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20 General Medicine Calculators – Body Size, Labs & Fluids

General calculators cover body size, reference weight, body composition, laboratory and specimen arithmetic, fluid estimates, infusion arithmetic, and unit conversion. Tools that use similar measurements may still answer different questions.

Choose the calculator whose formula, units, population, and specimen context match the task. A BMI estimate, BSA estimate, ideal or adjusted body-weight formula, and body-composition model are not interchangeable.

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Choose a calculator by task

Use the task groups below to find the calculator whose formula, intended population, required measurements, and purpose match the task.

Body size, weight, and composition

Select among adult size indices, formula-specific reference weights, body surface area, blood volume, and predicted body-composition estimates.

Adult BMI Calculator (Body Mass Index)Calculate adult BMI from metric or US customary inputs and view the CDC adult screening category with non-diagnostic interpretation limits.IBWDevine Ideal Body Weight Calculator for AdultsCalculate the adult Devine ideal body weight reference from height and the formula’s male or female coefficient, with kg and lb output.AjBW40Adult Adjusted Body Weight Calculator (AjBW 0.4)Calculate fixed adult AjBW40 dosing-weight arithmetic from total and externally determined ideal body weight, with drug- and protocol-specific limitations.Body Surface Area (BSA) CalculatorCalculate estimated body surface area from height and weight using the Mosteller, Du Bois, or Haycock formula.Estimated Blood Volume Calculator (Adult Nadler & Lemmens)Estimate reference blood volume with the adult Nadler or Lemmens formula, or perform arithmetic using an explicitly supplied external mL/kg protocol coefficient.Adult Fat-Free Mass (Boer) and FFMI CalculatorEstimate adult fat-free mass with the Boer equation and derive FFMI from height using metric or US units.Adult Relative Fat Mass (RFM) CalculatorEstimate adult whole-body fat percentage from height and waist circumference using the published Woolcott–Bergman RFM equation.

Waist and body-shape indices

Use the measure that matches the required waist, hip, height, weight, or body-shape inputs and its stated adult evidence boundary.

Waist-to-Height Ratio (WHtR) CalculatorCalculate WHtR from waist circumference and standing height in centimeters or inches, with NICE applicability and measurement limits.Adult Waist-to-Hip Ratio (WHR) CalculatorCalculate adult waist-to-hip ratio from standardized waist and hip circumferences in metric or US units.Body Roundness Index (BRI) CalculatorCalculate the Thomas 2013 geometric Body Roundness Index from height and waist circumference in centimeters or inches, with measurement and interpretation limits.A Body Shape Index (ABSI) CalculatorCalculate the original raw ABSI from height, weight, and waist with explicit raw-versus-z-score and measurement limits.Body Adiposity Index (BAI) CalculatorCalculate the original Bergman BAI from height and hip circumference, with DXA validation limits and no automatic body-fat category.

Laboratory and specimen arithmetic

Match units, collection timing, and specimen identity before calculating a laboratory difference, ratio, or conventional adjustment.

Corrected Calcium Calculator (Legacy 0.8 Formula)Reproduce the conventional 0.8 albumin-adjusted total-calcium estimate in conventional or SI units, with current guidance against routine reporting.Serum Anion Gap Calculator (Without Potassium)Calculate serum anion gap from sodium, chloride, and bicarbonate or total CO₂ using the standard formula without potassium.Stool Osmotic Gap CalculatorEstimate stool osmotic gap from stool sodium and potassium using the conventional assumed 290 mOsm/kg model, with collection and interpretation limits.A/G RatioAlbumin/Globulin Ratio (A/G) CalculatorCalculate A/G ratio from albumin plus reported globulin or same-panel total protein, with separate units and a complete laboratory arithmetic audit.Gamma Gap Calculator (Protein Gap)Calculate the serum protein gap from total protein and albumin with independent units and a complete arithmetic audit.

Fluids, infusion, and unit conversion

Separate maintenance-fluid estimation from gravity drip-rate arithmetic and unit conversion; each tool requires different inputs and answers a different question.

Maintenance Fluids Calculator (Pediatric & Adult)Calculate pediatric 4-2-1 and Holliday–Segar 100/50/20 maintenance-water estimates or adult NICE daily and hourly reference ranges.IV Drip Rate Calculator (gtt/min & mL/hr)Calculate whole-drop gravity IV rate and mathematical flow rate from volume, time, and tubing drop factor.Medical Lab Unit Converter (mg/dL ↔ mmol/L)Convert 10 common serum and plasma laboratory values between conventional and SI reporting units using analyte-specific factors.

How to choose a calculator in this category

  • Confirm whether the task calls for BMI, BSA, a formula-specific reference weight, or a body-composition estimate before choosing a tool.
  • For laboratory arithmetic, use compatible units and measurements from the required specimen and clinical time point.
  • Use maintenance-fluid tools only for their stated estimation context; drip-rate tools convert an already specified volume, time, and tubing factor.

Important limits

  • Formula-derived body measures are estimates and do not replace direct measurement or a complete nutritional or clinical assessment.
  • Laboratory differences and ratios cannot establish a diagnosis without assay, specimen, trend, and clinical context.
  • These calculators do not prescribe a fluid, infusion, medication, or patient-specific treatment.

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