Length unit conversions
Whenever you measure how tall someone is, how long a path is, or how thick a sheet of paper is, you need a unit of length. There are many of them — and on this page you will learn how to switch between the most common ones without losing your way.
What you will learn
In this topic you will work with two families of units:
- Metric system – used almost everywhere in the world: mm, cm, dm, m, km.
- Imperial system – still common in the United States and the United Kingdom: inch, foot, yard, mile.
For both families the trick is the same: going to a smaller unit you multiply, going to a larger unit you divide.
Topics in this guide
- Metric system – how mm, cm, dm, m and km relate to each other.
- Imperial units – the inch, foot, yard and mile and their conversion factors.
- Converting larger to smaller – the multiplication side of the table.
- Converting smaller to larger – the division side of the table.
- Mixed-unit arithmetic – adding, subtracting and comparing values written in different units.
Why do unit conversions matter?
You meet length conversions all the time, often without thinking about it:
- A label says 3 cm, but the box you have to fill is 40 mm wide. Will it fit?
- The recipe asks for 0.5 m of string and you only have a 70 cm piece. Is that enough?
- A road sign says 12 km, your map app reports 8 mi. Is that the same place?
Once you know the conversion rules you can compare any two distances quickly.
A quick mental model
Think of the units like steps on a ladder:
- Going down the ladder (large → small) means you have more pieces, so you multiply.
- Going up the ladder (small → large) means you can bundle pieces together, so you divide.
The metric ladder uses jumps of ×10 between mm – cm – dm – m, and a big jump of ×1000 between m and km. The imperial ladder uses different jumps: ×12, ×3 and ×1760.
Practise it
When you have read the articles above, try the exercises below. Each one focuses on one specific skill so you can practise step by step:
- Convert from a larger unit to a smaller one
- Convert from a smaller unit to a larger one
- Fill in the missing unit
- Convert two units to one
- Arithmetic with mixed units
- Compare lengths in different units
- Compare lengths in mixed units
- Imperial units – smaller to larger
- Imperial units – larger to smaller
- Compare imperial units