The Ratio Rule That Prevents Most Unit Mistakes

Updated Sep 8, 2026 · View standard page

Ratios compare quantities. That comparison only makes sense when the quantities are written in compatible units.

Take the ratio 2 m : 50 cm. Writing 2:50 and simplifying to 1:25 is wrong because the 2 and 50 are not measured in the same unit.

Convert before you simplify

Change 2 m to 200 cm:

200 cm : 50 cm = 200 : 50 = 4 : 1

Now the units cancel cleanly and the ratio is 4:1.

Time ratios work the same way

For 1 hour : 20 minutes, convert 1 hour to 60 minutes:

60 : 20 = 3 : 1 Habit worth keeping: write the unit beside every number until both sides match. Remove the units only after the conversion is complete.

What if the units are already the same?

Then you can simplify immediately. For 45 cm : 30 cm, divide both sides by 15 to get 3:2.

A practical check

Ask what your final ratio means in words. A ratio of 4:1 for 2 m to 50 cm says the first length is four times the second. That is clearly true, so the result passes a common-sense check.