SI Units and Conversions: mg, mcg, g, mL Practice

Every calculation in CBT Part A depends on one skill: moving cleanly between units. Get conversions wrong and every later step is wrong too, however good your arithmetic. This chapter is pure conversion practice.

The ladder is short. Learn it as a ladder, not as isolated facts:

• 1 kg = 1,000 g
• 1 g = 1,000 mg
• 1 mg = 1,000 mcg (micrograms)
• 1 mcg = 1,000 nanograms
• 1 L = 1,000 mL

Each rung is a factor of 1,000. Nothing in UK drug calculation uses 10 or 100 between mass units. So the only question is which direction you’re moving.

Going down the ladder to a smaller unit, multiply. 0.5 g becomes 500 mg. Going up to a larger unit, divide. 250 mcg becomes 0.25 mg. Said differently: smaller unit, bigger number.

Where the exam sets traps

The CBT rarely asks a naked conversion. It hides one inside a dose question. A prescription written in grams, stock strength in milligrams. Convert first, then calculate, and the question is easy. Calculate first and you’ll produce an answer that’s out by a factor of 1,000, and there will be a matching wrong option waiting for you. There always is.

Watch for these specific patterns:

• Decimal misreads. 0.125 mg and 0.0125 mg look similar at speed. Write the number down on your whiteboard before you work with it.
• Two-step conversions. Nanogram questions force two hops (nanograms to mcg to mg). Take them one rung at a time.
• The mcg notation rule. UK convention writes micrograms in full or as mcg, never μg, because handwritten μg gets misread as mg, a thousand-fold error. The exam can test the convention itself, not just the arithmetic.

A worked example

Prescription: digoxin 0.25 mg orally. Stock: 125 mcg tablets.

• Convert the prescription to the stock’s unit: 0.25 mg × 1,000 = 250 mcg.
• Divide: 250 ÷ 125 = 2 tablets.

Two lines. The candidates who get this wrong don’t fail the division; they skip the first line, compute 0.25 ÷ 125, and then try to make sense of 0.002 tablets. If your answer is absurd, the units are where you look first.

A sanity check worth building into every question: doses of solid oral medication are almost always between half a tablet and four tablets. An answer of 20 tablets or 0.01 tablets means a conversion error, not an unusual prescription.

The method behind all of this is covered in the drug calculation formulas chapter of the main guide. When you’re ready, the five questions below mirror how Part A actually words these.