Units (Psychology)

Converting Units

A dimension is a measurement of a particular kind, for example height or weight. Scientists use the SI Units to record measurements.

Knowing the dimensions of an object is important when performing calculations. You cannot add or subtract two amounts if they are not of the same dimension.

• For example, “£$1 + 1$kg” is undefined and has no meaning.
• However, $100\text{kg} - 50\text{kg} = 50$kg.

Although, you can multiply or divide quantities that are not of the same dimension, for example, metres per second (m/s).

Sometimes you are given two measurements of the same kind, for example weight, but the units are not the same (e.g. pounds (lbs) and kilograms). To perform calculations with them you will need to convert them into the same units.

For example, to convert pounds (lbs) into kilograms, you should know that there are approximately $0.454$kg in one lb. So, to convert pounds to kilograms you multiply the weight in pounds by $0.454$. As an example, to convert $20$lb to kg you calculate: $0.454 \times20 \text{lbs} \approx 9.1$kg.

SI Unit Prefixes

This below table will help with converting units.

Prefix	Name	Size
k	kilo	1000
d	deca	One-tenth, $10^{-1}$
c	centi	One-hundredth, $10^{-2}$
m	milli	One-thousandth, $10^{-3}$
μ	micro	One-millionth, $10^{-6}$

Examples

• $1$ kg $ = 1000$ g
• $1$ mg $ = 0.001$ g
• $1 $cm $ = 0.01$ m

Worked Example

Test Yourself

Try our Numbas test on background mathematics.

Try our Numbas test on Units of measurement.

See Also

For more information on all of the topics in this section see numerical reasoning.

For a guide at performing some of the calculations seen in this section using your calculator see using your calculator.