increased by carrying fat around your waist as opposed to on your hips and thighs. Carrying excess weight—anywhere—puts you at greater risk for heart disease, the researchers concluded.
To calculate your body fat percentage
The body adiposity index (BAI) is a new test proposed as an alternative to calculating your BMI. The BAI can be used to find your percentage of body fat.
How it works: To calculate it yourself, first convert your measurements into metric numbers. Here’s how: Hip circumference (in inches) x 2.5 = hip circumference in centimeters. Height (in inches) x 0.025 = height in meters. Then plug your metric measurements into the BAI formula: [(Hip circumference in centimeters) / (Height in meters x square root of height in meters)] - 18.
For example, if you’re a 150-pound woman with 30-inch hips and you’re 5'5", the math would look like this:
Hips: 30 x 2.5 = 75 cm
Height: 65 x 0.025 = 1.625 m
[75 / (1.625 x 1.275)] - 18 = approximately18% body fat
American Council on Exercise says an “acceptable” range for body fat for women is 25 to 31%, and for men 18 to 25%. You’re considered obese if your body fat is 32% or greater for women, and 26% or greater for men.
If you don’t want to pick up a calculator, let online tools take care of the math. Enter your height, weight, hip measurement (taken at the level of maximum extension around the bottom area), gender, and age in an online calculator, like this one.
You can also try a new feature included on many bathroom scales, Bioelectrical Impedance Analysis (BIA) technology. BIA is a common way to estimate your percentage of body fat. The device, which starts at around $100, sends a mild electric current up through one foot to the waist, then down the other leg to measure your percentage of body fat.
How it can help you manage your weight: Researches say the BAI is more accurate than the BMI because it takes into account body fat without factoring in weight. “This method has promise as it takes into account many factors: gender, age, height, weight, and hip circumference,” says Burron.
How accurate is it? This new test has been checked against a Dual Energy X-ray Absorptiometry (DEXA) machine, which is regarded as one of the most accurate ways to calculate body fat, and the BAI numbers were proven to be accurate, at least in Hispanic and African American populations. More research needs to be done with other ethnicities before it’s widely considered a valid measure of body fat. “Until this method has been scientifically determined by the medical community to be an accurate body composition assessment, I will stick with the more common methods such as BMI, body fat analysis, and waist-to-hip ratio,” says Burron.
And in terms of the BIA scale, the device reportedly is very sensitive to hydration (dehydration can cause it to overestimate the percentage of body fat), which can affect its accuracy.
To determine how many calories you need
The Basal Metabolic Rate (BMR), also known as Resting Metabolic Rate, calculator measures the number of calories your body would burn if you sat still all day and didn’t eat anything.
Once you know how fast or slow your body converts fuel into energy, you can use that information to determine the number of calories you need to consume to lose, gain, or maintain your weight based on your daily activity levels. Don’t worry if your metabolism needs a boost. Cardiovascular exercise can increase your BMR, improve your fitness, and make your body a more efficient calorie burner.
How it works: The formula used to calculate your BMR takes into account your weight, height, age, and gender. The Harris-Benedict equation is the most widely used formula for BMR, though a 2003 study published in the Journal of the American Dietetic Association stated that a newer formula, the Mifflin equation, was 10% more accurate than the Harris-Benedict equation.
To calculate your BMR using the Mifflin formula, follow these equations:
Men: (10 x weight in kg) + (6.25 x height in cm) - (5 x age) + 5
Women: (10 x weight in kg) + (6.25 x height in cm) - (5 x age) – 161
Lazy? Use this online calculator. Or try a more simple formula for quickly determining how many calories to eat on any given day to maintain your current weight: On days when you’re taking it easy and not exercising, multiply your weight (in pounds) by 15 and subtract 500 from that number. And on days that you're active, use the same calculation, but multiply your weight by 18.
How it can help you manage your weight: The number you get from the equation gives you the total number of calories you need to eat to maintain your current weight. To lose weight, shave off 500 calories daily to lose 1 pound per week, or 300 calories daily to lose 2 pounds in a month. “Modifying calories according to activity levels is a good health habit. It [helps people realize] that on low-activity days, fewer calories are needed to maintain weight,” says Burron. “It emphasizes the calories in/calories out formula.”
How accurate is it? According to the Journal of the American Dietetic Association, the Mifflin test used in many BMR formulas provided accurate estimates of actual resting metabolic rates in nonobese and obese individuals. However, Burron says to take these equations with a grain of salt. “It’s hard to predict activity levels, and if you’re planning your caloric needs for the day and things go awry, you may be eating too much or too little,” she says.
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