Medication Dosage Calculator
Compute a renal-function-adjusted medication dosage using patient weight, the prescribed mg/kg dose, and Cockcroft-Gault creatinine clearance scaled to a reference of 120 mL/min. Provides a rough dose estimate, not a substitute for clinical pharmacy review.
Last updated: May 2026
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About this calculator
The formula combines two clinical equations into a single output: Adjusted Dose (mg) = (medicationType × weight) × CrCl / 120, where medicationType is the per-kg dose in mg (5, 10, 15, or 2 mg/kg), weight is in kg, and CrCl is the Cockcroft-Gault creatinine clearance computed inline as ((140 − age) × weight) / (72 × creatinine) × (0.85 if female, else 1.0). The base dose is medicationType × weight (standard mg/kg dosing); this is then scaled by CrCl/120 to reduce the dose proportionally for patients with reduced renal function, using 120 mL/min as a reference for full clearance. A patient with CrCl 120 receives 100% of the base dose; CrCl 60 receives 50%; CrCl 30 receives 25%. Edge cases: the formula gives proportional reduction across the entire CrCl range, but real renal dosing follows drug-specific thresholds — for many drugs, no dose adjustment is needed until CrCl < 60, then a step-wise reduction at 60, 30, and 15 mL/min cut-offs. The Cockcroft-Gault formula itself has limitations: it uses actual body weight (over-estimates CrCl in obese patients; use ideal or adjusted weight for those), assumes steady-state creatinine (invalid in acute kidney injury), and is unreliable at extremes of muscle mass. Serum creatinine must be in mg/dL — patients in countries reporting µmol/L should divide by 88.4 first. Always cross-check against the drug's specific renal-dosing reference (Lexicomp, UpToDate, drug label) before administering.
How to use
Example 1 — gentamicin-style dosing in a normal-function adult. medicationType 5 (5 mg/kg), weight 70 kg, age 45, creatinine 1.0 mg/dL, gender male. Step 1: base dose = 5 × 70 = 350 mg. Step 2: CrCl = ((140 − 45) × 70) / (72 × 1.0) × 1.0 = (95 × 70) / 72 = 6,650 / 72 ≈ 92.4 mL/min. Step 3: adjustment factor = 92.4 / 120 ≈ 0.77. Step 4: adjusted dose = 350 × 0.77 ≈ 269.5 mg. Verify: a 70 kg patient with mild renal impairment (CrCl ~92 instead of full 120) receives ~77% of the unadjusted dose — close to a typical gentamicin dose of 5 mg/kg, slightly reduced for clinical safety ✓. Real gentamicin dosing protocols would round to 270 mg and check peak/trough levels. Example 2 — significantly reduced renal function, elderly female. medicationType 15 (15 mg/kg), weight 60 kg, age 75, creatinine 1.8 mg/dL, gender female. Step 1: base dose = 15 × 60 = 900 mg. Step 2: CrCl = ((140 − 75) × 60) / (72 × 1.8) × 0.85 = (65 × 60) / 129.6 × 0.85 = 3,900 / 129.6 × 0.85 = 30.1 × 0.85 ≈ 25.6 mL/min. Step 3: adjustment factor = 25.6 / 120 ≈ 0.213. Step 4: adjusted dose = 900 × 0.213 ≈ 191.6 mg. Verify: CrCl 25.6 mL/min represents stage 4 chronic kidney disease (15–29) — significant renal impairment. Reducing dose from 900 mg to ~192 mg (21% of base) follows the proportional logic of the formula; real drug-specific guidance might recommend a more conservative cut-off (50% reduction at CrCl 30–50, hold dose at CrCl <30) or a longer dosing interval rather than a smaller dose ✓.
Frequently asked questions
How does this differ from drug-specific renal dosing protocols?
This calculator applies a single proportional scaling (dose × CrCl/120) across the entire creatinine-clearance range, but real renal dose adjustments are usually step-wise and drug-specific. Many drugs require no adjustment until CrCl < 60 mL/min, then 50% dose at CrCl 30–59, 25% at CrCl 15–29, and discontinuation or alternative at CrCl < 15. Other drugs use frequency adjustment (extend the dosing interval from every 8h to every 12h or 24h) rather than dose reduction. Some drugs (vancomycin) target a serum concentration and are dosed using pharmacokinetic models, not simple weight-based formulas. The proportional formula here is a coarse estimate; for actual prescribing always consult drug-specific resources like Lexicomp Renal Dosing, the Sanford Guide, UpToDate, or the drug's package insert. The package insert is the authoritative legal source — calculators are convenient but should never replace the formal dosing reference, particularly for narrow-therapeutic-index drugs (aminoglycosides, vancomycin, digoxin, lithium, anticonvulsants).
Why use creatinine clearance instead of GFR for drug dosing?
Historical convention. Most drug-dose pharmacokinetic studies, particularly older ones that established renal dose-adjustment thresholds, used Cockcroft-Gault creatinine clearance as the renal function measure. Modern eGFR equations (CKD-EPI, MDRD) are now preferred for staging chronic kidney disease and cardiovascular risk stratification because they are validated in larger and more diverse populations and report values normalised to body surface area (mL/min/1.73 m²). However, since the dose-adjustment thresholds for most drugs were established using Cockcroft-Gault, switching to CKD-EPI eGFR could shift dosing decisions in ways the original studies didn't validate. The FDA continues to recommend Cockcroft-Gault for drug dosing despite the broader switch to CKD-EPI in other contexts. The two can differ by 10–20 mL/min in the same patient, particularly at extremes of body composition; for borderline dosing decisions, the choice of formula can affect whether a patient gets full or reduced dose. Always check which formula your hospital's renal-dosing protocol uses.
Why must I use actual body weight in Cockcroft-Gault, and when do I adjust?
The original Cockcroft-Gault equation was validated using actual body weight, so most published renal-dosing thresholds implicitly assume actual weight is the input. However, the formula systematically over-estimates CrCl in obese patients (BMI > 30) because excess fat tissue does not contribute to creatinine production the way muscle does. For these patients, many sources recommend using ideal body weight (Devine) or adjusted body weight (IBW + 0.4 × (actual − IBW)) instead of actual weight, but practice is inconsistent. The formula also over-estimates CrCl in significantly malnourished or very elderly patients with low muscle mass, where creatinine production is reduced and serum creatinine looks 'normal' despite poor renal function. In acute kidney injury, the steady-state assumption underlying Cockcroft-Gault is violated and CrCl computed from a rising or falling creatinine is unreliable; in those cases, use clinical judgement or a kinetic GFR model. For drug dosing decisions in these populations, defer to pharmacy or a clinical decision-support system rather than this calculator.
What are the common mistakes when calculating medication dosages?
The biggest mistake is mixing creatinine units — Cockcroft-Gault expects mg/dL, but many labs (especially outside the US) report µmol/L, off by a factor of ~88.4. The second is using actual body weight in obese patients, which inflates apparent CrCl and produces overly aggressive doses; use ideal or adjusted body weight for BMI > 30 unless the dosing reference specifies otherwise. The third is applying this proportional formula to drugs that need step-wise adjustment — many drugs require no dose change until CrCl < 60, then specific reductions at defined thresholds, not continuous proportional scaling. People also forget the female 0.85 correction (or apply it twice), and forget that the formula assumes steady-state creatinine — useless in acute kidney injury where the patient's true GFR is changing hourly. Some drugs target a serum level (vancomycin, aminoglycosides) and require trough monitoring regardless of formula-derived dose; relying on calculator output alone for these drugs is dangerous. Finally, drug-drug interactions can affect clearance independently of CrCl — always check interaction databases.
When should I not use this calculator?
Do not use it for actual prescribing decisions — it is a back-of-envelope estimator only and provides simpler logic than real drug-specific renal-dosing protocols. For prescribing, use Lexicomp Renal Dosing, the Sanford Guide for antimicrobials, drug package inserts, or your hospital's clinical decision support system. It is not appropriate for acute kidney injury, where creatinine is non-steady-state and Cockcroft-Gault is invalid; use clinical pharmacy consult for AKI dosing. Do not use it for narrow-therapeutic-index drugs (vancomycin, aminoglycosides, digoxin, lithium, warfarin, methotrexate, immunosuppressants) where dose adjustment requires pharmacokinetic modelling and therapeutic drug monitoring. It is unsuitable for pediatric patients — pediatric dosing uses age and weight-based or BSA-based protocols, not Cockcroft-Gault. The proportional CrCl/120 scaling here is a simplification; real adjustments are step-wise at thresholds like CrCl 60, 30, 15 mL/min, not continuous. Finally, do not use this calculator for drugs that require non-renal adjustment (hepatic, dialytic, age-based) or for routes other than systemic (topical, intra-articular, intrathecal) where the formula doesn't apply at all.