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Drug Dosing Calculators

All Drug Dosing calculators

Dosing is where pharmacology meets the individual patient, and the drugs gathered here are the ones where getting it wrong hurts: narrow therapeutic index agents whose clearance depends on kidney function, body composition and interacting drugs. This library collects the evidence-based dosing methods clinicians actually use at the bedside, from vancomycin AUC targeting to lithium maintenance, with honest notes on what each method assumes.

Medically reviewed by , physician.

In short: Free drug dosing calculators for the high-risk drugs: vancomycin AUC-guided regimens, gentamicin and aminoglycoside pharmacokinetics, digoxin loading and maintenance, and lithium initiation and adjustment. Each page explains the method, the targets and the monitoring.

What drug dosing covers, and the clinical questions these tools answer

Drug dosing calculators answer a deceptively simple question: how much of this drug, how often, for this patient. For most drugs the answer is a fixed tablet strength and the question barely needs asking. The drugs in this library are the exceptions. Vancomycin, the aminoglycosides, digoxin and lithium all share the same dangerous combination: a narrow gap between effective and toxic concentrations, clearance that varies widely between patients, and serious harm when the dose misses. That is why each of them has a published dosing method with a name attached, and why those methods survive in guidelines decades after they were derived.

Vancomycin dosing changed fundamentally with the 2020 consensus guideline, which moved the target from trough concentrations to the 24-hour area under the curve. The vancomycin calculator implements the Matzke pharmacokinetic method to size a starting regimen for an AUC of 400 to 600, the range associated with efficacy against MRSA without excess kidney injury. Gentamicin offers a choice of philosophies: the Hartford extended-interval method that exploits concentration-dependent killing with one large daily dose, the traditional multiple-daily method, and the low synergy doses used with a cell-wall-active partner in endocarditis. The aminoglycoside pharmacokinetics calculator goes further, using the Sawchuk-Zaske method to individualise the elimination rate and volume of distribution from the patient's own measured peak and trough.

Digoxin is the classic narrow-index drug of cardiology, and the calculator compares the three published approaches side by side: Jelliffe's total body stores method, Koda-Kimble's clearance-based maintenance dose, and Jusko's volume-based loading dose, with adjustment for the interacting drugs that change digoxin clearance. Lithium closes the set: the Winter method estimates lithium clearance from creatinine clearance, sizes the daily dose for a target level, and adjusts it from a measured steady-state trough, because lithium's therapeutic range of 0.6 to 1.0 mEq/L sits uncomfortably close to toxicity.

When clinicians and students use these calculators

The vancomycin calculator is used at the moment of prescribing, usually in the emergency department or on the ward when serious MRSA infection is suspected and the first dose cannot wait for pharmacy. It gives the loading dose for serious infection and a maintenance regimen sized to the patient's kidney function, with the explicit understanding that measured levels and Bayesian software take over from the second day. The gentamicin calculator serves the same prescribing moment for Gram-negative sepsis, pneumonia and the synergy regimens of endocarditis, where the choice between extended-interval and traditional dosing depends on the infection, the patient's kidneys and local practice.

The aminoglycoside pharmacokinetics calculator is used a day or two later, when the first measured peak and trough are back and the question is no longer what to start but what the patient's own numbers say about clearance and distribution. It is the bridge between empiric dosing and true individualisation. Digoxin dosing comes up in heart failure and atrial fibrillation management, often when renal function has changed or an interacting drug such as amiodarone has been added, and the calculator shows how the different methods agree or disagree so the prescriber can choose deliberately. Lithium dosing is used in psychiatry at initiation and at every dose change, because the difference between 0.6 and 1.2 mEq/L is the difference between prophylaxis and poisoning, and the calculator makes the pharmacokinetic reasoning explicit.

How to interpret results, and what they change in practice

Every calculator in this library reports the same logical chain: patient size and kidney function in, drug clearance and half-life out, then a dose sized to a published target. For vancomycin the target is an AUC24 of 400 to 600, and the calculator reports the predicted AUC of the rounded regimen so you can see whether it lands inside the range before prescribing. For gentamicin the targets are the Hartford peak above 20 mcg/mL with a trough below 1, or the traditional peak of 5 to 10 with a trough below 2, and the interval stretches as creatinine clearance falls. The aminoglycoside calculator reports the patient-specific elimination rate and volume of distribution, which are the numbers that let you redesign the regimen rather than just nudge it.

Digoxin results need reading as a comparison: Jelliffe, Koda-Kimble and Jusko rarely give identical numbers, and the spread between them is information, telling you how sensitive the dose is to the method's assumptions. The practical target is a level of 0.5 to 1.0 ng/mL in heart failure and 0.8 to 1.5 in atrial fibrillation, with lower preferred in both. Lithium results centre on the daily dose for the chosen target, usually 0.6 to 0.8 mEq/L for maintenance, with the per-dose amount for the chosen interval and an estimated time to steady state of about five half-lives, so you know when the first meaningful level can be drawn.

Limitations and pitfalls

All of these methods estimate; none of them measures. Creatinine-based estimates of kidney function are unreliable in the unstable patient, the very elderly with low muscle mass, and anyone whose creatinine is changing day to day, and every calculator here inherits that weakness. The Matzke vancomycin method gives a starting regimen, not a substitute for levels; the guideline is explicit that Bayesian software with measured concentrations is preferred for the individual patient. Hartford extended-interval gentamicin is not for endocarditis synergy, pregnancy, or significant renal impairment, and the traditional method needs real peaks and troughs, not assumptions.

Digoxin's published methods disagree most exactly where the patient is unusual: severe renal impairment, very low body weight, or interacting drugs that the simple clearance factor only approximates. Lithium dosing assumes stable renal function and steady state; a level drawn too early, during dehydration, or after a missed dose will mislead the adjustment badly. And across the whole library, the calculators assume adult patients with the usual pharmacokinetics: obesity, critical illness, pregnancy and dialysis each break the assumptions in ways no bedside tool can fully correct, which is why every result here is a starting point for clinical judgement and therapeutic drug monitoring, never the final word.

How to use this library

Start from the drug and the clinical moment. If you are starting vancomycin for suspected MRSA, use the vancomycin calculator for the loading dose and the first maintenance regimen, then plan the levels. If you are starting gentamicin, choose the method first: extended-interval for most Gram-negative infections, traditional where local practice or the patient demands it, synergy for endocarditis. When levels come back, move to the aminoglycoside pharmacokinetics calculator and let the patient's own numbers set the regimen. For digoxin, enter the interacting drugs honestly and compare the three methods before choosing; for lithium, pick the target for the phase of illness and respect the time to steady state. Document the method, the target and the result: dosing is one of the few places where the reasoning is as important as the number.

Frequently asked questions

What is AUC-guided vancomycin dosing?

AUC-guided dosing targets a 24-hour area under the concentration-time curve to MIC ratio of 400 to 600, rather than a trough concentration of 15 to 20 mcg/mL. The 2020 guideline made this switch because trough targeting pushed many patients into nephrotoxic exposures without better outcomes. The Matzke method estimates clearance from creatinine clearance to size the starting regimen.

When is extended-interval gentamicin appropriate?

Extended-interval (Hartford) dosing, typically 7 mg/kg once daily, suits most adult Gram-negative infections in patients with reasonable renal function, exploiting the drug's concentration-dependent killing and post-antibiotic effect. It is not appropriate for endocarditis synergy dosing, pregnancy, burns, or creatinine clearance below 20 mL/min, where traditional or individualised dosing is preferred.

What is the Sawchuk-Zaske method?

The Sawchuk-Zaske method individualises aminoglycoside dosing from a measured steady-state peak and trough. It back-calculates the patient's own elimination rate constant and volume of distribution, then uses those to design a regimen that hits the desired peak and trough, rather than relying on population estimates of clearance.

What digoxin level should I target?

For heart failure, 0.5 to 1.0 ng/mL, with the lower end preferred since higher levels increase mortality without added benefit. For atrial fibrillation rate control, 0.8 to 1.5 ng/mL. Levels should be drawn at least 6 to 8 hours after the last dose, ideally just before the next one, and at steady state.

What lithium level is therapeutic?

For maintenance prophylaxis of bipolar disorder, 0.6 to 0.8 mEq/L; for acute mania, up to 1.0. Levels of 1.2 to 1.5 are borderline toxic and above 1.5 frankly toxic. Troughs are drawn 12 hours after the evening dose, at steady state after about five days on a stable dose.

Are these calculators a substitute for therapeutic drug monitoring?

No. They size starting regimens and interpret levels using published pharmacokinetic methods, but every result assumes stable physiology and population-average parameters. Measured drug levels, clinical response and renal function trends must guide ongoing therapy, with pharmacist or clinical pharmacology input for complex patients.

Medical disclaimer

These calculators are educational tools for clinicians, students and informed readers. They do not provide medical advice, and no dose or result should replace the judgement of a qualified health professional who has seen the patient. Dosing decisions for narrow therapeutic index drugs should be confirmed with therapeutic drug monitoring and local pharmacy guidance. If you are unwell or worried about a result, seek professional care promptly.

Further reading

  1. IDSA Vancomycin Dosing Guidelines
  2. American College of Cardiology