Convert feet to kilometres

The foot belongs to the US customary and imperial systems. It holds 12 inches, and three feet make a yard. Since 1 July 1959 the inch has been exactly 25.4 millimetres, which fixes the foot at 0.3048 metres[1]. In the United States it is the everyday unit for elevation and building height, and most of the world's pilots read their altitude in it.

A kilometre is 1,000 metres. The metre is the SI base unit of length, defined through the speed of light: it is the distance light travels in a vacuum in 1/299,792,458 of a second[2]. Australia measures every road distance in kilometres, the UK still signposts roads in miles, and in the United States kilometres turn up mainly in 5K and 10K races.

The two units meet in the metre. A foot is 0.3048 metres, and moving the decimal point three places to the left turns that into 0.0003048 kilometres, the factor this page works with. For the step in metres alone, use the feet to meters converter.

Multiply the number of feet by 0.0003048. Dividing by 3,280.84 gives the same answer, because that many feet fit into one kilometre.

Say the seat-back map shows 37,000 feet. 37,000 × 0.0003048 = 11.2776 km, so the plane is a little over 11 kilometres up. Any other value works the same way: 5,000 feet comes to 1.524 km and 12,000 feet to 3.6576 km.

For a quick estimate in your head, treat each 1,000 feet as 0.3 km. Thirty-five thousand feet then gives 35 × 0.3 = 10.5 km against an exact 10.668 km. The shortcut runs about 1.6 percent low, which is fine for small talk on a flight but not for a calculation you want to keep.

The reverse direction has its own page, the km to feet converter, and the Swap order button takes you there with your result.

A flight level is a height in hundreds of feet measured against a standard pressure of 1013 hPa, or 29.92 inHg[3]. With every altimeter set to that value, all aircraft share one reference, and the vertical gap between them holds even when the air pressure on the ground changes. As a result, FL350 is not always exactly 10.668 km above the sea, because the true height shifts with the weather.

The International Civil Aviation Organization has recommended a move to SI units since 1979, including metres for flight levels[3]. Only a few countries use metre levels, among them China and Mongolia, and Russia switched to feet above the transition altitude on 17 November 2011[3]. The km to feet converter shows the Chinese metre levels in feet.

Aviation works out temperature at height from the International Standard Atmosphere. The model starts at 15 °C at sea level and cools by 6.5 °C for every kilometre up to 11 km[10]. From 11 to 20 km the temperature stays at −56.5 °C[10].

That is why a height in feet has to become kilometres first. 35,000 feet is 10.668 km, so the temperature drops by 6.5 × 10.668 = 69.3 °C to about −54.3 °C, or roughly −66 °F. At 30,000 feet, which is 9.144 km, the model gives −44.4 °C.

The 11 km boundary sits at 36,089 feet. Above it the standard temperature stops falling, so the model gives the same −56.5 °C at 39,000 and at 41,000 feet. The real air outside the window can be warmer or colder, because the model describes average conditions.

For a column of heights, one formula dragged down does the job. Excel's CONVERT function takes the syntax CONVERT(number,from_unit,to_unit)[11], with "ft" for the foot and "km" for the kilometre. For a height in cell B2 the formula reads =CONVERT(B2,"ft","km").

Google Sheets and LibreOffice Calc handle the same job with plain multiplication: =B2*0.0003048. Before you fill the whole column, put 10,000 into B2 and check that the result shows 3.048.