Lessons · Pharmacy · the metric ladder: grams, milligrams, micrograms
The metric ladder: one step is a thousand
Every step down the metric ladder multiplies by a thousand: 1 g = 1,000 mg, and 1 mg = 1,000 mcg.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
A prescriber writes 0.5 g and the bottle on the shelf says 250 mg. Nobody converts that for you, and the whole rest of the fill hangs on getting it right. The most dangerous mistakes in a pharmacy are not hard sums; they are a thousand in the wrong place.
How to think about it
Write the unit you have and the unit you want, side by side. Ask which one is smaller. Going to the smaller unit multiplies by 1,000, which moves the decimal point three places to the right; going to the bigger unit divides by 1,000, three places left. Say the direction out loud before you touch the calculator.
Worked example
1 g = 1,000 mgOne step down the ladder. Grams into milligrams multiplies.
0.5 g = 500 mgHalf a gram. The decimal point moved three places right.
1 mg = 1,000 mcgThe next step down, the same thousand.
300 mcg = 0.3 mgGoing back up divides by 1,000: three places left.
Your turn
A script is written for 2.5 g and the shelf is labelled in milligrams. Write the milligrams.
2.5 g = mg
Solve one, graded on the server
The trap
Mixing up mg and mcg. They differ by a thousand, they look almost the same handwritten, and a number that is a thousand times too big still looks like a number. Read the unit twice and write it on every line.