Aviation is a global industry – which can present some complications when it comes to units of measurements. Aircraft manufacturers from Europe often use metric units in their designs (centimetres, litres, kilograms) but many of the engines attached to the front of the aircraft are manufactured in America where imperial units (inches, US gallons, pounds) are more common. This often results in a strange mix of units used accross the aviation industry and you will find yourself needing to convert them.
Atmospheric pressure is expressed in hectopascals (hPa). Older textbooks may refer to millibars (mbar or mb) – one millibar is equal to one hectopascal.
Standard atmospheric pressure is 1013.2 hPa.
Across the UK and Europe you will find a mix of kilograms (kg) and pounds (lb) being used. A kilogram is equal to 2.2 pounds, so to convert kilograms to pound multiply by 2.2 and to convert pounds to kilograms divide by 2.2. It can be useful to memorise this conversion.
Runway lengths and meteorological visibility are typically expressed in metres (m) or kilometres (km). Flight distances are measured in nautical miles (nm), which is the distance between each minute of latitude on the Earth.
Altitude is a distance above mean sea level expressed in feet (ft).
Height is a distance above the ground expressed in feet (ft). If you are flying over a hill, your height is your distance above the hill where as your altitude is your distance above the mean sea level.
Elevation is the distance a certain point is above sea level, e.g. an airport, hill or skyscraper.
Most speed is expressed in knots (kts). A knot is one nautical mile per hour. Vertical speeds, such as rate of climb or descent, are expressed in feet per minute (ft/m or fpm).
Temperature is typically express in degrees celsius (°C), though you may find some aircraft manuals (from American manufacturers) using fahrenheit. You do not need to memorise how to convert between the two for any of the PPL exams.
Litres (L) and US Gallons (USG) are the most common units used to express volume, but Imperial Gallons (Imp. Gal) are occasionally used. One US gallon is 3.8 lites, so to convert US gallons to litres multiply by 3.8. To convert litres to US gallons divide by 3.8.