Dosage Calculator — Weight-Based mg/kg Drug Dosing
Use this free dosage calculator to compute weight-based drug doses using the mg/kg method. Enter the patient weight, prescribed dose per kilogram, and dosing frequency. Optionally enter the medication concentration to calculate the volume in mL per dose. This tool is for educational purposes only— always verify calculated doses with a licensed healthcare professional before administration.
Weight-Based Dosage Calculator
Enter the patient weight, prescribed dose per kilogram, dosing frequency, and optional concentration to calculate the total and per-administration dose.
e.g. 1 = once daily, 2 = twice daily (BID), 3 = three times daily (TID), 4 = four times daily (QID)
Enter the liquid medication concentration to calculate volume per dose (mL).
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Frequently Asked Questions
What is weight-based (mg/kg) dosing?
Weight-based dosing is a method of calculating drug doses proportional to a patient's body weight, expressed as milligrams of drug per kilogram of body weight (mg/kg). This approach ensures that lighter and heavier patients receive doses appropriate to their size, which is especially critical in pediatric medicine, where children vary widely in body mass. For example, a dose of 10 mg/kg for a 20 kg child gives 200 mg, while the same dose for a 70 kg adult gives 700 mg.
How do I calculate the dose per administration?
First calculate the total daily dose: Daily Dose (mg) = Weight (kg) × Dose per kg (mg/kg). Then divide by the number of daily administrations: Dose per Administration = Daily Dose / Frequency. For example, a 70 kg patient receiving 10 mg/kg twice daily (BID) would receive: Daily Dose = 70 × 10 = 700 mg; Dose per Administration = 700 / 2 = 350 mg per dose.
How do I calculate the volume of a liquid medication to administer?
To calculate the volume in mL, divide the dose in mg by the concentration in mg/mL: Volume (mL) = Dose per Administration (mg) / Concentration (mg/mL). For example, if the dose per administration is 350 mg and the medication is available as 50 mg/mL, the volume to administer is 350 / 50 = 7 mL. Always confirm you are using the correct concentration, as many medications are available in multiple strengths.
Why is weight-based dosing important for children?
Children are not simply small adults. Their pharmacokinetics — how drugs are absorbed, distributed, metabolized, and excreted — differ significantly from adults and change with age and development. Weight-based dosing helps account for these differences. A fixed adult dose would be dangerously too large for a small child, and may also be insufficient for a larger teenager. Weight-based dosing reduces the risk of underdosing (ineffective treatment) and overdosing (toxicity).
What is the maximum dose and why does it matter?
Most drugs have a maximum single dose and a maximum daily dose, regardless of how high the weight-based calculation goes. For example, acetaminophen is typically limited to 1,000 mg per dose and 4,000 mg per day for adults, even if the weight-based calculation exceeds that. This calculator does not automatically apply maximum dose caps, so users must always verify calculated doses against the drug's approved labeling and clinical guidelines.
How do I convert lbs to kg for dosing?
To convert pounds to kilograms, multiply by 0.453592. For example, 154 lbs × 0.453592 ≈ 69.85 kg. This calculator performs the conversion automatically when you select 'lbs' as the weight unit. Always use kilograms for dosing calculations, as most drug prescribing information uses kg.
When should adjusted body weight (ABW) be used instead of actual body weight?
For most drugs, actual body weight (ABW) is used. However, for some drugs in obese patients, actual body weight may lead to overdosing because fatty tissue has poor blood supply and low drug distribution. In these cases, a prescriber may specify adjusted body weight (ABW) or ideal body weight (IBW). This is particularly relevant for drugs with narrow therapeutic windows, such as aminoglycosides, vancomycin, and certain chemotherapy agents. The prescriber or clinical pharmacist should determine which weight to use.
Is this dosage calculator suitable for clinical use?
No. This calculator is designed for educational and informational purposes only. It performs straightforward mathematical calculations based on user-supplied inputs and does not account for renal function, hepatic impairment, drug interactions, maximum dose limits, patient allergies, or clinical context. Clinical dosing decisions must be made by qualified healthcare professionals using validated clinical tools, current prescribing references, and individualized patient assessment.
About the Dosage Calculator
Weight-Based Dosing Formula
Weight-based dosing (mg/kg) is the standard approach for calculating drug doses in pediatric patients and in adults where body size significantly affects drug distribution, metabolism, or toxicity. The formula ensures the dose is proportional to the patient's body mass.
Daily Dose (mg) = Weight (kg) × Dose per kg (mg/kg)
Dose per Administration (mg) = Daily Dose / Frequency (times/day)
Volume per Dose (mL) = Dose per Administration / Concentration (mg/mL)
Where:
- Weight (kg)— Patient body weight in kilograms. Pounds are automatically converted using: kg = lbs × 0.453592
- Dose per kg (mg/kg)— The prescribed dose in milligrams per kilogram of body weight, as specified by clinical guidelines or a prescribing physician
- Frequency— Number of administrations per 24-hour period (e.g., 1 for once daily, 2 for BID, 3 for TID, 4 for QID)
- Concentration (mg/mL)— Optional. The strength of a liquid formulation, used to calculate the volume (in mL) to draw up or administer
Always use actual body weight unless otherwise directed. In some cases (e.g., obese patients or certain chemotherapy regimens), adjusted body weight (ABW) or ideal body weight (IBW) may be specified by the prescriber.
Common Pediatric Doses
The following table lists approximate weight-based doses for commonly used medications in pediatric patients. These are general guidelines only — always verify with current clinical references (e.g., Lexicomp, Micromedex, or the BNF for Children).
| Drug | Typical Dose | Frequency | Indication |
|---|---|---|---|
| Acetaminophen (Paracetamol) | 10–15 mg/kg | Every 4–6 hours (max 5×/day) | Pain, fever |
| Ibuprofen | 5–10 mg/kg | Every 6–8 hours | Pain, fever, inflammation |
| Amoxicillin | 25–45 mg/kg/day | Divided every 8–12 hours | Bacterial infections |
| Azithromycin | 10 mg/kg (day 1), then 5 mg/kg | Once daily for 5 days | Respiratory infections |
| Prednisolone | 1–2 mg/kg/day | Once or twice daily | Asthma, croup, inflammation |
| Ceftriaxone (IV) | 50–100 mg/kg/day | Once daily | Serious bacterial infections |
| Ondansetron | 0.1–0.15 mg/kg | Every 8 hours (as needed) | Nausea and vomiting |
Note: Maximum dose caps apply to all weight-based calculations. For example, acetaminophen is typically capped at 1,000 mg per dose regardless of weight. Always check product monographs and clinical guidelines for dose limits.
Frequency Guide
Medical dosing frequencies use standardized abbreviations derived from Latin. The following table maps common frequency terms to the number of daily administrations:
| Abbreviation | Latin Origin | Meaning | Times/Day |
|---|---|---|---|
| QD / OD | Quaque die | Once daily | 1 |
| BID | Bis in die | Twice daily | 2 |
| TID | Ter in die | Three times daily | 3 |
| QID | Quater in die | Four times daily | 4 |
| Q4H | Quaque 4 hora | Every 4 hours | 6 |
| Q6H | Quaque 6 hora | Every 6 hours | 4 |
| Q8H | Quaque 8 hora | Every 8 hours | 3 |
| Q12H | Quaque 12 hora | Every 12 hours | 2 |
Enter the numeric value (e.g., “2” for BID, “3” for TID) into the frequency field in the calculator above.
Safety Checks
Weight-based dosage calculations require careful attention to safety. Before administering any medication, verify the following:
- Maximum Dose Limits— Most drugs have a maximum single dose and a maximum daily dose. For example, ibuprofen is typically capped at 400–600 mg per dose and 2,400 mg/day in adults, regardless of weight. Always check the product monograph.
- Correct Weight Used— Use the patient's current actual body weight in kilograms. In obese patients, some drugs (e.g., aminoglycosides, vancomycin) require adjusted body weight. The prescriber should specify.
- Concentration Verification— Liquid medications are available in multiple concentrations (e.g., acetaminophen 160 mg/5 mL vs. 500 mg/5 mL). Always confirm which concentration is being used before calculating volume.
- Renal and Hepatic Impairment— Kidney or liver dysfunction may require dose reduction or extended dosing intervals. Consult a pharmacist or clinical guidelines for renally-adjusted dosing.
- Drug Interactions— Multiple concurrent medications may interact. Use a drug interaction checker (e.g., Drugs.com, Lexicomp) or consult a pharmacist.
- Allergy and Contraindication Review— Check for documented allergies and contraindications before prescribing or administering any drug.
- Double-Check Calculations— Always have a second person independently verify high-alert medication calculations (e.g., chemotherapy, anticoagulants, insulin) before administration.
Ten-Fold Errors: A common and dangerous calculation error involves a decimal point misplacement, resulting in doses that are 10 times too high or too low. Always double-check your arithmetic and verify units (mg vs. mcg, mL vs. L).
Medical Disclaimer
This dosage calculator and the information on this page are provided for educational and informational purposes only. The results are mathematical calculations and do not constitute medical advice. Drug dosing is highly individualized and must account for the patient's age, weight, renal and hepatic function, concurrent medications, allergies, clinical indication, and other factors. Never administer a medication based solely on the output of this calculator. Always verify calculated doses with a licensed pharmacist, physician, or nurse practitioner before administration. If you are a patient or caregiver, consult your healthcare provider for personalized dosing guidance.