A patient in acute bronchospasm requires a weight-based nebulised medication. The patient weighs 82 kg. Before drawing up the drug, the crew needs the total milligrams and the volume to draw from the stock concentration to confirm the dose is correct and within protocol limits.
Prehospital Drug Dose (mg/kg)
Medication
Imperial mode auto-converts lb→kg
Total mg = Weight (kg) × Dose (mg/kg)
Volume (mL) = Total mg ÷ Concentration (mg/mL)
Example (morphine): 80 kg · 0.1 mg/kg · 10 mg/mL stock → 8 mg → 0.8 mL
💡 Always confirm drug, dose, route and patient weight with your partner before administration. Document time given.
Reference: NPS MedicineWise · ACSQHC Medication Safety
Clinical safety:
🇦🇺 Verify with facility drug formulary and senior clinician · Meets AHPRA/ACSQHC standards
1 What this calculator does
Calculates the total dose in milligrams and the volume in millilitres to administer from a known drug concentration (mg/mL) and patient weight. Supports weight-based dose calculation for any drug requiring mg/kg dosing. Supports metric and imperial weight entry.
2 Formula & professional reasoning
Total dose (mg) = Patient weight (kg) x Dose (mg/kg)
Volume (mL) = Total dose (mg) / Stock concentration (mg/mL)
Always double-check: confirm drug, dose, route, concentration and weight with partner before administration
Weight-based dosing ensures the administered dose is proportional to the patient's body mass, minimising both underdosing (therapeutic failure) and overdosing (adverse effects). The volume calculation converts the required dose from milligrams to a measurable volume in the syringe or nebuliser, using the specific concentration of the available stock. In prehospital settings where multiple concentrations of the same drug may be carried, explicitly stating the concentration used prevents critical errors.
3 Worked examples
⚠️ Illustrative example only — not clinical or professional instruction.
Basic
Illustrative weight-based dose calculation
Given: Patient weight: 82 kg | Dose: 0.01 mg/kg | Stock concentration: 1 mg/mL | Route: IV
Working:
Total dose: 82 x 0.01 = 0.82 mg | Volume: 0.82 / 1 = 0.82 mLAnswer: Total: 0.82 mg | Volume: 0.82 mL from 1 mg/mL stock
💡 Illustrative example only. Verify all clinical decisions with your agency protocol, medical director and partner clinician. Meets AHPRA/ACSQHC prehospital care standards. Enter the specific drug dose from your agency protocol into the calculator. This is an illustrative calculation only showing the mg/kg formula -- do not use this as a drug dose reference.
Standard
Illustrative paediatric weight-based calculation
Given: Patient weight: 18 kg (paediatric) | Dose: 15 mg/kg | Stock: 50 mg/mL | Route: oral
Working:
Total dose: 18 x 15 = 270 mg | Volume: 270 / 50 = 5.4 mLAnswer: Total: 270 mg | Volume: 5.4 mL from 50 mg/mL stock
💡 Illustrative example only. Verify all clinical decisions with your agency protocol, medical director and partner clinician. Meets AHPRA/ACSQHC prehospital care standards. Paediatric weight-based dosing requires verified weight (use Broselow estimator if actual weight unavailable) and confirmed concentration. Double-check all paediatric calculations with a second clinician before administration.
Advanced
Illustrative concentration mismatch check
Given: Drug needed: 60 mg | Stock A: 50 mg/mL | Stock B: 10 mg/mL
Working:
Volume from Stock A: 60 / 50 = 1.2 mL | Volume from Stock B: 60 / 10 = 6.0 mL | Same dose, 5x different volumeAnswer: Stock A: 1.2 mL | Stock B: 6.0 mL | Confirm which concentration is in the kit before drawing up
💡 Illustrative example only. Verify all clinical decisions with your agency protocol, medical director and partner clinician. Meets AHPRA/ACSQHC prehospital care standards. Drug concentration errors are a leading cause of prehospital medication incidents. Always read the label of the specific ampoule in your hand, not the label of the box or protocol chart. The calculator requires you to enter the concentration of the stock you are actually using.
4 Sanity check
Five rights of medication administration
Right patient | Right drug | Right dose | Right route | Right time
Some agencies add: right documentation and right to refuse.
Double-check calculation with partner before administration
Both clinicians should independently verify: drug name, dose in mg/kg, total mg, stock concentration, calculated volume and route
Weight estimation when actual weight is unavailable
Use Broselow tape for paediatric patients | For adults: use Broselow chart for patients unable to self-report | Document that weight is estimated
Concentration on the ampoule may differ from the box
Always read the concentration from the specific ampoule being drawn up -- not the box label, protocol chart or memory
5 Common errors
| Error | Cause | Consequence | Fix |
|---|---|---|---|
| Using an estimated or incorrect patient weight without documentation | Accepting verbal weight without verification or using a visual estimate without a structured tool | Dose error proportional to weight error -- a 25% weight error produces a 25% dose error | Obtain actual weight when possible. For paediatric patients, use the Broselow tape. For adults unable to self-report, use Broselow adult chart or IDEAL body weight estimation. Document clearly that weight is estimated and the method used. |
| Not confirming the concentration of the specific ampoule before calculating volume | Assuming the concentration matches the protocol reference or the box | 10-fold or other major dosing error if a different concentration is inadvertently used | Before entering the concentration into the calculator, read the mg/mL value from the specific ampoule in hand. Confirm with your partner. This is especially critical for drugs that come in multiple concentrations. |
| Not verifying the calculated volume against a plausibility check | Accepting the calculated volume without a sanity check | A calculation or entry error produces an implausible volume that is administered without question | Apply a plausibility check to every calculated volume: an adult IV dose in the mL range 0.1-20 mL is typically within plausible range. A calculated dose of 0.02 mL or 50 mL from a standard syringe should trigger an immediate recheck of the inputs. |
| Not documenting the time of administration and dose given | Prioritising patient care over contemporaneous documentation in a high-acuity scene | Incomplete medication record -- subsequent clinicians cannot safely give additional doses | At minimum, document drug name, dose in mg, volume administered, route, time and clinician ID before leaving the scene or immediately on transport. Use the partner system -- one clinician administers, one documents. |
6 Reference & regulatory links
🇦🇺 Australia
7 Professional workflow
Common tools used alongside this one:
Paeds Weight Estimator (Broselow)
Paeds Weight Estimator provides the estimated weight input for paediatric drug dose calculations
→
Cardiac Arrest Drug Doses
Cardiac Arrest Drug Doses calculator provides weight-based doses for cardiac arrest drugs using the same formula
→
Mean Arterial Pressure (MAP)
Mean Arterial Pressure monitors the haemodynamic response to drug administration during resuscitation
→