Medication Dosage Calculator

About Medication Dosage Calculator

The Medication Dosage Calculator solves a problem most online calculators sidestep: real prescribing rarely fits one formula. A 22 lb toddler with an ear infection needs a weight-based mg/kg amoxicillin dose. A 9-month-old with a fever and no clinic scale needs an age-based estimate using Fried's Rule. A patient on chemotherapy or vasoactive drips needs a body surface area dose in mg/m² using the Mosteller formula. Most calculators ship one of these methods. This tool gives you all three side by side so you can pick the right one and cross-check the others, then converts the recommended mg into actual mL of liquid suspension on a measuring syringe — the step where parents and even nurses make the most errors.

What makes this calculator different

How to use the Medication Dosage Calculator

1. Pick a calculation method — weight (default for most pediatric meds), age (when no weight is available), or BSA (chemotherapy and ICU dosing).
2. Choose a medication from the library. The card highlights, the safety note loads with max single and max daily caps, and the suspension concentration auto-fills.
3. Enter the patient's weight (required) and, depending on method, age and/or height. Use kg/cm or lb/in — toggle the units pill at the top of the patient section.
4. Confirm or edit the suspension concentration (e.g. 160 mg / 5 mL is the standard for children's acetaminophen). Override if your bottle reads differently.
5. Pick a frequency. The calculator divides daily totals across the schedule and checks against the maximum daily dose.
6. (Optional) Override the library mg/kg with a custom value if your reference (Lexicomp, BNF, hospital protocol) uses different numbers.
7. Hit Calculate. Read mg per dose, mL on a syringe, where the dose lands on the safety meter, the daily total vs cap, and the side-by-side method comparison.

The four classic age-based dosing rules

Clark's Rule — uses weight in pounds against a "standard" 150 lb adult:

\( \text{Pediatric dose} = \text{Adult dose} \times \dfrac{\text{Weight (lb)}}{150} \)

Young's Rule — for children 2–12 years old, uses age:

\( \text{Pediatric dose} = \text{Adult dose} \times \dfrac{\text{Age (years)}}{\text{Age (years)} + 12} \)

Fried's Rule — for infants under 2, uses age in months:

\( \text{Pediatric dose} = \text{Adult dose} \times \dfrac{\text{Age (months)}}{150} \)

Cowling's Rule — uses the patient's next birthday over 24:

\( \text{Pediatric dose} = \text{Adult dose} \times \dfrac{\text{Age at next birthday}}{24} \)

These rules pre-date modern mg/kg dosing and are crude approximations — they ignore body composition, organ maturity, and pharmacokinetics. They survive in clinical practice as quick mental sanity checks and for situations where weight is genuinely unavailable.

Body surface area: the Mosteller formula

For chemotherapy, intensive care, and any drug whose prescribed amount is in mg/m², body surface area is the dosing variable. The Mosteller formula is the most widely used because it is simple to compute by hand:

\( \text{BSA (m}^2\text{)} = \sqrt{\dfrac{\text{Height (cm)} \times \text{Weight (kg)}}{3600}} \)

An average adult is taken as 1.73 m². Pediatric BSA is much smaller — a 5-year-old is roughly 0.73 m², an infant is roughly 0.3 m². Dosing as mg/m² rather than mg/kg matters for drugs whose toxicity scales with surface area (skin, mucosa, bone marrow turnover) rather than body mass.

Common pediatric medications quick-reference table

Why mg/kg dosing wins for pediatric prescribing

Children are not small adults — they have higher water content, faster organ turnover, and a smaller margin for error. Weight-based mg/kg dosing scales linearly with body mass and was validated against pharmacokinetic data for most modern drugs. Age-based rules ignore that a chubby 8-year-old and a thin 8-year-old need different absolute doses. BSA dosing is more accurate than weight for drugs with narrow therapeutic windows but requires a height measurement that is rarely available outside hospital. In day-to-day pediatric practice the algorithm is: measure weight in kg, multiply by mg/kg, cap at the maximum single dose, divide by the prescribed frequency, and convert to mL of suspension.

Dosing errors to avoid

• mL vs teaspoons — the FDA strongly recommends dosing only in mL. A "teaspoon" can mean anywhere from 3 to 7 mL depending on the spoon. Use the dosing syringe that comes with the medication.
• Concentration mismatch — children's acetaminophen (160 mg / 5 mL) and infant drops (160 mg / 5 mL post-2011, but 80 mg / 0.8 mL pre-2011) look similar but had different strengths. Always read the bottle label.
• Adult vs pediatric formulations — adult ibuprofen (200 mg/tab) and pediatric (100 mg / 5 mL) require different math. Don't crush adult tablets without confirming a pediatric dose-equivalent.
• Frequency confusion — q6h means literally every 6 hours (4 doses/day including overnight). QID often means 4 daytime doses spread over waking hours. The two are not always interchangeable for steady drug levels.
• Missing the max daily cap — acetaminophen at 15 mg/kg q4h would give 6 doses × 15 = 90 mg/kg/day, exceeding the 75 mg/kg pediatric max. The calculator catches this.
• Aspirin in children — never give aspirin to anyone under 18 with a viral illness due to Reye syndrome risk. The library entry exists for adult reference; pediatric calculations should use acetaminophen or ibuprofen instead.

Frequently Asked Questions

How do you calculate a medication dose by weight?

Multiply the patient's weight in kilograms by the medication's mg/kg dose. For children's acetaminophen at 12.5 mg/kg, a 20 kg child gets 20 × 12.5 = 250 mg per dose. For drugs given as a daily total divided across multiple doses (such as amoxicillin 45 mg/kg/day), multiply weight by mg/kg/day, then divide by the number of doses per day.

What is Clark's Rule for pediatric dosing?

Clark's Rule estimates a pediatric dose by scaling the adult dose against an "average adult" weight of 150 pounds. The formula is pediatric dose = adult dose × weight (lb) ÷ 150. It is a rough approximation used when only the child's weight is known and a mg/kg reference is not available — modern practice prefers mg/kg whenever the drug has published pediatric dosing.

When do you use Young's Rule versus Fried's Rule?

Young's Rule is for children ages 2 to 12: pediatric dose = adult dose × age ÷ (age + 12). Fried's Rule is for infants under 2 years: pediatric dose = adult dose × age in months ÷ 150. Both are historical age-based rules — modern pediatric practice prefers weight-based mg/kg dosing whenever weight is known. The calculator shows all of them so you can see how much they differ for the same patient.

What is the Mosteller formula for body surface area?

The Mosteller formula computes BSA as the square root of (height in cm × weight in kg ÷ 3600), in m². It is the most widely used BSA formula in clinical practice for chemotherapy dosing, ICU drug calculations, and any medication where the prescribed dose is in mg/m². An average adult is approximately 1.73 m².

Is this calculator safe to use for actual patient dosing?

This calculator is educational reference only. It uses widely published pediatric dosing ranges and standard formulas, but real prescribing decisions must account for the patient's renal and hepatic function, drug interactions, allergies, age, and the specific clinical context. Always confirm doses with a licensed healthcare provider, the medication's package insert, or a clinical reference such as Lexicomp, the Harriet Lane Handbook, or the BNF for Children before administering.

How do I convert milligrams to milliliters of liquid suspension?

Divide the prescribed dose in milligrams by the suspension concentration in milligrams per milliliter. Concentrations on bottles are usually given as mg per 5 mL — for example children's acetaminophen at 160 mg / 5 mL is 32 mg/mL. So a 250 mg dose is 250 ÷ 32 = 7.8 mL. The calculator does this automatically and shows the math under "Step 2: Convert mg to liquid volume."

Why does my calculated dose differ from what the doctor prescribed?

Pediatric mg/kg ranges are wide because they cover several conditions and severities. A doctor may prescribe at the low end for a mild condition or the high end for a serious infection (e.g., high-dose amoxicillin at 80–90 mg/kg/day for acute otitis media), may round to a convenient measuring volume (5 mL = 1 teaspoon), or may apply a different mg/kg reference. Small differences are normal — large differences should always be discussed with the prescriber before changing the dose.

What does "mg/kg" mean on a medication bottle?

It means milligrams of medication per kilogram of patient body weight, per dose unless explicitly noted as per day. A label saying "10 mg/kg q6h" means: weight (kg) × 10 = mg per dose, repeated every 6 hours. A label saying "40 mg/kg/day divided q8h" means: weight × 40 = total daily mg, divided into 3 doses 8 hours apart.

Should I round the calculated mL to the nearest 0.5 mL?

Yes — most oral syringes are graduated in 0.1 or 0.5 mL increments. The calculator shows a "practical" rounded value next to the exact mL. For thin-margin medications (digoxin, narrow-therapeutic-index drugs), round only as much as the syringe demands and confirm with a pharmacist. For acetaminophen and ibuprofen, rounding to the nearest 0.5 mL is clinically fine.

Can I use this calculator for adult patients?

Yes — most adult medications use the same library entries. For a typical adult weighing 70 kg, the weight-based method gives the same recommendation as a clinician's mg-per-dose. The BSA method is most useful for adult chemotherapy or ICU dosing where the prescribed dose is in mg/m². Aspirin's library entry is adult-only — never give it to a child under 18 with a viral illness due to Reye syndrome risk.