Dosage Calculator

What this calculator does — and the strict scope

This calculator handles the arithmetic of weight-based pediatric dosing — multiplying patient weight by a prescribed mg/kg figure, dividing by frequency to get a single-dose amount, optionally dividing by liquid concentration to get a volume per dose. It exists so that nursing students, pharmacy students, and allied-health students can practise the arithmetic in a clean form before they encounter the same math in clinical training, exams, and (eventually) practice.

It does not handle:

• Adult dosing
• IV drip rates or infusion calculations
• BSA-based dosing (used in oncology)
• Renal/hepatic adjustment
• Drug-drug interactions
• Validating whether the prescribed mg/kg is appropriate for the indication and patient

For any of those, use Lexicomp, Epocrates, your institutional formulary, the drug’s prescribing information, or a clinical pharmacist. This calculator is the tip of the iceberg; clinical dosing is everything below the waterline.

Why the scope is so tight

Two reasons.

First, the arithmetic is the trivial part of dosing. The hard parts are: choosing the right mg/kg figure (validated by indication, age band, organ function), checking against drug-drug interactions and allergies, confirming the formulation available, and verifying the dose makes clinical sense in the patient’s overall state. A calculator that does the arithmetic correctly but doesn’t validate any of the above produces more harm than help if a user mistakes it for clinical guidance.

Second, pediatric dosing errors are dangerous. Children have a narrower margin of safety than adults for many medications — they clear drugs differently at different ages, they have less metabolic reserve, and adult-extrapolated doses are frequently wrong (some medications need higher mg/kg in children than adults because clearance is faster; others much lower because organ systems aren’t yet mature). A web calculator presenting itself as “answers your pediatric dosing question” would invite errors that the institutional safety system (pharmacist double-check, EHR alerts, drug interactions screening) exists specifically to catch.

So: this is a teaching tool. The disclaimers above the form, the warning beside the result, and the standard-issue health disclaimer at the bottom of the page are all there to reinforce the same message. They are deliberately redundant.

How weight-based dosing works conceptually

Most medications in children are prescribed as mg per kilogram of body weight, either per dose or per day:

• mg/kg/day (the more common convention): the total daily dose is weight × the figure, divided across the frequency. Amoxicillin 90 mg/kg/day TID for a 20 kg child = 1800 mg/day = 600 mg per dose three times daily.
• mg/kg/dose (used for some single-administration medications): the per-administration dose is weight × the figure. Acetaminophen 15 mg/kg/dose Q4-6h for fever: 20 kg × 15 = 300 mg per dose.

The calculator handles both conventions via the dosing-basis toggle.

For liquid medications, the formulation comes with a concentration on the label, like “Amoxicillin 250 mg / 5 mL”. To find the volume per dose: single-dose mg ÷ concentration. 600 mg per dose ÷ (250 mg/5 mL = 50 mg/mL) = 12 mL per dose.

These are mechanical conversions — exactly what students learn in dosage-and-calculations modules. The calculator surfaces them in one place to practise the workflow.

What references look like in clinical practice

In the US, the standard authoritative pediatric dosing references are:

• Lexicomp Pediatric & Neonatal Dosage Handbook — comprehensive pediatric drug reference, indication-specific dosing
• Harriet Lane Handbook — Johns Hopkins pediatric reference, widely carried by residents
• AAP Red Book — infectious-disease-specific, the standard for antibiotic and antiviral dosing
• Sanford Guide — antimicrobial therapy reference
• Epocrates — mobile-first drug reference, widely used at the point of care
• UpToDate — clinical-decision-support platform with dosing embedded in disease-specific topic reviews
• The medication’s FDA prescribing information (available at DailyMed) — the source of truth all the above are built from

Most US hospitals also maintain an institutional pediatric formulary that incorporates local protocols, drug shortages, and pharmacist review on top of the published references.

The classic dosing-error patterns

Knowing what not to do helps reinforce the proper workflow:

Using the wrong weight. A 20 lb child is not a 20 kg child — that’s a 2.2× error. The calculator above has a weight-unit toggle precisely because mis-typing the unit is the most common error pattern.

Confusing per-dose and per-day prescriptions. A “5 mg/kg dose given Q6h” produces 4 doses/day at 5 mg/kg each = 20 mg/kg/day total. If you misread it as 5 mg/kg/day divided Q6h, each dose is only 1.25 mg/kg — 75 % underdose. The calculator’s dosing-basis toggle makes the distinction explicit.

Skipping the concentration check on liquids. “12 mL per dose” only holds for the specific concentration on the label. Pediatric suspensions of the same drug come in multiple strengths. Always read the label.

Trusting the calculator without the reference check. This is the biggest one. The arithmetic step is the easy part; the prescription choice is the hard part. Always validate the prescribed mg/kg against an authoritative reference before computing the dose.

Privacy

The calculator runs JavaScript multiplication and division on your device. Patient weight, prescribed dose, frequency, concentration — every value stays in your browser tab. No fetch calls, no analytics, no server-side logging.

That said: even if this were a perfectly private clinical-grade tool, you should not be entering real patient information into any public website for clinical decision-making. Use your EHR, your institutional formulary, and your pharmacist. This tool is for practising the arithmetic, on hypothetical or de-identified examples.