A Number and a Unit

You find a note on the kitchen table: "The new table is 2." Two what? Two hand spans would be a tiny table. Two steps would be a big one. Or a recipe says: "Add 3 of salt." Three spoons, or three cups? One is tasty, the other is terrible.

What is missing from these notes?

Two parts

Every measurement has two parts:

a number - how many;
a unit - how many of what.

"2" alone tells you nothing. "Hand spans" alone tells you nothing either. "2 hand spans" tells you exactly how long the table is.

Measuring is counting units

In the Your Foot, My Foot lesson we saw that a unit is an amount everyone agrees to measure with. When you measure, you count how many of that unit fit.

A table that is 6 hand spans long is a table you can cover with 6 spans laid end to end, one after the other.

A bigger unit, a smaller number

A cubit, from your elbow to your fingertip, is about as long as two of your hand spans. So the same table is 6 spans or 3 cubits long.

The same table measured twice: 6 hand spans fit along it, or 3 cubits. The bigger unit gives the smaller number

The table didn't change. Only the unit did. A bigger unit gives a smaller number, and a smaller unit gives a bigger one. That is why a measurement without its unit is useless: "3" and "6" can be the very same table.

Different things, different units

How long a table is, is one kind of thing you can measure. How heavy a bag of rice is, is another. How long a song lasts is another. How hot your tea is, is another. Each kind of thing you can measure is called a quantity.

Four quantities and a tool for each: length with a ruler, how heavy with a scale, how long it takes with a clock, and how hot with a thermometer

Each quantity needs its own kind of unit. You can't say how heavy a bag of rice is in steps, or how long a song lasts in hand spans. In the rest of this course we will meet the units for each of these quantities.

Try it at home

Measure a table in hand spans, then in pencil lengths (lay a pencil down again and again).
Which number is bigger? Which unit is bigger? Check that the bigger unit gave the smaller number.

Words to know

number and unit - the two parts of every measurement, like "6" and "hand spans"
quantity - a kind of thing you can measure, like length, how heavy something is, time or how hot something is

Big idea

A measurement is a number and a unit: the unit says what you counted, and the number says how many. The same thing gives a smaller number in a bigger unit, and each quantity has its own kind of unit.