Dosage Calculator

Volume Required = (SR × VS) ÷ SS, or (SR × VS) the 'what you need part', over SS 'what you have'. The formula gives you the exact amount of medication to administer:

• SR (Strength Required) - the dose prescribed.
• SS (Stock Strength) - the strength of the medication you have.
• VS (Volume of Stock) - the amount the drug comes in.

VS (Volume of Stock) - For tablets/capsules: this is always 1, for liquids/injections: this is the mL amount supplied.

• Example (tablet) - this is always 1:
  
  • Order = 250 mg
  • Stock = 500 mg tablet
  • VS = 1
  • Volume Required = (SR × VS) ÷ SS
    
    • (250 × 1) ÷ 500 = 0.5 tablets
• Example (liquid)- this is the mL amount supplied:
  • Order = 25 mg
  • Stock = 50 mg/mL
  • VS = 1 mL
  • Volume Required = (SR × VS) ÷ SS
    • (25 × 1) ÷ 50 = 0.5 mL 

Let's get to the heart of the formula so we inderstand it instead of just memorising it.

• What you need (SR × VS) is a combination of two numbers, here is why:
  • Rx: Give 250mg of Amoxicillin. In stock you have Amoxicillin 250mg/5mL.
  • SR (Strength Required) - the dose you need to give ie "Give 250mg of Amoxicillin".
  • VS (Volume of Stock) - the volume of the 'what you have' medicine comes in, in this example it's "per 5mL".
  • By multiplying SR × VS you are working out 'total drug amount' that your dose requires based on the container you have.
• What you have (SS) is the concentration of the medicine sitting on the shelf:
  • SS (Stock Strength) - carrying on with our example, it will be "250mg", as Amoxicillin we have comes as 250mg/5mL.
  • In this example where Amoxicillin 250mg is ordered we would give:
    • (Strength Required x Volume of Stock) ÷ Stock Strength = Volume Required
    • (250mg x 5mL) ÷ 250mg = 1mL

Volume (mL) = Dose (mg) ÷ Concentration (mg/mL). 

This formula tells you exactly how much liquid to pour or draw up:

• Dose (mg) - the amount of medication prescribed by the doctor.
• Concentration (mg/mL) - the strength of the liquid as written on the bottle label.
• Volume (mL) - the final amount of liquid you need to administer.

Now lets take a look at some examples:

• Example (Oral Suspension):
  
  • Order = 100 mg of Ibuprofen
  • Stock = 20 mg/mL (Concentration) 
  • Formula: Volume = Dose ÷ Concentration (100) ÷ 20 = 5 mL
• Example (Syrup):
  
  • Order = 15 mg
  • Stock = 5 mg/mL
  • Formula: Volume = Dose ÷ Concentration (15) ÷ 5 = 3 mL

Let's get to the heart of the formula so we inderstand it instead of just memorising it.

• • The Dose (mg) is the total weight of the drug required for the patient's treatment.
  • The Concentration (mg/mL) tells you how crowded the drug particles are in the liquid.
  • If a syrup is 5 mg/mL, it means for every tiny milliliter of liquid, there are 5 milligrams of medicine floating in it.
  • By dividing the Dose by the Concentration, you are calculating how many of those "milliliter units" are needed to add up to the total weight required.

Injection Volume = Desired Dose (mg) ÷ Vial Strength (mg/mL)

This formula helps you determine how much fluid to draw into the syringe:

• Desired Dose (mg) - the amount of drug the patient needs.
• Vial Strength (mg/mL) - the concentration found on the vial label.
• Injection Volume (mL) - the amount to draw up in the syringe.

Now lets take a look at some examples:

• Example (Standard Vial):
  
  • Order = 80 mg of Gentamicin
  • Stock = 40 mg/mL vial
  • Formula: Injection Volume = Dose ÷ Strength (80) ÷ 40 = 2 mL
• Example (High Concentration):
  
  • Order = 10 mg of Morphine
  • Stock = 10 mg/mL ampoule
  • Formula: Injection Volume = Dose ÷ Strength (10) ÷ 10 = 1 mL

Let's get to the heart of the formula so we inderstand it instead of just memorising it.

• • The Desired Dose is what the doctor has ordered.
  • The Vial Strength is the variable that changes most often.
    • One manufacturer might supply a drug as 10mg/mL, while another supplies it as 50mg/5mL (which is still 10mg/mL, but the label looks different). 
    • By dividing what you need (Dose) by the strength of what you have (Vial Strength), you convert the medical order into a precise physical action: pulling back the plunger to a specific mark on the syringe.

1. Choose your calculation mode by selecting one of the 4 modes
   1. Need/have - work out the dose using the standard “what you need over what you have” formula.
   2. mg/kg - calculate the dose based on patient weight.
   3. mg → mL - convert a dose in milligrams into millilitres using the vial concentration.
   4. Injection Volume  - work out how many millilitres to draw up for a required dose.
2. Enter your numbers. Each field clearly tells you what to enter.
3. Click "Calculate" to get the final result. The calculator will give the formula used and breakdown of the calculation steps. 
4. If you need to redo, click "Reset" to start fresh.

This drug dosage calculator helps you work out the right amount of medicine to give.

It is based on standard clinical dose calculations, including “need over have,” mg/kg dosing, mg to mL conversion, and injection volume.

Always double-check concentrations and prescriptions, and do not rely on the calculator alone for clinical decisions.

**All information is private, no data is saved or shared.**

Q1. Does need/have work for tablets?

• Yes. Set volume of stock (VS) to 1.

Q2. Does this calculator work for injections?

• Yes. Use mg to mL or Injection Volume mode.