Health calculators

Why it matters

Body mass index (BMI) is a "calculated measure of weight relative to height", and the CDC describes it as a "screening tool for estimating weight status" and not a tool for diagnosing disease.

The Mifflin–St Jeor equation estimates basal metabolic rate (the energy the body needs at complete rest), a number useful for estimating daily calories but still a statistical estimate and not a direct measurement.

Converting blood glucose units (mg/dL and mmol/L) and glycated haemoglobin (% and mmol/mol) is necessary because laboratories around the world do not always use the same unit.

The estimated glomerular filtration rate (eGFR) measures how well the kidneys filter the blood, and is one of the most used numbers for monitoring kidney function, and is usually calculated from the blood creatinine result with age and sex.

How to use it

Body mass index: BMI = weight in kilograms ÷ (height in metres)², which is the metric formula given in the CDC guide for calculating body mass index.

Adult weight classification (age 20 and over) according to the CDC: below 18.5 underweight, 18.5 to below 25 healthy weight, 25 to below 30 overweight, 30 or higher obesity (and the CDC divides obesity itself into class 1 at 30–<35, class 2 at 35–<40, and class 3 (severe) at 40 or higher).

Basal metabolic rate by the Mifflin–St Jeor equation as given in a reference table (Endotext, chapter on dietary treatment of obesity, Table 12), for adults over 19 years (weight in kilograms and height in centimetres): for men RMR = (9.99 × weight) + (6.25 × height) − (4.92 × age) + 5, and for women RMR = (9.99 × weight) + (6.25 × height) − (4.92 × age) − 161.

Blood glucose conversion: the NIDDK document "Diabetes in America" states verbatim "Conversion: mg/dL x 0.0555 = mmol/L", meaning that mmol/L equals mg/dL multiplied by 0.0555.

Converting glycated haemoglobin to estimated average glucose (eAG): according to the formula published from the ADAG study (Nathan and colleagues), as given in an article hosted on PubMed Central of the US National Library of Medicine: eAG (mg/dL) = 28.7 × A1C − 46.7.

Converting the glycated haemoglobin unit between percentage (NGSP/DCCT) and the international mmol/mol unit (IFCC), according to the official NGSP equation: IFCC (mmol/mol) = (10.93 × percentage%) − 23.50, and conversely NGSP (%) = (0.09148 × IFCC mmol/mol) + 2.152.

Estimated glomerular filtration rate (eGFR) by the 2021 CKD-EPI equation without a race coefficient, as given verbatim and identically in both the NKF and the NIDDK: eGFR = 142 × min(creatinine/κ,1)^α × max(creatinine/κ,1)^-1.2 × 0.9938^age × 1.012 (an additional factor for females only), where κ=0.7 and α=-0.241 for females, and κ=0.9 and α=-0.302 for males (creatinine in mg/dL, age in years).

eGFR stages (ml/min/1.73m²) according to the US National Kidney Foundation classification: 90 or higher: normal or high; 60–89: mildly decreased; 45–59: mildly to moderately decreased; 30–44: moderately to severely decreased; 15–29: severely decreased; below 15: kidney failure.

Limits of this information

The CDC states explicitly: "BMI is a screening measure and is not intended to diagnose disease or illness" (body mass index is a screening tool and not intended for diagnosing a disease).

The CDC adult weight classification thresholds are for those aged 20 and over, and do not apply to children and adolescents, for whom a different growth chart is used.

The Mifflin–St Jeor equation is a statistical estimate of the average of a group of people, so an individual's actual consumption may differ from the calculated figure.

Unit conversions (blood glucose and glycated haemoglobin) are purely arithmetic operations that do not change the medical interpretation of the result; the clinical decision remains with the treating doctor based on the test and the full context.

All calculations run locally in the visitor's browser; the page does not store or send the numbers anyone enters to any server.

This coefficient assumes that kidney function is stable; the source (NIDDK/NKDEP) states: 'Creatinine-based estimates of kidney function are only useful when renal function is stable' — that is, it is not accurate during acute kidney injury (AKI) or when the creatinine level is changing.

Using these equations is not recommended in people with muscle mass or diet that differs greatly from usual (such as amputees, people with paralysis, bodybuilders, or the severely obese), according to the source text: 'Persons with extremes in muscle mass and diet' are excluded from the equation's accuracy, and creatinine clearance is preferred instead.

This equation is not used for children; the source explicitly states to use the Bedside Schwartz equation for those under 18: 'the Bedside Schwartz equation should be used' for patients under 18.

Pregnancy is one of the conditions in which using these equations is not recommended because functional creatinine is unstable during it: 'This includes pregnant women' among the groups in whom use of the equation is excluded.

The result is an estimate and not a direct measurement of kidney function — the source: 'the reported eGFR is the best estimate of a patient's GFR; it is not the patient's actual GFR', and accuracy varies between individuals.

The equation requires calibrated creatinine values (IDMS-traceable/standardised) from the laboratory; uncalibrated results may give an inaccurate eGFR value.

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