Different ideal weight formulas give different results because each equation starts with a different reference weight and adds a different amount for every inch above 5 feet. These formulas were developed as height-based reference tools, not as universal definitions of the healthiest weight for every person. Their results are usually best treated as estimates to compare rather than precise targets.
Why ideal weight formulas produce different estimates
Common formulas use the same basic information—height and a sex category—but apply different numerical assumptions. Devine, Robinson and Miller each assign their own starting weight at 5 feet and their own adjustment for additional height.
As a result, two formulas can process the same height correctly and still return different answers. The difference is not necessarily an error. It reflects how each formula was constructed.
These equations also simplify body weight into a relationship with height. They do not directly assess muscle mass, body fat, bone structure, health history or individual weight preferences.
How the Devine, Robinson and Miller formulas differ
For adults who are at least 5 feet tall, the three formulas use a base weight at 5 feet and add weight for every inch above that point. The calculator reports the results in kilograms, with unit conversions available where applicable.
| Formula | Male equation | Female equation |
|---|---|---|
| Devine | 50 kg + 2.3 kg per inch over 5 ft | 45.5 kg + 2.3 kg per inch over 5 ft |
| Robinson | 52 kg + 1.9 kg per inch over 5 ft | 49 kg + 1.7 kg per inch over 5 ft |
| Miller | 56.2 kg + 1.41 kg per inch over 5 ft | 53.1 kg + 1.36 kg per inch over 5 ft |
The base weights and height adjustments explain most of the variation. For example, Miller begins with a higher reference weight at 5 feet but adds less weight for each extra inch. Devine begins lower but uses a larger increase per inch.
The formulas may therefore be close at one height and farther apart at another. No single equation becomes automatically “correct” simply because it returns the highest, lowest or middle estimate.
A practical example for a 5 ft 10 in adult
Consider a male who is 5 feet 10 inches tall. This height is 10 inches above the 5-foot starting point used by the formulas.
| Formula | Calculation | Estimated reference weight |
|---|---|---|
| Devine | 50 + (2.3 × 10) | 73.0 kg |
| Robinson | 52 + (1.9 × 10) | 71.0 kg |
| Miller | 56.2 + (1.41 × 10) | 70.3 kg |
| Average | (73.0 + 71.0 + 70.3) ÷ 3 | About 71.4 kg |
All three results are mathematically consistent with their equations, but they are not identical. The average provides a convenient summary of the formulas. It does not make the result a measured or medically determined ideal weight.
Rounding and unit conversion can also create small differences between calculators. A result converted from kilograms to pounds may vary slightly depending on when rounding is applied.
Reference weight, healthy weight range and target weight are not the same
A formula result is a single height-based reference estimate. A healthy weight range is broader and may be presented using another screening method, such as body mass index, or discussed in the context of a person’s health and circumstances.
The CDC describes adult BMI as a screening measure based on weight relative to height and places results into BMI categories. BMI is not the same as an ideal-weight formula, even though both use height and weight-related calculations.
A target weight is a planning value. It may reflect current weight, personal preferences, body composition, health considerations and what is practical to maintain. It does not need to match the exact output of a historical formula.
For someone with substantial muscle mass, a formula may provide limited context because it cannot distinguish muscle from fat. The same limitation applies when two people have the same height but different frames, fitness levels or body composition.
What the ideal weight calculator can and cannot tell you
The ideal weight calculator applies the Devine, Robinson and Miller equations to adults who are 5 feet or taller. It displays each estimate separately and calculates their average.
What it can tell you
- How three commonly cited height-based equations compare.
- The reference weight produced by each formula.
- The average of the three estimates.
- How changes in height affect the formula outputs.
What it cannot tell you
- Your measured body-fat percentage or muscle mass.
- Whether a particular weight is healthiest for you.
- How frame size, ethnicity, medical history or fitness level may affect personal weight considerations.
- Which target will be comfortable, practical or sustainable.
- Whether a change in weight is medically appropriate.
The output is therefore a reference point rather than an assessment. The average is especially easy to misinterpret: it summarises three equations, but it does not remove their shared limitations.
How to interpret several different results
Start by looking at the spread between the estimates. If the results are close, that shows the formulas make broadly similar height-based predictions for that input. It does not prove that the middle value is your personal optimum.
Useful ways to interpret the results include:
- Keep each number labelled as an estimate rather than a measured value.
- Review the separate formulas instead of relying only on the average.
- Compare the estimates with a broader healthy weight range rather than treating one number as a strict boundary.
- Consider factors the formulas leave out, including body composition, health history and personal preference.
- Use a target weight as a flexible planning value, not as a test of success or health.
Greater precision in the display does not mean greater personal accuracy. A result such as 71.4 kg comes from arithmetic, but the underlying equation remains a simplified estimate.
Frequently asked questions
Which ideal weight formula is the most accurate?
There is no universally most accurate formula for every individual. Devine, Robinson and Miller are all simplified height-based reference equations, and none directly measures body composition or personal health.
Why does the calculator show an average?
The average provides one summary of the three results. It can make comparison easier, but it is not a separate clinical standard and should not be assumed to be more individually accurate.
Can two people of the same height have different suitable weights?
Yes. People of the same height can differ in muscle mass, frame, body composition, medical history, fitness level and preferences. The formulas do not incorporate these differences.
Do these formulas work for adults shorter than 5 feet?
The equations described here are used by this calculator for adults who are 5 feet or taller. Applying them below their stated starting height would require subtracting the per-inch adjustment and may not be supported by the tool.
Should a target weight match the formula average?
Not necessarily. The average can be one reference point, while a target weight is a personal planning value that may take account of broader health information, present circumstances and preferences.
Ultimately, different ideal weight formulas give different answers because their base values and height adjustments differ. Reading the results as nearby historical reference estimates—rather than competing claims about one perfect body weight—provides the clearest interpretation.