Dosage Practice

Injection Dosage Calculation: mL to Draw up for IM and Subcut Doses

The vial label gives a concentration, for example 80 mg in 2 mL. Divide the dose ordered by the drug amount on the label and multiply by the label's volume, exactly as for oral liquids. Volumes under 1 mL are rounded to hundredths.

The Formula

mL to draw up = (desired ÷ have) × vial volume

Same desired-over-have idea; the volume is what is in the vial, not the whole vial.

How to work it out

  1. 1 Read the vial as a concentration: drug amount per volume.
  2. 2 Convert units if the order and the vial differ.
  3. 3 Divide the ordered dose by the vial's drug amount, multiply by the vial's volume.
  4. 4 Round to the nearest tenth for a 3 mL syringe, or the nearest hundredth for volumes under 1 mL in a 1 mL syringe. Check the volume suits the site: usually up to 1 mL subcutaneous, and up to 3 mL in a large adult muscle.

Worked Examples

Example 1

Ordered: 40 mg IM. Available: 80 mg per 2 mL. How many mL?

  1. 40 mg ÷ 80 mg = 0.5.
  2. 0.5 × 2 mL = 1 mL.

Answer: 1 mL

Example 2

Ordered: 0.3 mg subcut. Available: 1 mg per mL. How many mL?

  1. 0.3 mg ÷ 1 mg × 1 mL = 0.3 mL.
  2. Under 1 mL, so it is drawn up in a 1 mL syringe.

Answer: 0.3 mL

Try five free problems

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Problem 1 of 3No sign-up needed

Ordered: 25 mg IM. Available: 50 mg per mL. How many mL will you draw up?

Round to the nearest tenth

mL

Common Mistakes

  • Treating a multi-dose vial as one dose

    A 10 mL vial of 50 mg/mL contains 500 mg. The calculation uses the concentration, 50 mg per 1 mL, not the vial total.

  • Rounding a small dose to tenths

    0.47 mL stays 0.47 mL in a 1 mL syringe. Rounding it to 0.5 mL is a 6% overdose.

  • Ignoring the site limit

    An answer of 4 mL for a deltoid injection is arithmetically possible but cannot be given in one shot. Exams expect you to notice.

Questions Students Ask

Which syringe do I use?

Under 1 mL: a 1 mL syringe marked in hundredths. 1 to 3 mL: a 3 mL syringe marked in tenths. Match the rounding to the syringe.

What is the maximum volume per site?

Textbook limits are about 1 mL subcutaneous, 1 mL deltoid, and up to 3 mL in the ventrogluteal or vastus lateralis of an adult. Follow your institution's policy.

Keep practicing in lesson 6: Injections (IM and Subcut)

Fresh numbers every time. Ten correct answers finish the lesson. The practice round and the first five problems of lesson 1 are free.

Open lesson 6