MATHEMATICAL CONVERSIONS - IMPERIAL - METRIC

The automotive trade uses the S.I units of measurements.

The term ‘S.I.’ refers to a metric system of measurement called ‘SYSTEM INTERNATIONAL’ units.

The most common units of measurement that you will come across in the motor trade are listed below

LENGTH                 -        MILLIMETRE (mm)
CAPACITY             -        LITRE (l)
FORCE                    -        NEWTON (N)
POWER                   -        WATT (W)
PRESSURE             -        KILOPASCAL (kPa)
TORQUE                 -        NEWTON METRE (Nm)
VACUUM               -        MILLIMETRE OF MERCURY (mm Hg)
TEMPERATURE    -        DEGREES CELSIUS (oC)
TIME                       -        SECOND (s)
SPEED                     -        KILOMETRE PER HOUR (km/h)
MASS                      -        KILOGRAM (kg)
AREA                      -        SQUARE CENTIMETRE (cm2)

While there may seem to be quite a number of different units around, its only by taking a little time now in learning the different units and their symbols that will save you time and trouble later on.

Most text books, reference books, tune-up books and workshop manuals will have information on some and if not all the units that you require.

Listed below are the main units and their symbols.

Remember to look at the example column as this will give you a practical example of the unit.

 

S.I. MEASUREMENT IN AUTOMOTIVE MECHANICS

QUANTITY TO BE MEASURED	UNITS USED IN MOTOR VEHICLE MEASUREMENTS	SYMBOLS USED TO REPRESENT UNITS	EXAMPLE MEASUREMENT
LENGTH - distance between two points. SMALL   MEDIUM   LARGE	Millimetre   Metre   Kilometre	mm   m   km	Any measurement of length on a vehicle or part. Short distances moved by a vehicle. Long distances travelled by a vehicle
Note:   Centimetres are normally not used for length measurements in the automotive trade.
AREA - the size of a surface. SMALL   LARGE	Square Centimetre   Square Metre	cm2   m2	The area on the top of a piston. The area of a workshop floor.
VOLUME - spaces occupied by or within a body. SMALL   LARGE	Cubic Centimetre   Cubic Metre	cc or cm3   m3	Volume of a cylinder. Volume inside part of a vehicle.
VOLUME - quantity of liquid or gas. SMALL   LARGE	Millilitre   Litre	ml   l	Amount of liquid in small container or reservoir. Amount of liquid in a large container or tank.

 

QUANTITY TO BE MEASURED	UNITS USED IN MOTOR VEHICLE MEASUREMENTS	SYMBOLS USED TO REPRESENT UNITS	EXAMPLE MEASUREMENT
MASS - quantity of matter in a body. SMALL MEDIUM LARGE	gram kilogram tonne	g kg t	Mass of small parts. Mass of large parts. Mass of the whole vehicle.
TIME - how long it takes for something to happen. SMALL   MEDIUM   LARGE	Second   Minute   Hour	s   min   hr	How long it takes to accelerate from 0 to 100 km/h. How long it takes an engine to warm up. How long it takes a vehicle to travel a long distance.
SPEED - how fast a thing is moving. FOR A DISTANCE ROTATIONAL	kilometre per hour revs per minute	km/h rpm	How fast a vehicle is travelling. How fast an engine is turning over.
FORCE - how hard something is pushing or pulling.	Newton	N	The force developed when a spring is compressed.
PRESSURE - a force acting on an area	Kilopascal	kPa	The pressure in a tyre.

QUANTITY TO BE MEASURED	UNITS USED IN MOTOR VEHICLE MEASUREMENTS	SYMBOLS USED TO REPRESENT UNITS	EXAMPLE MEASUREMENT
TORQUE - how hard something is turning or being turned.	Newton metre	Nm	The turning force being developed by an engine.
POWER - how hard something is working. SMALL LARGE	Watt Kilowatt	W kW	The output of a light globe. The output of an engine.
TEMPERATURE - how hot or cold something is.	Degrees Celsius	oC	The temperature of the engine cooling water.
VACUUM - where pressure is below atmospheric.	Millimetres of Mercury or Kilopascals	mm Hg or kPa	The vacuum in an inlet manifold.
FUEL CONSUMPTION - the rate at which a vehicle uses fuel.	Litres per 100 kilometres	1/100km	How many litres of fuel a vehicle uses every 100 kilometres.

 

Prefixes commonly used in automotive measurements.

Milli    (m)	a one thousandth of	There are one thousand millimetres in a metre.
centi    (c)	a one hundredth of	There are one hundred centimeters in a metre.
kilo      (k)	one thousand times	There are one thousand metres in a kilometre.

                                                        
CONVERSIONS                                                                

There may be times when you have to convert one unit of measurement into another unit of measurement, for example from IMPERIAL TO METRIC or the other way round.

It may be that the gauge you are using is in IMPERIAL measurement and the manufacturer has given you the measurement in a METRIC unit.  In this case you have to convert the readings to IMPERIAL.

In order to help you do this conversion, listed below are the common units of measurements and their conversion factors.

 

METRIC AND IMPERIAL CONVERSIONS

LENGTH

Small          - millimetre (mm)         To convert millimetres to inches divide then by 25.4

                   - inches (in)                   To convert inches to millimetres multiply them by 25.4

There are 25.4 millimetres in every inch.

Large          - kilometre (km)            To convert kilometres to miles divide them by 1.61.

                   - mile                             To convert miles to kilometres multiply them by 1.61.

There are approximately 1.61 kilometres in every mile.

AREA

Small          - square millimetres(mm2) To convert square millimetres to square inches divide  them by 645.

                   - square inches (in2)        To convert square inches to square millimetres  multiply them by 645.

There are approximately 645 square millimetres in every square inch.

Large          - square metres (m2)        To convert square metres to square feet divide them by 0.093.

                   - square feet (ft2)             To convert square feet to square metres multiply  them by 0.093.
There are approximately 0.093 square metres in every square feet.

VOLUME

Small          - cubic centimetre (cc)    To convert cubic centimetres to cubic inches divide them by 16.4

                   - cubic inch (in3)             To convert cubic inches to cubic centimetres multiply  them by 16.4
 
There are approximately 16.4 cubic centimetres in every cubic inch.

Large          - Litres (l)                      To convert litres to gallons divide them by 4.55

                   - Gallon (G)                  To convert gallons to litres multiply them by 4.55

There are approximately 4.55 litres in very gallon.

 

MASS

Small          - gram (g)                      To convert grams to pounds divide them by 454

                   - pound (lb.)                  to convert pounds to grams multiply them by 454

There are approximately 454 grams in every pound.

                   - Kilogram (kg)             To convert kilograms to pounds divide them by 0.454

                   - Pounds (lb.)                To convert pounds to kilograms multiply them by 0.454

There are approximately 0.454 kilograms in every pound.

Large          - tonne (t)                      To convert tonnes to tons divide them by 1.02

                   - ton                              To convert tons to tonnes multiply them by 1.02

There are 1.02 tonnes in every ton.

Because tonnes and tons are so close to each other we don’t usually convert them.

 

SPEED

Kilometre per hour (km/h)               To convert kilometres per hour to miles per hour divide them by 1.61

Miles per hour (mph)                       To convert miles per hour to kilometres per hour multiply them by 1.61.
There are approximately 1.61 ‘kilometres per hour’ in every ‘mile per hour’.
FORCE

Newton (N)                                      To convert Newtons to pounds-force divide them by 4.45

Pounds-force (lbf)                            To convert pounds-force to Newtons multiplythem by 4.45

There are approximately 4.45 Newtons in every pound of force.

 

PRESSURE

Kilopascal (kPa)                              To convert kilopascals to pounds-force per square inch   divide them by 6.89.

Pounds-force per square inch (psi)  To convert pounds-force per square inch to kilopascals   multiply them by 6.89.

There are approximately 6.89 kilopascals in every pounds-force per square inch.

 

TORQUE

Newton metre (Nm)                         To convert Newtons to foot pounds divide by 1.36.

Foot pounds (lbf.ft )                        To convert foot pounds to Newtons multiply them by 1.36

There are approximately 1.36 Newtons metres in every foot pound.

NOTE: Foot pounds is the name given to ‘Pounds-force foot’.

 

POWER

Kilowatt (kW)                                  To convert kilowatts to horse power divide them by 0.746.

Horse Power (hp)                             To convert horse power to kilowatts multiply them by 0.746.

There is approximately 0.746 kilowatts in every horse power.

TEMPERATURE

Degrees Celsius (oC)                       To convert oC to oF divide by 0.555 and then add 32.

Degrees Fahrenheit (oF)                  To convert oF to oC take away 32 and then multiply by 0.555
There is approximately 0.555 degrees Celsius in every degree Fahrenheit and 0o C is equal to 32 F.

VACUUM

Millimetres of mercury (mm Hg)    To convert millimetres of mercury to inches of mercury  divide by 25.4

Inches of mercury (inch Hg)           To convert inches of mercury to millimetres of mercury multiply by 25.4

There are 25.4 millimetres of mercury in every inch of mercury.