BMI Calculator — Free BMI, BMR & TDEE Guide
What It Solves
Most body measurement tools stop at BMI — a single number that tells you if your weight is in a healthy range for your height. The BMI calculator goes further. It computes your BMI, BMR (Basal Metabolic Rate), TDEE (Total Daily Energy Expenditure), ideal weight range, and estimated body fat percentage using the US Navy method. You get a complete metabolic profile from one set of inputs: height, weight, age, gender, and activity level. No additional tools, no clinical visits, no guesswork.
The Real Problem
Someone trying to understand their body faces a fragmented world of health metrics. BMI calculators abound, but BMI alone tells you nothing about your metabolism. BMR calculators exist on separate sites. TDEE calculators ask for the same inputs but use different formulas. Body fat estimation requires a separate tool entirely. To get a full picture, a person would visit four different websites, enter their data four times, and manually cross-reference four sets of results. That friction means most people never see the full picture — they check their BMI, see "normal" or "overweight," and move on without understanding their calorie needs or body composition. The result is incomplete information that leads to poorly informed health decisions.
How to Use It
Open the BMI calculator. You can use metric (cm/kg) or imperial (ft/in/lbs) — toggle at the top. Enter your height, weight, age, and select your gender and activity level (sedentary through extra active). The calculator instantly shows five results. BMI: your body mass index with the WHO category and a color-coded bar. BMR: your basal metabolic rate in calories per day using the Mifflin-St Jeor formula. TDEE: your total daily energy expenditure based on your activity multiplier. Ideal weight range: the healthy BMI range for your height in both units. Body fat percentage: an estimate using the US Navy circumference method (requires optional neck, waist, and hip measurements for maximum accuracy). Everything updates in real time as you adjust any input.
BMI: 145 ÷ (66)² × 703 = 23.4 — Normal range (18.5-24.9).
BMR (Mifflin-St Jeor): (10 × 65.8) + (6.25 × 168) - (5 × 30) - 161 = 658 + 1050 - 150 - 161 = 1,397 calories/day.
TDEE (moderately active × 1.55): 1,397 × 1.55 = 2,165 calories/day.
Ideal weight range for 5'6": 118-155 lbs (53.5-70.3 kg).
Body fat (US Navy): If neck=13", waist=30", hips=38" → approximately 27.5% body fat.
The Weight Loss Journey Tracker
Carlos, 42, was told by his doctor to lose weight. He started at 220 lbs, 5'10". He used the calculator weekly to track his BMI, BMR, and body fat. Week one: BMI 31.6 (obese), BMR 1,845 cal/day, TDEE (sedentary) 2,214 cal/day. He set a daily calorie target of 1,800 based on his TDEE minus 400 for a modest deficit. Week eight: BMI 29.2 (overweight), BMR 1,790 cal/day (lower because he weighed less), TDEE 2,148 cal/day. He adjusted his target to 1,750. The key insight was watching his BMR drop as he lost weight — his body needed fewer calories at a lower weight, so he had to reduce his intake to continue losing at the same rate. After six months: BMI 26.1 (overweight, trending toward normal), body fat from 32% to 24%. The calculator gave him a single dashboard to track the four metrics that mattered, recalculated every week with no extra effort.
The Athlete's Recomposition Plan
Maya is a 28-year-old CrossFit athlete, 5'8", 165 lbs. Standard BMI calculators would flag her at 25.1 (overweight) — but she had 22% body fat, well within the athletic range. The body fat measurement saved her from the wrong conclusion. Her BMR (Mifflin-St Jeor) came to 1,526 cal/day, but her TDEE (very active × 1.725) was 2,632 cal/day. She wanted to maintain weight while building muscle, so she targeted 2,600-2,700 calories daily with 140g protein. Every two weeks, she checked her body fat trend: 22% → 21.5% → 20.8% over three months, while her weight stayed nearly the same. The scale said "no change," but the body fat trend showed steady recomposition. Without the full metrics — especially the body fat estimate — she would have seen a plateau and might have cut calories unnecessarily.
Limitations
Every metric in this calculator has limitations. BMI doesn't distinguish muscle from fat, so muscular individuals may be miscategorized as overweight or obese. The Mifflin-St Jeor BMR formula is accurate within ±10% for about 70% of the population — the other 30% may have a BMR that differs significantly from the prediction. TDEE multipliers (1.2 for sedentary, 1.55 for moderate, etc.) are population averages that may not reflect your actual activity. The US Navy body fat formula has a margin of error of about ±3% for most people and is less accurate for very lean or very obese individuals. All results are estimates, not clinical measurements. For precise body composition, DEXA scans, hydrostatic weighing, or BIA devices are more accurate. Always consult a healthcare professional before making significant changes to your diet or exercise.
FAQ
What is the difference between BMI, BMR, and TDEE?
BMI (Body Mass Index) measures weight relative to height for a general health screening category. BMR (Basal Metabolic Rate) is the calories your body burns at complete rest just to stay alive. TDEE (Total Daily Energy Expenditure) adds your activity level on top of BMR to show your total daily calorie burn. The calculator gives you all three so you understand both your body composition category and your energy needs.
How is BMR calculated with the Mifflin-St Jeor formula?
For men: BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age(y) + 5. For women: BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age(y) - 161. This formula was developed in 1990 and is considered the most accurate for the general population, replacing the older Harris-Benedict equation.
How does the US Navy body fat method work?
The US Navy method estimates body fat percentage using circumference measurements. For men: neck and waist measurements. For women: neck, waist, and hip measurements. The formula accounts for your height and these circumferences to estimate fat mass. It's one of the most practical field methods — no calipers or specialized equipment needed beyond a tape measure.
What is my ideal weight range?
Your ideal weight range is the weight that corresponds to a BMI of 18.5 (lower end) to 24.9 (upper end) for your height. For a 5'9" person, that is 125-168 lbs. The calculator shows this range in both kilograms and pounds. Keep in mind that ideal weight varies by muscle mass, bone density, age, and ethnicity — the range is a guideline, not a prescription.
Is this medical advice?
No. The BMI, BMR, TDEE, and body fat estimates provided by this calculator are for informational and educational purposes only. They are not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making any decisions about your health, diet, or exercise routine.
Conclusion
Use this calculator when you want more than just a BMI number — when you're tracking weight loss, planning a recomposition, or simply curious about your body's energy needs. The combination of BMI, BMR, TDEE, ideal weight, and body fat gives you a metabolic dashboard that most standalone calculators can't match. Don't use it as a diagnostic tool or a substitute for professional medical advice. If you need a simpler weight-only assessment, try the ideal weight calculator. For daily calorie and macro targets, the calorie calculator builds on these same principles. Know your numbers, but trust your doctor.
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