Ideal Body Weight (IBW) Calculator

What Is Ideal Body Weight?

Ideal body weight (IBW) is a reference weight calculated from a person's height and sex using a published equation. It answers a narrow question: given this height and sex, what reference weight do standard formulas produce? That is the entire job of the number.

The word “ideal” is a historical label, and it misleads many readers. IBW was never developed as an appearance standard or a personal weight goal. B.J. Devine published his formula in 1974 to standardize gentamicin dosing — the formula gave pharmacists a consistent reference weight so that drug doses could be compared across patients of different sizes. G.J. Hamwi's 1964 formula came from diabetes care, where dietitians needed a quick reference for estimating caloric needs. In both cases, the formula served a calculation, not a judgment about how someone should look.

That origin explains a key property: two people of the same height and sex receive the same IBW even when one is heavily muscled and the other is not. The formulas know nothing about body composition, frame size, age, or fitness. Clinicians accept that limitation because the number is used as a dosing input, where consistency matters more than individual body description. Some critical-care references call the same quantity “predicted body weight” to avoid the misleading word “ideal” altogether.

The 4 IBW Formulas Compared

All four formulas share one structure: a base weight at 5 feet (60 inches / 152.4 cm), plus or minus a fixed amount for every inch above or below 5 feet. For heights under 5 feet, the per-inch amount is subtracted — an extrapolation the original authors never validated, which is why this calculator flags it. The formulas differ only in the base weight and the per-inch increment, and the male and female versions use different constants.

Hamwi's original statement used pounds: 106 lb + 6 lb per inch over 5 ft for men, 100 lb + 5 lb per inch over 5 ft for women. Converted at 2.20462 lb/kg, those equal roughly 48.1 kg + 2.72 kg/in and 45.4 kg + 2.27 kg/in, which is why rounded metric tables show 48 + 2.7 and 45.5 + 2.2. Robinson and Miller published in the same year (1983) as competing refinements of Devine; Miller chose a higher base weight with a smaller per-inch slope, so the two formulas agree near 5 feet and diverge at taller heights.

For a 5'10" man, the four results span 70.3 to 75.0 kg — a spread of under 5 kg that is normal and expected. The comparison table in the calculator above shows this spread for any height, which is precisely the point: no single formula is accepted as the one correct answer, and the range itself is the useful information. Clinicians evaluating related size-based metrics often check a lean body mass calculator alongside IBW, since lean mass — not total weight — drives the dosing of several drug classes.

Clinical Uses of IBW

IBW exists because several clinical calculations need a standardized weight — a number that is reproducible for a given height and sex, independent of how much a particular patient weighs today. Three areas of practice use it most.

Drug Dosing Adjustments

Devine's formula was published specifically for gentamicin, an aminoglycoside antibiotic with a narrow therapeutic window. Clinical pathways for aminoglycosides still reference IBW in their dosing calculations, particularly when a patient's actual weight differs substantially from the reference weight. For other weight-based therapies — including some chemotherapy protocols and anticoagulants — institutional protocols variously reference actual body weight, ideal body weight, or adjusted body weight (IBW plus a fraction of the excess weight), depending on how the drug distributes in tissue. The choice of weight scalar is a clinical decision made within those protocols, not something a calculator determines.

Mechanical Ventilation: The ARDSNet Trial

The most consequential use of an IBW-derived number comes from critical care. The ARDSNet trial (New England Journal of Medicine, 2000) randomized patients with acute lung injury and acute respiratory distress syndrome (ARDS) to traditional tidal volumes (12 mL per kg of predicted body weight) or lower tidal volumes (6 mL per kg of predicted body weight). Predicted body weight in the trial was calculated with Devine-based equations: 50 + 0.91 × (height in cm − 152.4) for men and 45.5 + 0.91 × (height in cm − 152.4) for women — the 0.91 is simply Devine's 2.3 kg per inch converted to centimetres.

The result changed ventilator practice worldwide: hospital mortality was 31.0% with 6 mL/kg versus 39.8% with 12 mL/kg, a relative reduction of about 22%. Lung-protective ventilation at 6 mL/kg of predicted body weight remains a standard of care in ARDS, and it is one of the clearest demonstrations that the weight used for dosing should reflect lung size (which scales with height) rather than total body weight.

Contrast Media Dosing

Some radiology protocols scale iodinated contrast dose to lean or ideal body weight rather than total body weight, particularly in patients with obesity, to limit contrast load while preserving image quality. As with drug dosing, the specific weight scalar follows institutional protocol.

IBW vs BMI vs Body Fat Percentage

Three numbers are often confused because all involve weight and all are expressed relative to the body. They answer different questions, use different inputs, and serve different purposes. Conflating them is one of the most common misunderstandings in this topic.

A concrete example shows why the distinction matters. A 5'10" man has a Devine IBW of 73 kg. If he weighs 95 kg with substantial muscle mass, his BMI is 29.3 (in the overweight range) and his IBW-based “excess” is 22 kg — yet his body fat percentage might be a lean 15%. IBW and BMI both describe size; only body fat percentage describes composition. For general health screening, clinicians generally start from BMI and, where composition matters, from a body fat percentage estimate. The medical calculators hub groups these related tools together.

Limitations of IBW

The formulas are simple by design, and that simplicity creates well-documented blind spots. Clinical pathways account for these by pairing IBW with other measures or by switching to adjusted body weight. The main limitations:

• Muscular builds. IBW cannot distinguish muscle from fat. A strength athlete at 5'10" may weigh 95 kg with low body fat while the formula returns 73 kg — the 22 kg “excess” is lean tissue the formula was never designed to see. Dosing protocols that reference IBW in muscular patients adjust accordingly rather than treating the gap as adiposity.
• Older adults. The formulas were derived from younger adult populations and use standing height, which decreases with age through disc compression and kyphosis. Sarcopenia (age-related muscle loss) further changes what a given weight means. Clinical pathways in geriatric dosing interpret IBW alongside frailty and renal function rather than in isolation.
• Amputees. A missing limb removes mass the formula assumes is present. Published segment tables (e.g., Osterkamp, Journal of the American Dietetic Association 1995) assign approximate proportions of body weight to each segment — roughly 16% for a full leg, 5% for a full arm, with hand, forearm, foot, and lower leg broken out separately — so that an adjusted reference weight can be calculated. The segment percentages vary slightly between published tables.
• Pregnancy. IBW does not apply during pregnancy. Gestational weight gain is expected and physiologically normal, and dosing in pregnancy follows obstetric-specific guidance rather than standard IBW equations.
• Children and adolescents. None of the four formulas was derived or validated in pediatric populations. Pediatric dosing uses weight-based (mg/kg of actual weight) or body-surface-area-based methods instead.
• Obesity. When actual weight substantially exceeds IBW, dosing on total body weight may overestimate the dose for drugs that distribute poorly into adipose tissue, while dosing on IBW alone may underestimate it. Clinical pathways commonly use adjusted body weight as a middle ground:

The 0.4 correction factor is the most widely used, though institutional protocols vary and the drugs to which adjusted body weight applies are protocol-specific. Where dosing depends on body surface area rather than weight alone, clinicians use a body surface area calculator (for example, the Du Bois equation) as the complementary tool.

Worked Example

Profile: a man, 5 feet 10 inches tall. Walk through the Devine formula step by step.

Step 1 — convert height to inches. 5 feet × 12 = 60 inches, plus 10 inches = 70 inches.

Step 2 — find inches over 5 feet. 70 − 60 = 10 inches over the 5-foot baseline.

Step 3 — apply the Devine equation for men. IBW = 50 + 2.3 × 10 = 50 + 23 = 73.0 kg.

Step 4 — convert to pounds. 73.0 × 2.20462 = 160.9, so ≈ 161 lb.

The same height and sex through all four formulas shows the expected spread:

The four estimates span 70.3 to 75.0 kg (155 to 165 lb). Enter 5 ft 10 in in the calculator above — or choose the “5'10" male” preset — to reproduce every number in this table, including the inline breakdown line and the range gauge.

Frequently Asked Questions

Clinical Sources & Citations

1. Devine BJ. Gentamicin therapy. Drug Intelligence & Clinical Pharmacy. 1974;8:650-655. (Original Devine IBW equation for drug dosing.)
2. Robinson JD, Lupkiewicz SM, Palenik L, et al. Determination of ideal body weight for drug dosage calculations. Am J Hosp Pharm. 1983;40(6):1016-1019.
3. Miller DR, Hayes KC, Duffy TC. Personal communication, 1983. (As cited in Pai MP, Paloucek FP. The origin of the “ideal” body weight equations. Ann Pharmacother. 2000;34(9):1066-1069. doi:10.1345/aph.19364)
4. Hamwi GJ. Therapy: changing dietary concepts. In: Danowski TS, ed. Diabetes Mellitus: Diagnosis and Treatment. New York, NY: American Diabetes Association; 1964:73-78.
5. The Acute Respiratory Distress Syndrome Network. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med. 2000;342(18):1301-1308. doi:10.1056/NEJM200005043421801
6. Pai MP, Paloucek FP. The origin of the “ideal” body weight equations. Ann Pharmacother. 2000;34(9):1066-1069. (Review of the derivation and history of all four equations.)

Editorial review statement: This page was reviewed by the Heart Score Calculator editorial team in October 2026 for formula accuracy, citation integrity, and readability. Content is updated when guidelines change.

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