Ideal Weight Calculator

The 5 formulas — where they came from

Ideal body weight (IBW) formulas have a history rooted firmly in clinical pharmacology, not nutrition or fitness. The oldest and still most-cited is the Hamwi formula (1964), developed by George Hamwi as a bedside rule of thumb for estimating weight in hospitalised patients before standardised height-weight tables were widely available. Hamwi's baseline — 48 kg (106 lbs) for a 5-foot male, 45.5 kg (100 lbs) for a 5-foot female — was chosen purely from clinical observation, not from regression analysis on population data.

The most clinically dominant formula arrived a decade later. In 1974, B.J. Devine published a paper in Drug Intelligence and Clinical Pharmacy specifically to estimate creatinine clearance for aminoglycoside dosing — gentamicin in particular. His formula was never validated against population health outcomes; it was a pharmacokinetic convenience. Yet it spread throughout clinical practice and remains the formula most pharmacists and intensivists use today, including for ventilator tidal-volume targets in the landmark ARDSNet ARDS protocol (6 ml per kg of IBW).

Robinson (1983) and Miller (1983) were published the same year, both as regression analyses on NHANES population data attempting to produce more statistically grounded estimates than Devine. Robinson produces the most conservative (lowest) IBW; Miller the highest of the four formula-based methods. The fifth method, BMI 22, simply back-calculates weight from the midpoint of the "healthy" BMI range (18.5–24.9) — a transparent but ethnicity-sensitive approach.

They all disagree by 5–15 lbs — and that's fine

For a 5'7" male (170.2 cm), the five formulas produce results spanning roughly 67 kg to 73 kg — a spread of about 6 kg (13 lbs). For a 5'4" female, the spread is similar. This disagreement is not a sign of scientific failure; it reflects the fact that "ideal" body weight has no single biological definition. Each formula was calibrated on a different population, for a different clinical purpose, and with different regression constraints.

The consensus average shown in the calculator is the mean of all five results. In the absence of a specific clinical indication (such as aminoglycoside dosing, where Devine is specified by most pharmacy protocols), the average gives a defensible central estimate that is less sensitive to the quirks of any single formula. Think of the range rather than fixating on any single number.

Why doctors use ideal body weight (drug dosing, ventilators — NOT diet goals)

IBW entered clinical medicine as a solution to a pharmacokinetic problem: many drugs distribute primarily in lean tissue (fat-free mass), not adipose tissue. For an obese patient, dosing by total body weight would overdose based on a compartment the drug barely enters. Aminoglycosides (gentamicin, tobramycin, amikacin), digoxin, and several chemotherapy agents use IBW or adjusted body weight (ABW = IBW + 0.4 × (actual − IBW)) to normalise dosing across body sizes.

The ARDSNet trial (NEJM, 2000) established that ventilating acute respiratory distress syndrome patients at 6 ml/kg of predicted body weight (equivalent to IBW by Devine) rather than actual body weight dramatically reduced mortality. A 100 kg obese patient with the lungs of a 70 kg person cannot safely receive tidal volumes calculated on 100 kg — this is one of the most clinically consequential applications of IBW. Outside of these pharmacological and mechanical ventilation contexts, IBW is rarely a meaningful metric in modern clinical care.

The healthier framing — body composition + waist circumference > a number

Scale weight — whether compared to IBW, BMI, or any other table — conflates muscle, fat, bone, and water into a single number that says almost nothing about health risk on its own. Two people at identical IBW can have radically different body fat percentages, insulin sensitivity, and cardiovascular fitness. The metrics with the strongest independent associations with metabolic health outcomes in the literature are:

• Waist circumference: Men below 94 cm, women below 80 cm — WHO thresholds for low metabolic risk. Waist-to-height ratio below 0.5 is an even stronger predictor.
• Body fat percentage: Estimated via DEXA, bioelectrical impedance, or skinfold calipers. Healthy ranges: men 10–20%, women 18–28% (varies by age).
• Cardiorespiratory fitness (VO2 max): One of the strongest independent predictors of all-cause mortality — stronger than BMI, blood pressure, or cholesterol in some analyses.
• Fasting glucose and HbA1c: More directly linked to metabolic disease than body weight.

What to do if your scale is far from ideal weight

If your current weight is 20–40 kg above your calculated IBW, treat the gap as context, not an overnight target. A clinically meaningful and achievable first milestone is 5–10% body weight reduction. Research consistently shows that losing 5–10% of body weight significantly improves blood pressure, fasting glucose, triglycerides, sleep apnoea severity, and joint load — independent of whether it brings you anywhere near IBW.

At a sustained 500 kcal/day deficit, accounting for metabolic adaptation (which typically reduces actual losses to 60–75% of theoretical over 12+ weeks), a realistic rate is 0.3–0.4 kg per week, or roughly 15–20 kg per year. Staged goals of 5–10% reduction every 3–6 months, with maintenance phases between, produce better long-term outcomes than aggressive linear cuts aimed at IBW in one phase. Use the weight loss calculator to model realistic timelines based on your TDEE.

When ideal weight is meaningless

Several populations fall outside the assumptions baked into every IBW formula:

• Athletes and strength sportspeople: Competitive powerlifters, rugby players, and bodybuilders carry 10–25 kg more lean mass than the general population these formulas were calibrated on. Being 30 kg above IBW may simply mean you have large quadriceps.
• Older adults (65+): Sarcopenia — age-related muscle loss — means that many elderly individuals at or below IBW are actually underpowered in lean mass. Aiming for IBW by restricting calories in this group can accelerate functional decline. Mild overweight (BMI 25–27) in older adults is associated with lower mortality in multiple cohort studies.
• Pregnant and breastfeeding women: IBW has no role here. The IOM recommends specific gestational weight gain targets based on pre-pregnancy BMI, ranging from 11–16 kg for normal-weight women. Post-partum weight return should be gradual and guided by a healthcare provider.
• South Asian and East Asian populations: These groups develop metabolic complications (insulin resistance, type 2 diabetes, cardiovascular risk) at lower BMI and body fat levels than European-ancestry populations. The WHO recommends lower BMI cut-offs (23 for overweight, 27.5 for obesity). BMI-22-based IBW is especially misleading here.
• Pediatric populations: IBW formulas are derived from adult populations. Children require age-adjusted growth chart percentiles, not adult IBW formulas.

Related reading

• BMI calculator — understand what BMI does and doesn't measure
• Body fat calculator — estimate body fat % from measurements
• Weight loss calculator — realistic timeline to any weight goal
• Weight regain and set point biology — why the body defends its weight

Frequently asked questions

Why are there five different ideal weight formulas and which is most accurate?

The five formulas were each derived independently from different patient populations and time periods. Devine (1974) was created to estimate creatinine clearance for gentamicin dosing in hospitalised patients — not for fitness. Robinson (1983) and Miller (1983) were regression analyses on NHANES data attempting to refine Devine. Hamwi (1964) is the oldest and is still widely cited in nursing education. BMI 22 back-calculates from a "healthy" BMI midpoint. None is universally "most accurate" because ideal body weight is not a biological constant — it is a clinical tool. For drug dosing, Devine remains the most-validated formula. For population comparisons, BMI 22 is most transparent. The consensus average of all five reduces formula-specific bias.

Which ideal body weight formula do doctors actually use?

In clinical practice, Devine is the most commonly used formula, particularly for drug dosing (aminoglycosides, vancomycin, digoxin), ventilator tidal-volume settings (6 ml/kg IBW in ARDS protocols from the ARDSNet trial), and renal dose adjustments. Some institutions use Adjusted Body Weight (ABW) — a blend of actual and ideal body weight — for patients who are more than 20–30% above their IBW. The Robinson formula is sometimes preferred in paediatric adaptations. Outside of those clinical contexts, ideal body weight is rarely used by physicians; body composition analysis and metabolic markers are more actionable.

Is ideal body weight a fitness or beauty goal?

No. Ideal body weight is a clinical drug-dosing concept, not a fitness benchmark or appearance target. It was designed to normalise drug pharmacokinetics across body sizes — it says nothing about your athletic performance, health risk, or how you should look. Many highly-fit individuals (athletes, strength athletes, older adults with preserved muscle) are well above their calculated IBW. Many individuals at their IBW have poor metabolic health. If you want meaningful health metrics, focus on waist circumference (men <94 cm, women <80 cm for low risk), body fat percentage, blood pressure, fasting glucose, and cardiorespiratory fitness rather than any single weight number.

My ideal body weight is much lower than my current weight — what should I do?

First, remember that IBW is a clinical reference point, not a prescription. A realistic, evidence-based fat loss rate is 0.5–1% of body weight per week. At a 500 kcal/day deficit, most people lose 0.3–0.5 kg per week accounting for metabolic adaptation. If your scale weight is 20–30 kg above your IBW, a six-to-twelve month plan targeting 10% body weight reduction (a clinically meaningful milestone that improves blood pressure, glycaemic control, and joint load) is far more achievable and sustainable than aiming directly for IBW in one phase. Work with a registered dietitian to set staged goals. IBW is a number on a reference table — your health journey has more dimensions than a single formula.

Should elderly people, athletes, or pregnant women use ideal body weight?

For these groups, IBW is largely meaningless as a personal health target. Older adults naturally lose lean mass with age; hitting IBW by losing muscle (sarcopenia) would worsen metabolic and functional outcomes, not improve them. Strength and power athletes carry substantially more muscle than the general population the formulas were calibrated on — their IBW will be systematically understated. During pregnancy, healthy weight gain (Institute of Medicine guidelines: 11–16 kg for normal pre-pregnancy BMI) is the appropriate target. For certain ethnicities — particularly South and East Asian populations — standard IBW formulas may not reflect appropriate body composition targets; adjusted BMI thresholds (23 for overweight, 27.5 for obesity in South Asians) are recommended by the WHO.