Ideal Weight Calculator

Ideal Weight Calculator: Four Clinical Formulas, One Diagnostic Picture

TL;DR: The four standard ideal body weight formulas (Devine, 1974; Robinson, 1983; Miller, 1983; Hamwi, 1964) produce outputs that differ by up to 7 kg for the same person at the same height. All four were designed for clinical drug dosing, not personal fitness targets. For a 170 cm woman, the formula range spans 69.3 kg (Robinson) to 73.0 kg (Devine). For a 175 cm man, it spans 73.4 kg (Robinson) to 78.1 kg (Hamwi). This calculator runs all four simultaneously so you can see where they converge and where they diverge.

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Table of Contents

• What These Four Formulas Were Actually Designed to Do
• Six Groups Who Benefit Most From Running All Four Formulas
• The Four Formulas, Fully Explained
• How to Read Your Results in Five Steps
• Two IBW Calculations, Fully Worked
• Five Mistakes When Interpreting Formula Outputs
• FAQ
• Assumptions and Notes
• One Number Is Not Enough
• Further Reading

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What These Four Formulas Were Actually Designed to Do

In 1964, Dr. Grant Hamwi published the first widely adopted ideal body weight formula in the context of diabetes management and drug dosing. A decade later, Dr. B. J. Devine adapted the approach for pharmacokinetic calculations — specifically for gentamicin, an antibiotic whose toxicity at high doses made precise weight-based dosing critical. Both researchers were solving a clinical problem: certain drugs distribute through lean body mass rather than total body weight, and patients carrying significant fat mass above their lean weight were being dosed incorrectly.

Robinson (1983) and Miller (1983) each published modifications of the Devine formula independently in the same year, using different regression analyses of population data. Both produced slightly different base weights and per-inch increments. Neither was validated against long-term health outcomes.

This history matters for one practical reason: none of the four formulas answers the question "what weight will make me healthy?" They answer a narrower question: "what weight approximates lean body mass for drug dosing at a given height and sex?" Applying them to personal fitness targets requires understanding what they measure and what they do not.

What they do not measure: muscle mass, bone density, age beyond early adulthood, frame size, body fat distribution, or activity level. Two people at the same height, such as a 40-year-old marathon runner and a 40-year-old sedentary office worker, receive identical formula outputs despite having meaningfully different body compositions and different optimal weights.

Enter your height and sex above to see where all four formulas place you.

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Six Groups Who Benefit Most From Running All Four Formulas

• You are a patient or caregiver verifying a clinically assigned weight target. Many weight management programmes and hospital discharge plans assign target weights without stating which formula was used. Running all four formulas reveals the likely source: if your target is 68 kg at 165 cm female, it aligns with the Devine output of 67.5 kg. Knowing the derivation helps you assess whether the formula is appropriate for your body type.

• You want a height-based reference point before starting a structured diet. A formula cluster gives a long-range reference independent of current weight. If your formula cluster for 168 cm male is 68–76 kg and you currently weigh 95 kg, you know the distance to the cluster midpoint (approximately 25 kg) without needing a body composition scan. The cluster anchors long-range planning even when current weight and goal weight are far apart.

• You have been given IBW-based medication dosing and want to verify the figure. Aminoglycoside antibiotics, vancomycin, and certain weight-based chemotherapy regimens are dosed on IBW, specifically using the Devine formula in most protocols. Running the calculator confirms the Devine output for your height and sex, and checking it against current weight identifies whether you are within, above, or significantly above the dosing reference weight.

• You are a fitness professional assessing a client's starting point. For a personal trainer or dietitian, the four-formula cluster provides a clinical reference point that frames a client's current weight in relation to established benchmarks. A client at 88 kg and 172 cm male whose formula cluster is 68–76 kg is clearly above the clinical reference range; a client at 78 kg with the same measurements is within or just above it. The cluster contextualises the client's situation without implying a single prescriptive target.

• You are tracking weight loss progress and want a fixed reference that does not change with body weight. Unlike TDEE or BMI, formula IBW is a fixed number for a given height and sex. Using it as a long-range anchor while tracking percentage progress from current weight toward the formula cluster gives a stable reference point throughout a multi-month fat loss programme.

• You are comparing your weight to a family member of similar height and want an objective reference. Body weight comparisons between individuals are often distorted by differences in starting weight, activity history, and frame size. The four-formula cluster provides a shared, height-based reference that makes the comparison more objective, though it does not account for muscle mass or frame size (the primary limitation described in the mistakes section below).

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The Four Formulas, Fully Explained

All four formulas share the same structure: a base weight at exactly 5 feet (152.4 cm), plus an increment per inch of height above that baseline. They differ in the base weight and the per-inch increment, which is why their outputs diverge more at taller and shorter heights.

All formulas use height in inches above 60 (5 feet).
Convert: height_in = total height in inches
         inches_over_60 = height_in − 60

DEVINE (1974) — most clinically validated; standard for drug dosing protocols
  Male:   IBW = 50.0 + 2.3 × (height_in − 60)    kg
  Female: IBW = 45.5 + 2.3 × (height_in − 60)    kg

ROBINSON (1983) — modification of Devine; smallest per-inch increment
  Male:   IBW = 52.0 + 1.9 × (height_in − 60)    kg
  Female: IBW = 49.0 + 1.7 × (height_in − 60)    kg

MILLER (1983) — modification of Devine; smallest base weight for men
  Male:   IBW = 56.2 + 1.41 × (height_in − 60)   kg
  Female: IBW = 53.1 + 1.36 × (height_in − 60)   kg

HAMWI (1964) — oldest formula; largest per-inch increment for men
  Male:   IBW = 48.0 + 2.7 × (height_in − 60)    kg
  Female: IBW = 45.5 + 2.2 × (height_in − 60)    kg

Healthy BMI range for reference:
  Lower bound: 18.5 × height(m)²
  Upper bound: 24.9 × height(m)²

Formula Outputs at Common Heights: Women (kg)

Formula Outputs at Common Heights: Men (kg)

The spread between the lowest and highest formula output at any given height is a meaningful diagnostic signal. At 165 cm female, the spread is 3.3 kg (55.8–59.1 kg), indicating good formula agreement. At 170 cm female, the spread widens to 3.7 kg (69.3–73.0 kg). For men at 180 cm, the spread reaches 6.1 kg (77.9–84.0 kg) — the Hamwi formula's larger per-inch increment for men produces this divergence at taller heights. When the spread exceeds 5 kg, the formula cluster midpoint is more reliable than any individual formula output.

Genetic variation affecting body composition adds a layer of variability the formulas cannot capture. Variants in the LEPR gene (leptin receptor) affect fat storage set-points; variants in the ACTN3 gene (present in homozygous form in approximately 18% of the population) reduce fast-twitch muscle fibre expression, producing a lean body mass approximately 2–4 kg lower than average at the same height and sex. Both effects are invisible to height-and-sex formulas and contribute to the real-world variability around any formula output.

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How to Read Your Results in Five Steps

1. Read the four formula outputs together, not individually. The calculator produces four numbers; the relevant information is their range, midpoint, and agreement. If all four fall within 3 kg of each other, the cluster midpoint is a credible clinical reference. If the Hamwi output exceeds the others by 4 kg or more (common in taller men), note it as an outlier and focus on the Devine-Robinson-Miller cluster.

2. Compare the formula cluster to the healthy BMI range for your height. All formula outputs should fall within the healthy BMI range (18.5–24.9) for your height. If they do, the formulas and the BMI framework agree. If any formula output falls below your BMI lower bound (which can happen for shorter women with the Robinson formula), that output is producing an underweight target and should be disregarded.

3. Locate your current weight on the same scale. The most useful way to read the results is: current weight relative to the formula cluster and the BMI range together. Are you above the BMI upper bound (above 24.9 BMI)? Within the range but above the formula cluster? Within the formula cluster itself? Below it? Each position implies a different relationship to the clinical benchmarks.

4. Identify whether the formula outputs are relevant to your body type. If you have above-average muscle mass from consistent resistance training, calculate your lean body mass: weight × (1 − body fat percentage). If your lean body mass exceeds or approaches any formula output, that formula is producing a target below your lean mass baseline and is not a meaningful guide. Use the healthy BMI upper bound (24.9 × height(m)²) as your reference instead.

5. Note the formula-specific context for any clinical use. If you are checking IBW for medication dosing verification, the Devine formula is the correct clinical reference, not the average of all four. For personal health planning, the cluster midpoint is more useful than any single formula. For older adults (65+), the relevant comparison range shifts upward to BMI 23–27, making the formula upper bounds more appropriate references than the cluster midpoints.

Non-obvious insight: The Robinson and Miller formulas produce notably different results from each other despite both being described as "modifications of Devine." Robinson uses a smaller per-inch increment (1.9 kg for men vs. Devine's 2.3 kg) but a higher base weight (52 kg vs. 50 kg for men). Miller uses an even smaller increment (1.41 kg) but the highest base weight (56.2 kg for men). The crossover point where Robinson and Miller outputs equal each other is at approximately 180 cm for men and 172 cm for women. Below these heights Miller produces higher outputs; above them Robinson produces higher outputs. This is why the formula spread widens at the extremes of height.

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Two IBW Calculations, Fully Worked

Example 1: Woman Beginning a Weight Management Programme After Doctor's Referral, Age 41

Saoirse is 161 cm, weighs 79 kg, and has been told by her GP to "reach a healthier weight." She wants to understand what the four formulas say before setting a target.

Height: 161 cm = 63.4 inches. Inches over 60: 3.4

Devine:   45.5 + 2.3 × 3.4 = 45.5 + 7.8  = 53.3 kg
Robinson: 49.0 + 1.7 × 3.4 = 49.0 + 5.8  = 54.8 kg
Miller:   53.1 + 1.36 × 3.4 = 53.1 + 4.6 = 57.7 kg
Hamwi:    45.5 + 2.2 × 3.4 = 45.5 + 7.5  = 53.0 kg

Formula range:    53.0–57.7 kg
Cluster midpoint: 54.7 kg
Formula spread:   4.7 kg

Healthy BMI range at 161 cm (1.61 m):
  Lower: 18.5 × 1.61² = 48.0 kg
  Upper: 24.9 × 1.61² = 64.6 kg

Current weight: 79 kg (BMI 30.5 — above healthy range)
Distance to BMI upper bound: 14.4 kg
Distance to cluster midpoint: 24.3 kg

Saoirse's formula cluster (53.0–57.7 kg) sits in the lower third of her healthy BMI range. Her GP's instruction to "reach a healthier weight" is best interpreted as reaching the BMI upper bound of 64.6 kg first, a 14.4 kg reduction that brings her within the healthy BMI range and crosses the 10% clinical benefit threshold. The formula cluster midpoint at 54.7 kg is 24.3 kg away and represents a longer-range reference, not a first-step target. For practical planning, her first milestone is 73 kg (5% reduction from 79 kg), her second is 71 kg (10% reduction), and her entry into the healthy BMI range is 64.6 kg.

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Example 2: Man Preparing for an Occupational Medical Weight Assessment, Age 34

Levi is 183 cm, weighs 93 kg, and has an upcoming medical assessment for a civil service role that references IBW standards. He wants to know his formula outputs in advance.

Height: 183 cm = 72.0 inches. Inches over 60: 12.0

Devine:   50.0 + 2.3 × 12.0 = 50.0 + 27.6 = 77.6 kg
Robinson: 52.0 + 1.9 × 12.0 = 52.0 + 22.8 = 74.8 kg
Miller:   56.2 + 1.41 × 12.0 = 56.2 + 16.9 = 73.1 kg
Hamwi:    48.0 + 2.7 × 12.0 = 48.0 + 32.4 = 80.4 kg

Formula range:    73.1–80.4 kg
Cluster midpoint: 76.5 kg
Formula spread:   7.3 kg (wide — Hamwi outlier)
Cluster ex-Hamwi: 73.1–77.6 kg, midpoint 75.2 kg

Healthy BMI range at 183 cm (1.83 m):
  Lower: 18.5 × 1.83² = 62.0 kg
  Upper: 24.9 × 1.83² = 83.5 kg

Current weight: 93 kg (BMI 27.8 — above healthy range)
Distance to BMI upper bound: 9.5 kg
Distance to cluster midpoint: 16.5 kg

At 183 cm, Levi's Hamwi output (80.4 kg) sits 7.3 kg above Miller (73.1 kg) — a spread wide enough to treat Hamwi as a high outlier at this height. His current weight of 93 kg is 9.5 kg above his healthy BMI upper bound but within a range where body composition matters significantly. Before any assessment, Levi should check whether the civil service standard references a specific formula. If Devine is used (the most common clinical choice), his assessment reference weight is 77.6 kg, putting him 15.4 kg above it. If the standard is BMI-based (below 24.9), reaching 83.5 kg would meet the requirement — a more achievable near-term target.

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Five Mistakes When Interpreting Formula Outputs

Selecting one formula as "the right one" and ignoring the others. The Devine formula is the most clinically validated for pharmacological purposes, but this does not make it the most accurate for any individual's personal health weight. Robinson produces consistently lower outputs; Hamwi produces higher outputs for tall men. Using only one formula without checking the cluster midpoint introduces unnecessary imprecision. The four-formula cluster is more informative than any single output.

Assuming the formula output is a target rather than a reference. A formula IBW is a clinical reference derived from population statistics, not a prescribed target for any individual. Being above the formula cluster does not mean you are unhealthy; being at the cluster midpoint does not mean you are healthy. A 175 cm man at 82 kg with 14% body fat is in excellent health despite sitting above his Devine IBW of 76.9 kg. The formula does not know he has 70.5 kg of lean mass.

Ignoring the formula outputs when they fall below the BMI lower bound. For shorter women, the Robinson and Devine formulas can produce outputs that fall below BMI 18.5. At 152 cm, the Robinson formula output for a woman is 49.0 kg (the base weight with zero inches over 60), which corresponds to BMI 21.2, within the healthy range. At 155 cm, Robinson gives 55.8 kg (BMI 23.2). The formulas hold within the BMI healthy range for most adult heights, but checking this against the BMI lower bound is a necessary verification step.

Applying adult IBW formulas to adolescents or children. All four formulas are validated for adults, typically those 18 years and older. Children and adolescents have fundamentally different body composition trajectories depending on growth stage and pubertal status. The CDC BMI-for-age percentile charts are the appropriate reference for under-18s; adult IBW formulas applied to a 15-year-old produce incorrect and potentially harmful outputs.

Not considering how the formula spread changes interpretation at extreme heights. At 170 cm, the four-formula spread for men is 3.2 kg (68.9–72.1 kg), tight enough that any formula gives a reliable reference. At 190 cm, the spread widens to over 9 kg (Robinson 82.0 kg vs. Hamwi 94.8 kg), at which point the formula cluster is so wide that only the Devine output (88.5 kg) and the healthy BMI range (66.9–90.0 kg) are reliable guides. For people at the taller end of the height distribution, formula agreement deteriorates and the BMI range becomes the more useful primary reference.

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Assumptions and Notes

• Margin of error: All four IBW formulas assume average skeletal mass and muscle-to-fat ratios for a given height and sex. They are most accurate for adults of typical body composition and least accurate for highly muscular individuals, very short or very tall adults (outside 155–185 cm), older adults (for whom BMI 23–27 is a more appropriate reference than the formula cluster midpoint), and individuals with conditions affecting body composition such as oedema or sarcopenia. The healthy BMI range displayed alongside the formula outputs is based on WHO standards and uses the standard population range of 18.5–24.9; South and East Asian populations have a recommended lower action threshold of BMI 23.
• Professional disclaimer: IBW outputs from this calculator are for informational and reference purposes only and do not constitute medical or dietary advice. Drug dosing that uses IBW should be calculated by a qualified healthcare professional using the Devine formula specifically. Weight management targets for clinical conditions should be set in consultation with a physician or registered dietitian.

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One Number Is Not Enough

Saoirse's four formula outputs spanned 4.7 kg. Levi's spanned 7.3 kg. In both cases, a single formula would have given a false impression of precision — the impression that there is one correct answer at a given height and sex. There is not.

What the cluster gives instead is a range: a clinically derived window that, combined with the healthy BMI range, tells you where multiple independent references converge. Where they converge is the most credible reference. Where they diverge widely is a signal to weight individual factors — frame size, muscle mass, age — more heavily than any formula output.

Enter your height and sex above to see where all four formulas land for your measurements.

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Further Reading

• Goal Weight Calculator: Add Frame Size to Your IBW Reference and Set a Personalised Target Weight Range
• Weight Loss Calculator: Convert Your Formula IBW Into a Calorie Deficit Plan and Timeline
• BMI Calculator: Cross-Reference Your IBW Formula Outputs Against the Full BMI Classification Scale