Three numbers come up in almost every conversation about weight and food: your BMI, how many calories you need a day, and your macros, the split of those calories between protein, carbohydrate and fat. Each answers a different question. BMI is a quick screen of weight for height. Calorie needs estimate how much energy your body uses. Macros describe what that energy is made of.
This guide explains what each number means, how it is calculated, where it comes from, and where it falls short. Every figure below is either worked out live by our calculators or taken from a health agency or a peer-reviewed paper, linked where it appears.
These are estimates for general information, not medical advice. If you have a health condition, are pregnant, or are planning a big change in what you eat, talk to a doctor or registered dietitian.
What BMI tells you (and what it doesn’t)
Body mass index is your weight divided by the square of your height. In metric units that is kilograms divided by meters squared; in US units, CDC multiplies pounds divided by inches squared by 703.CDC
For adults 20 and older, CDC sorts the result into categories that apply “regardless of age, sex, or race”:CDC
| Category | BMI |
|---|---|
| Underweight | less than 18.5 |
| Healthy weight | 18.5 to less than 25 |
| Overweight | 25 to less than 30 |
| Obesity | 30 or greater (class 1: 30 to less than 35; class 2: 35 to less than 40; class 3: 40 or greater) |
CDC’s own example is an adult 5 feet 8 inches tall who weighs 200 pounds. Our calculator gives the same answer CDC does: a BMI of 30.4, in the Obesity (class 1 obesity) category. At that height, the healthy-weight range runs from 122 lb to under 164 lb.
That last point is worth noticing. A “healthy” BMI is not one weight but a band of weights for each height. The older “ideal body weight” formulas, such as Devine’s 1974 equation, give a single number instead; for a woman 5 ft 4 in tall they give 120–129 lb, inside a CDC healthy range of 108–146 lb. Researchers who reviewed those formulas noted they fail to recognize that there is a range of healthy weights.Am J Clin Nutr
What BMI can’t see
CDC is clear about the limits. BMI does not measure body fat directly, does not distinguish fat from muscle and bone, and does not show what type of fat you have or where it sits.CDC A muscular athlete can have a BMI in the overweight range. CDC also calls BMI “just one measure of health”: it is a screening tool that a health care provider interprets alongside other information.
For children and teens, adult categories don’t apply. Because they are still growing, CDC compares a child’s BMI with others of the same age and sex and reports a percentile, using its Child and Teen BMI Calculator for ages 2 through 19.
If you want a second view of body composition, the U.S. Navy tape-measure method estimates body fat percentage from your neck, waist (and hips for women) and height. It has its own error margin of about 3 to 4 percentage points of body fat,Frontiers in Physiology, and it uses your measurements rather than weight alone.
23.6
- CDC category
- Healthy weight
- Healthy weight from
- 125 lb
- Healthy weight up to
- under 169 lb
- Change to reach healthy range
- Within the range
- BMI is a screening measure, not a diagnosis. It does not distinguish fat from muscle and bone, or show where fat is carried (CDC).
- This is an estimate for general information, not medical advice. Talk to a doctor or other qualified health professional about your own situation.
Your BMI: 23.6 HEALTHY WEIGHT
CDC adult BMI categories at 5 ft 9 in
BMI ranges from CDC. Weights show where each range starts and ends for your height. Your category is highlighted.
| Category | BMI | Weight at your height |
|---|---|---|
| Underweight | Under 18.5 | Under 125 lb |
| Healthy weight | 18.5 to under 25 | 125 lb to under 169 lb |
| Overweight | 25 to under 30 | 169 lb to under 203 lb |
| Obesity, class 1 | 30 to under 35 | 203 lb to under 237 lb |
| Obesity, class 2 | 35 to under 40 | 237 lb to under 271 lb |
| Obesity, class 3 (severe) | 40 or more | 271 lb or more |
How many calories you burn: BMR and TDEE
Your daily calorie need is built from two pieces.
Basal metabolic rate (BMR) is the energy you use at rest, just to keep your body running. It can’t be measured at home, so it is predicted from an equation. A widely used one is Mifflin–St Jeor, published in 1990 after its authors measured resting energy expenditure in 498 healthy adults aged 19 to 78, both normal weight and with obesity:Am J Clin Nutr
- Men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
- Women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
The same study found the original Harris–Benedict equations overestimated measured resting energy by about 5%. Our calculator offers the 1984 Roza and Shizgal revision of Harris–Benedict as an alternative.
Total daily energy expenditure (TDEE) adds everything else you do: walking, working, exercising. It is estimated by multiplying BMR by a physical activity level (PAL). We use the lifestyle categories from the FAO/WHO/UNU expert report on human energy requirements:FAO/WHO/UNU
| Lifestyle | PAL range | Example value |
|---|---|---|
| Sedentary or light activity | 1.40–1.69 | 1.53 |
| Active or moderately active | 1.70–1.99 | 1.76 |
| Vigorous or vigorously active | 2.00–2.40 | 2.25 |
The report’s own description of an active lifestyle includes someone with a desk job who does about an hour of moderate to vigorous exercise, such as jogging or cycling, every day.
Here is how that plays out for a 35-year-old woman, 5 ft 5 in and 150 lb. Her BMR is about 1,376 calories a day. With a desk job and little exercise, her estimated maintenance is 2,106 calories. If she added an hour of moderate to vigorous exercise every day and moved into the active category, it would be about 2,422.
Calories to maintain your weight: 2,686 CALORIES A DAY (ESTIMATE)
Treat these as starting points. The equations predict the average for people like you, and Roza and Shizgal put the precision of Harris–Benedict at about ±14% for normally nourished adults.Am J Clin Nutr
Calorie targets for weight loss
To lose weight you need to take in less energy than you use. NHLBI’s long-standing guidance is that cutting 500 to 1,000 calories a day from your current intake produces a weight loss of about 1 to 2 pounds a week.NIH / NHLBI
For the woman above, that means about 1,606 calories a day to lose about a pound a week, or 1,106 for about two pounds.
A few guardrails from the same sources:
- Slower tends to last. CDC notes that people who lose about 1 to 2 pounds a week are more likely to keep the weight off than people who lose weight more quickly.CDC
- Don’t go too low. NHLBI suggests 1,000 to 1,200 calories a day for most women and 1,200 to 1,600 for men on a weight-loss diet, and says diets under 800 calories a day should not be used routinely. Our calculator flags any target below those levels.
- Expect a plateau. NHLBI’s clinical guidelines found that weight loss at 1 to 2 pounds a week commonly lasts for up to 6 months, then slows, because a smaller body uses less energy.NIH / NHLBI Re-run the calculator with your new weight.
We don’t publish weight-gain surpluses, because we haven’t found an agency guideline that sets one.
Macros: splitting calories into protein, carbs and fat
Calories come from three macronutrients. FDA’s nutrition labeling rules use general factors of 4 calories per gram for protein, 4 for carbohydrate and 9 for fat.FDA (eCFR) So to turn a calorie split into grams you divide each share by 4 or 9.
How much of each? The National Academies’ Food and Nutrition Board set Acceptable Macronutrient Distribution Ranges (AMDRs): the range of intake associated with reduced risk of chronic disease while providing enough essential nutrients.National Academies For adults:
| Macronutrient | Adult AMDR | Other reference |
|---|---|---|
| Protein | 10–35% of calories | RDA 0.8 g per kg of body weight |
| Fat | 20–35% of calories | — |
| Carbohydrate | 45–65% of calories | RDA 130 g a day |
On 2,000 calories at 20% protein, 30% fat and 50% carbohydrate, that works out to 100 g of protein, 250 g of carbohydrate and 67 g of fat, all inside the ranges.
The ranges are wide on purpose. There is no single correct split; a higher-protein or lower-fat pattern can both sit inside them. What the AMDRs flag is the extremes. Push protein to 30% and fat to 35% on 2,500 calories and carbohydrate falls to 35%, below the 45% floor:
Protein a day: 188 GRAMS · 219 G CARBS · 97 G FAT
Children have different ranges: fat 30–40% at ages 1–3 and 25–35% at 4–18, and protein 5–20% and 10–30% for those groups. In 2024 the National Academies published a letter report on rethinking the AMDR framework;National Academies until new DRIs are issued, the ranges above are the published values.
Putting the three numbers together
The three numbers work best in order:
- Check where you are with BMI. Use the BMI calculator to see your category and the healthy-weight range for your height. If you are well inside it, maintenance calories may be all you need. If you are outside it, the range gives you a realistic target band rather than a single number.
- Estimate your calories. Use the calorie calculator for your maintenance figure, and, if weight loss is the goal, a target that is 500 to 1,000 calories lower and not below the NHLBI floors.
- Split them into macros. Enter that calorie target in the macro calculator and choose a protein and fat share within the adult ranges. Add your weight to compare protein with the 0.8 g/kg RDA.
- Recheck as you go. Your BMR falls as your weight falls, so update your weight every few weeks and recalculate.
Hydration is part of the same daily picture. The National Academies’ Adequate Intake of total water, from food and drinks combined, is 3.7 liters a day for men and 2.7 liters for women 19 and older; the water intake calculator shows the values for other ages and during pregnancy.
Limitations to keep in mind
- Population tools, individual people. BMI categories, the Mifflin–St Jeor equation, PAL values and AMDRs were all developed from groups of people. They describe averages and ranges; your body may differ.
- Not for everyone. These adult tools don’t apply to children, and the National Academies set different intakes for pregnancy and breastfeeding. Roza and Shizgal also found the Harris–Benedict equations unreliable in malnourished patients. Your clinician can tailor the numbers to you.
- Measurements matter. Every result is only as good as the height, weight and measurements you enter.
- Numbers are a starting point. None of these calculations diagnose anything. A doctor or registered dietitian can interpret them alongside your health history.
Frequently asked questions
Is BMI a good measure of health?
BMI is a screening measure, not a diagnosis. CDC notes it is moderately to strongly associated with other measures of body fat, but it can’t tell fat from muscle or show where fat is carried, so a health care provider interprets it with other information.
What’s the difference between BMR and TDEE?
BMR is the energy your body uses at rest. TDEE is BMR multiplied by your physical activity level, so it includes everything you do in a day. TDEE is the number to compare with how much you eat.
How many calories should I cut to lose a pound a week?
NHLBI guidance is that a deficit of 500 calories a day leads to a loss of about 1 pound a week, and 1,000 calories a day to about 2 pounds, with diets under 800 calories a day not to be used routinely.
What macro split should I follow?
Any split inside the National Academies’ adult ranges, 10–35% protein, 20–35% fat and 45–65% carbohydrate, is within the acceptable range. Choose one that fits the food you enjoy and any advice from your doctor or dietitian.
Why do different calculators give different calorie numbers?
They use different BMR equations and activity multipliers. We use Mifflin–St Jeor and the FAO/WHO/UNU activity levels, both linked above, and show every step so you can compare.