When trying to determine a healthy weight, you will encounter two common measurements: Ideal Body Weight (IBW) and Body Mass Index (BMI). Both claim to tell you whether your weight is healthy, yet they often give seemingly different answers. So which one should you trust?
The short answer is that both have value, and understanding their differences helps you use them correctly. This guide breaks down how each measurement works, their strengths and limitations, and how to use them together for a complete picture of your health.
What Is Ideal Body Weight (IBW)?
Ideal body weight is a calculated estimate of what you should weigh based primarily on your height. Several formulas exist, each developed for different purposes:
- Robinson Formula (1983): The most commonly used in clinical settings today
- Devine Formula (1974): Originally created for medication dosing calculations
- Miller Formula (1983): Tends to give lower estimates, better for smaller frames
- Hamwi Formula (1964): Popular among dietitians for nutrition planning
All these formulas share a common structure: they start with a base weight for a certain height (typically 5 feet) and add a set amount for each additional inch of height. Most also account for sex, with different calculations for men and women.
Example (Robinson Formula for Women):
Ideal Weight = 49 kg + 1.7 kg for each inch over 5 feet
For a 5'5" woman: 49 + (5 x 1.7) = 57.5 kg (127 lbs)
Calculate your ideal weight using multiple formulas with our Ideal Weight Calculator.
What Is BMI?
Body Mass Index is a simple ratio of your weight to your height squared. Unlike ideal weight formulas, BMI does not give you a target number - it places you in a category.
BMI Formula:
BMI = Weight (kg) / Height (m)²
Or in imperial units:
BMI = (Weight in lbs x 703) / Height in inches²
BMI Categories
| BMI Range | Category | Health Risk |
|---|---|---|
| Under 18.5 | Underweight | Increased |
| 18.5 - 24.9 | Normal/Healthy | Low |
| 25.0 - 29.9 | Overweight | Increased |
| 30.0 - 34.9 | Obese Class I | High |
| 35.0 - 39.9 | Obese Class II | Very High |
| 40.0+ | Obese Class III | Extremely High |
Calculate yours with our BMI Calculator.
Key Differences at a Glance
| Feature | Ideal Weight | BMI |
|---|---|---|
| Output | Specific weight (e.g., 127 lbs) | Category (e.g., "Normal") |
| Accounts for sex | Yes (different formulas) | No (same for both) |
| Inputs required | Height, sex | Height, weight |
| Body composition | Not considered | Not considered |
| Primary use | Personal goal setting | Population health screening |
| Medical applications | Medication dosing | Risk assessment |
Strengths and Limitations of Ideal Weight
Strengths
- Specific target: Gives you an actual number to aim for
- Sex-specific: Accounts for biological differences between men and women
- Multiple formulas: You can compare results and find a range
- Easy to understand: A weight in pounds is more intuitive than an index number
Limitations
- Based on old data: Many formulas were developed decades ago with limited populations
- Ignores body composition: Muscle and fat are treated the same
- No frame size consideration: Standard formulas do not adjust for small or large builds
- May be unrealistic: For some body types, the calculated ideal is never achievable or healthy
Strengths and Limitations of BMI
Strengths
- Simple and quick: Easy to calculate with just height and weight
- Widely validated: Decades of research linking BMI to health outcomes
- Non-invasive: No special equipment needed
- Free: Can be calculated by anyone
- Good population tool: Effective for assessing health risks across large groups
Limitations
- Ignores body composition: A muscular athlete and an unfit person of the same height and weight have identical BMIs
- No sex distinction: Women naturally have more body fat than men
- Age blind: Does not account for natural changes in body composition with aging
- Ethnicity issues: May not accurately assess health risks across all ethnic groups
- Wide ranges: A "healthy" BMI spans a 35+ pound range for most heights
When BMI and Ideal Weight Disagree
Sometimes your ideal weight falls outside the "healthy" BMI range, or you are at a healthy BMI but not at your ideal weight. Here is how to interpret these situations:
Scenario 1: Ideal Weight Gives a BMI Below 18.5
If a formula suggests an ideal weight that puts your BMI under 18.5, the formula may be too aggressive for your body type. Being underweight carries health risks including weakened immune function, bone loss, and nutritional deficiencies. In this case, aim for the lower end of the healthy BMI range (18.5-20) instead.
Scenario 2: At Healthy BMI but Above Ideal Weight
You might be at a BMI of 23 (healthy range) but above your calculated ideal weight. This is often fine, especially if you:
- Have a larger bone structure
- Carry more muscle mass
- Are older (slightly higher weight may be protective)
- Have healthy blood pressure, blood sugar, and cholesterol
Scenario 3: At Ideal Weight but Overweight BMI
Athletes and very muscular women may hit their ideal weight while showing an "overweight" BMI. If your body fat percentage is healthy (under 31% for women) and your health markers are good, the BMI classification is misleading in your case.
Scenario 4: Both Agree You Are Over or Under
When both measurements point in the same direction, it is a stronger signal that action may be beneficial. If both indicate overweight, focusing on gradual weight loss is likely appropriate. If both indicate underweight, increasing calorie intake deserves attention.
Which Should You Use?
The most practical approach is to use both together:
- Calculate your ideal weight range using our Ideal Weight Calculator - this gives you a specific target based on multiple formulas
- Check that your target falls within a healthy BMI using our BMI Calculator - this confirms your target is medically reasonable
- Consider body composition - if you strength train, accept that you may weigh more than formulas suggest while being healthier
- Measure waist circumference - if it is under 35 inches for women, your weight-related health risk is lower regardless of BMI
Beyond BMI and Ideal Weight: Other Metrics
Neither BMI nor ideal weight tells the whole story. Consider these additional measurements for a complete health picture:
Waist Circumference
Abdominal fat is more strongly linked to health risks than overall weight. For women, a waist over 35 inches indicates elevated risk for heart disease, diabetes, and metabolic syndrome regardless of BMI.
Waist-to-Hip Ratio
Divide your waist measurement by your hip measurement. For women, a ratio above 0.85 suggests increased health risk. This metric captures where you carry weight, which matters more than total weight for predicting health outcomes.
Body Fat Percentage
The most accurate indicator of body composition. For women, healthy ranges are:
- Athletes: 14-20%
- Fitness: 21-24%
- Acceptable: 25-31%
- Obesity: 32%+
Estimate yours with our Body Fat Calculator.
Health Markers
Ultimately, what matters most is your metabolic health:
- Blood pressure
- Fasting blood glucose
- Cholesterol levels (total, HDL, LDL, triglycerides)
- Inflammatory markers
You can have a high BMI and excellent health markers (metabolically healthy obesity) or a normal BMI with poor markers (metabolically unhealthy normal weight). Blood tests tell you more than any weight-based calculation.
Practical Recommendations
- Do not obsess over exact numbers: Both BMI and ideal weight are estimates with significant margins of error
- Think in ranges: A healthy weight zone is more realistic than a single target number
- Focus on trends: Whether you are moving toward or away from healthy ranges matters more than where you are on any given day
- Prioritise habits over numbers: Consistent exercise, balanced nutrition, adequate sleep, and stress management lead to healthy weight naturally
- Get regular check-ups: Blood work and clinical assessments tell you more about health than any calculator
Disclaimer: This article is for informational purposes only and does not constitute medical advice. Consult a healthcare professional for personalised guidance on healthy weight management.
Frequently Asked Questions
What is the main difference between ideal weight and BMI?
Ideal weight gives you a specific target weight in pounds or kilograms based on height and sometimes gender. BMI is a ratio of weight to height that places you in a category (underweight, normal, overweight, obese) rather than giving a single target number.
Which is more accurate: ideal weight or BMI?
Neither is perfectly accurate alone. BMI works well for population-level health assessments but can misclassify muscular individuals. Ideal weight formulas account for gender but not body composition. Using both together with waist circumference or body fat percentage gives the most complete picture.
Can you have a normal BMI but not be at your ideal weight?
Yes. The healthy BMI range of 18.5-24.9 spans a wide weight range. For a 5'5" woman, this covers 111-150 lbs. You could be at the top of the healthy BMI range but above your ideal weight, especially if you carry more fat and less muscle.
Why do doctors still use BMI if it has limitations?
BMI is quick, free, non-invasive, and works well for most of the population. At a public health level, it effectively identifies weight-related health risks. For individuals, doctors use BMI as a starting point alongside other measurements and clinical judgement.
Should I use ideal weight or BMI for my weight loss goal?
Use both. Calculate your ideal weight range as a specific target, then verify it falls within a healthy BMI range of 18.5-24.9. If your ideal weight gives a BMI under 18.5, adjust upward. If it gives a BMI over 25, that may still be healthy if you have significant muscle mass.
