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>Converting Between Systems>How Do You Convert Imperial to Metric?

How Do You Convert Imperial to Metric?

When you convert imperial units to metric units, you need to multiply each unit by a conversion factor. This entry gives you an overview of which conversion factor to use and which metric unit it produces.

Length

The most common imperial length unit outside of the US is inches. PC monitors and TV sizes are measured in inches. You could for example read in a store that a PC monitor is 24 inches or 24”. What does that mean? It means that the diagonal of the monitor is 24 inches long.

Monitor measuring 24 inches along the diagonal

So how long is that? That’s what you’ll learn in this entry.

For length you usually convert inches to centimeters, feet to centimeters, yards to meters and miles to kilometers or meters.


Imperial Unit	Conversion Factor	Metric Unit

$1$ in	$\cdot 2.54$	$2.54 cm \approx 2.5 cm$

$1$ ft	$\cdot 30.48$	$30.48 cm \approx 30 cm$

$1$ yd	$\cdot 0.9144$	$0.9144 m \approx 0.9 m$

$1$ mi	$\cdot 1609.3$	$1609.3 m \approx 1.6 km$

Example 1

So how many centimeters is the 24” monitor?

You find the conversion factor from the table above to be $2.54$ :

24 in \cdot 2.54 = 60.96 cm .

Meaning the monitor has a diagonal which is $60.96$ cm.

Example 2

An American football field is exactly 100 yards. How long is that in meters?

You find the conversion factor from the table above to be $0.9144$ :

100 yd \cdot 0.9144 = 91.44 m .

Area

For area you usually convert square inches to square centimeters, square feet to square centimeters, square yards to square meters, acres to square meters and square miles to square kilometers.


Imperial Unit	Conversion Factor	Metric Unit

$1$ in²	$\cdot 6.4516$	$6.4516 {cm}^{2} \approx 6.5 {cm}^{2}$

$1$ ft²	$\cdot 929$	$929 {cm}^{2} \approx 0.09 m^{2}$

$1$ yd²	$\cdot 0.8361$	$0.8361 m^{2} \approx 0.8 m^{2}$

$1$ acre	$\cdot 4046.9$	$4046.9 m^{2} \approx 4047 m^{2}$

$1$ mi²	$\cdot 2.59$	$2.59 {km}^{2} \approx 2.6 {km}^{2}$

Example 3

You measure you room to be 13 feet long and 8 feet wide. What is the area of your room in square feet and square meters?

Square feet:

13 ft \cdot 8 ft = 104 {ft}^{2}

Square meters:

You use the table and see that you need to multiply with 929 to get square centimeters, but since you want square meters, you multiply with $929 \cdot \frac{1}{100} = 0.0929$ :

104 {ft}^{2} \cdot 0.0929 = 9.6616 m^{2} .

Volume

For volume you usually convert fluid ounces to milliliters, pints to liters and gallons to liters.


Imperial Unit	Conversion Factor	Metric Unit

$1$ fl oz	$\cdot 29.574$	$29.574 mL \approx 30 mL$

$1$ pt	$\cdot 0.4732$	$0.4732 L \approx 0.5 L$

$1$ gal	$\cdot 3.7854$	$3.7854 L \approx 3.8 L$

Example 4

You travel to Germany where the soda bottle says $0.5 L$ . You remember that a bottle of similar size in USA is $20 fl oz$ . Which bottle has more soda?

You need to find out how many liters $20$ fl oz is:

20 fl oz \cdot 29.574 \approx 591.5 mL \approx 0.59 L .

Seems like the bottles in USA has more soda in them than the ones in Germany. The saying “everything is bigger in America” is true in this case.

Weight

You usually convert ounces to grams, pounds to kilograms and ton to metric tonne.


Imperial Unit	Conversion Factor	Metric Unit

$1$ oz	$\cdot 28.35$	$28.35 g \approx 28 g$

$1$ lb	$\cdot 0.4536$	$0.4536 kg \approx 0.45 kg$

$1$ ton	$\cdot 0.9072$	$0.9072 tonne \approx 0.9 tonne$

Example 5

Your dog weighs $45 lb$ . How much is that in kilograms?

The table above gives

45 lb \cdot 0.4536 \approx 20.4 kg .

Good boy!

Mass Density

Since density consist of two units, you can convert each of them separately and then combine them. Density in imperial is given by lb/ft³, while metric often uses g/cm³. You can convert lb/ft³ to g/cm³ by first converting pounds to grams and then feet to centimeters. One foot is $0.3048$ meters, so one foot cubed is $0.304 8^{3} = 0.0283 m^{3}$ : $\begin{array}{l} 1 lb & = 453.6 g \\ 1 {ft}^{3} & = 0.0283 m^{3} = 28 300 {cm}^{3} \\ 1 {lb/ft}^{3} & = \frac{453.6 g}{28 300 {cm}^{3}} = 0.016 {g/cm}^{3} \end{array}$

Rule

You convert from lb/ft³ to g/cm³ by multiplying lb/ft³ with $0.016$ .

Temperature

The US uses Fahrenheit, while most other countries use Celsius. Unlike most other units, you cannot multiply Fahrenheit with a conversion factor to get Celsius. Instead, you have to use this simple formula:

Formula

°C is temperature measured in Celsius and °F is temperature measured in Fahrenheit.

°C = \frac{°F - 32}{1.8}

Example 6

On a hot summer day, it’s $108 °F$ outside. What is that in Celsius?

You use the above formula:

\frac{108 - 32}{1.8} = \frac{76}{1.8} \approx 42.2 °C .

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