SI and Imperial Conversions
Ch. 3 Measurement
SI and Imperial Conversions

Sometimes we may need to convert
from SI units to Imperial, or from
Imperial to SI units. To do this, we will
require a chart that describes the basic
conversion factors:
Conversion Chart
Conversion Factors
Length
Imperial  SI
Capacity/ Volume
SI  Imperial
Imperial  SI
1 gal = 4.546 L
SI  Imperial
1 in = 2.54 cm
1 cm = 0.394 in
1 L = 0.22 gal
1 ft = 0.305 m
1 m = 3.281 ft
1 yd = 0.914 m
1 m = 1.094 yd
Imperial  SI
SI  Imperial
1 mi = 1.609 km
1 km = 0.621 mi
1 oz = 28.35 g
1 g = 0.0305 oz
1 lb = 0.454 kg
1 kg = 2.205 lb
1 lb = 454 g
1 g = 0.002 lb
Mass/Weight
*note: These conversions are approximate. oz = ounce, lb = pound, gal = gallon
To convert between units, we will use
proportions:
W ANT

1) Set up a fraction : H A V E Place an “x” and
the units for what you WANT.

2) Find the appropriate conversion on the
chart.

3) Set up a second fraction, with the units of
what you WANT in the numerator and what
you HAVE in the denominator

4) Cross multiply and divide to get the
correct answer.
Ex1. Carlos measures 14 cm but he needs to know
this length in inches. How many inches is 14 cm?

14cm = _______in ?

1) WANT inches HAVE centimeters
x _ in
14 cm

2) we are converting from SI  imperial, so
we will use: 1 cm = 0.394 in
Ex. 1 Continued

3)
0 .3 9 4 in

4)
x _ in
1cm
14 cm



0.394 in
1cm
14 x 0.394  1 = 5.52 in
*note: rounded to 2 decimal places*
There are 5.52 in. in 14 cm. OR ~ 5
1
2
in.
Ex2. You are on vacation in Hawaii and are going
on a mountain hike. You read in the guide book
that the hike is 3.5 miles. How many km is this?

1) WANT km HAVE mi
x _ km
3.5 m i

2) we are converting Imperial  SI so we
will use: 1 mi = 1.609 km
1.609 km

3)
1m i
1.609 km
x _ km

4)
3.5 m i
=
1m i
3.5 x 1.609  1 = 5.63 km
*note: rounded to 2 decimal places*

There are 5.63 km in 3.5 miles.
But what if…?

We may need to complete more than
one conversion if what we want is not
on the conversion factor chart.

For example, you may need to change
from Imperial SI first , then convert
the SI units to the desired unit.
Ex3. Edmund is measuring wood planks to make a
chair. He measures a piece to be 0.8 m, but he
needs to know how many inches this will be.

1) We WANT inches we HAVE meters.

BUT there is no conversion on the chart
from meters to inches. We must first convert
m  ft then ft in.

2) SI  Imperial so we will use: 1 m =
3.281 ft
x _ feet
0 .8 m
Ex. 3 Continued
3.281 feet

3)

4) x _
1m
feet
0 .8 m


=
3.281 feet
0.8 x 3.281 1 = 2.62 ft
1m
We must now convert from ftin
7
2.62 ft x 12 = 31.44 in OR ~3116 in
We could have converted the SI units first (from m 
cm) and then converted from SIImperial. Let’s try it!

1) 0.8 m = ____cm?
0.8 x 100 = 80 cm
KHDmdcm
x _ in
80 cm

2) SI  Imperial, so we will use:
1 cm = 0.394 in

3)
0 .3 9 4 in
1cm
x _ in

4)
80 cm
=
0 .3 9 4 in
1cm
31.52 in. OR ~31
80 x 0.394  1 =
1
2
in