How to Calculate Ideal Body Weight: Formulas and Examples

If you are figuring out ideal body weight, you may find several formulas online that produce slightly different answers. That can make it difficult to know which number to use.

Ideal body weight (IBW) is an estimated weight based mainly on height and, in many formulas, sex. It is useful as a reference point, but it is not a precise measurement of health or a required weight.

What Does Ideal Body Weight Mean?

Ideal body weight is a calculated estimate originally developed for practical and clinical purposes. Several equations have been proposed to estimate a person's desirable or ideal weight from height and sex.

A formula cannot account for every factor that influences a person's healthy weight. Muscle mass, bone structure, body-fat distribution, medical conditions, activity level, and other individual factors can all affect how body weight should be interpreted.

For this reason, calculating ideal body weight is best treated as a mathematical estimate rather than a personal medical prescription.

Common Ideal Body Weight Formulas

Several formulas can be used when figuring ideal body weight. The most commonly encountered include the Devine, Robinson, and Miller formulas.

Devine Formula

The Devine formula is one of the best-known IBW equations. It was published in 1974 and uses height above 5 feet.

For men:

IBW = 50 kg + 2.3 kg × each inch over 5 feet

For women:

IBW = 45.5 kg + 2.3 kg × each inch over 5 feet

Robinson Formula

The Robinson formula uses a different starting value and increment.

For men:

IBW = 52 kg + 1.9 kg × each inch over 5 feet

For women:

IBW = 49 kg + 1.7 kg × each inch over 5 feet

At 5 feet 8 inches:

• Man: 52 + (1.9 × 8) = 67.2 kg
• Woman: 49 + (1.7 × 8) = 62.6 kg

Miller Formula

The Miller formula also uses height above 5 feet.

For men:

IBW = 56.2 kg + 1.41 kg × each inch over 5 feet

For women:

IBW = 53.1 kg + 1.36 kg × each inch over 5 feet

At 5 feet 8 inches:

• Man: 56.2 + (1.41 × 8) = 67.5 kg
• Woman: 53.1 + (1.36 × 8) = 64.0 kg

These formulas demonstrate why there is no single universal ideal body weight.

How Height Is Used in the Calculation

Height is central to most traditional ideal body weight equations.

The formulas generally start with a base weight for someone who is 5 feet tall. Additional weight is then added for every inch above 5 feet.

Using the Devine formula for a man:

50 + (2.3 × 6) = 63.8 kg

For a woman:

45.5 + (2.3 × 6) = 59.3 kg

This calculation shows how the estimated ideal weight increases as height increases.

If height is provided in centimeters, it must first be converted into the units required by the formula. Since 1 inch equals 2.54 cm, a height of 170 cm is approximately 66.9 inches.

However, manually converting centimeters and applying several steps can be inconvenient. A calculator can reduce the chance of arithmetic or unit-conversion errors.

Male and Female Calculations

Traditional IBW formulas usually provide separate equations for men and women.

This is because the equations were developed using assumptions about body size and composition that differ between the sexes.

For example, using the Devine formula, consider two adults who are both 5 feet 10 inches tall. They are 10 inches above 5 feet.

Male calculation:

50 + (2.3 × 10) = 73.0 kg

Female calculation:

45.5 + (2.3 × 10) = 68.5 kg

The difference comes from the formula's sex-specific starting value.

Example: How to Calculate Ideal Body Weight

Suppose you want to figure out ideal weight for a man who is 5 feet 9 inches tall.

Step 1: Find the height above 5 feet

5 feet 9 inches is:

9 inches over 5 feet

Step 2: Apply the Devine formula

For men:

IBW = 50 + (2.3 × inches over 5 feet)

So:

IBW = 50 + (2.3 × 9)

IBW = 70.7 kg

The estimated ideal body weight is therefore approximately 70.7 kg using the Devine formula.

Now compare this with a woman who is also 5 feet 9 inches:

IBW = 45.5 + (2.3 × 9)

IBW = 66.2 kg

Why Do Different Formulas Give Different Results?

If you are figuring out ideal body weight, you may notice that different websites provide different numbers.

This happens because there is no single universally accepted ideal body weight equation.

The Devine, Robinson, and Miller formulas use different:

• Starting weights
• Height increments
• Sex-specific values
• Historical populations and assumptions

For a 5-foot-8-inch man, for example, the estimates are approximately:

Ideal Body Weight vs. Actual Body Weight

Your actual body weight is simply what you weigh at a particular point in time. Ideal body weight is a mathematical estimate.

These numbers serve different purposes.

For example, a person may weigh 80 kg while an IBW formula estimates 68 kg. That does not automatically mean the person is unhealthy or needs to lose 12 kg.

Healthcare professionals may also consider BMI, waist circumference, body composition, medical history, lifestyle, and other health indicators when assessing weight-related health risk.

BMI itself is a screening measure rather than a direct measurement of body fat. The CDC classifies adult BMI from 18.5 to less than 25 as the healthy-weight category, while also recognizing that BMI has limitations.

Limitations of Ideal Body Weight Formulas

Ideal body weight formulas are convenient, but they have important limitations.

They generally do not account directly for:

• Muscle mass
• Bone density
• Body-fat percentage
• Fat distribution
• Fitness level
• Pregnancy
• Certain medical conditions
• Medications
• Individual body-frame differences

A muscular person may weigh more than an IBW estimate while having a relatively low body-fat level.

This is why an ideal body weight calculation should not be used alone to diagnose obesity, determine whether someone is healthy, or set an aggressive weight-loss target.

If you have a medical condition, are pregnant, are under 18, or have unexplained changes in body weight, seek individualized advice from a qualified healthcare professional.

When Should You Use a Calculator Instead of Manual Calculation?

Manual calculation is useful for understanding the formula, while an ideal body weight calculator is convenient for a quick estimate.

A calculator is especially useful when:

• Your height is in centimeters rather than feet and inches.
• You want to avoid unit conversions.
• You want a result quickly.
• You want to compare results consistently.
• You are checking several different heights.
• You want to reduce arithmetic errors.

You can use the Howtigo Ideal Body Weight Calculator to enter your height and obtain an estimated result without calculating the formula manually.

Summary

If you are figuring ideal body weight, several established formulas can provide an estimate. The Devine formula is one of the most widely encountered, while Robinson and Miller are other commonly referenced equations.

The key points are:

• Ideal body weight is an estimate, not a perfect target.
• Traditional formulas use height and usually sex.
• Height above 5 feet is used to calculate additional weight in several common equations.
• Different formulas can produce different results.
• Actual body weight and ideal body weight are not the same thing.
• Muscle mass and body composition can make a simple IBW estimate misleading.
• BMI and other health measures provide additional context.
• A calculator is convenient when you want a quick, consistent result.

Calculate Your Ideal Body Weight with the Howtigo Ideal Body Weight Calculator

References

Centers for Disease Control and Prevention. (2024). Adult BMI categories. https://www.cdc.gov/bmi/adult-calculator/bmi-categories.html

Devine, B. J. (1974). Gentamicin therapy. Drug Intelligence & Clinical Pharmacy, 8(11), 650–655. https://doi.org/10.1177/106002807400801005

Robinson, J. D., Lupkiewicz, S. M., Palenik, L., Lopez, L. M., & Ariet, M. (1983). Determination of ideal body weight for drug dosage calculations. American Journal of Hospital Pharmacy, 40(6), 1016–1019.