Who decides what measurement units are used for altitude, and why is there no standard?

I know that some countries (Russia and China for instance) use meters to measure altitude while the rest of the world uses feet. Why isn't this standardized around the world?

I guess all modern cockpit instruments nowdays offer both unit system measurements, but this must be creating some confusion for pilots.

Who decides which measurement units should be used for aircraft altitude? The airlines? ATC? Each country?

Why are both units of measure used? Are there any pros or cons with using meters?

• FWIW, my impression of whether countries use feet or meters is a legacy of the cold war. Soviet bloc used meters, others feet. A bigger problem for pilots, in my opinion, is the differing transition altitudes between countries. Commented Jan 28, 2014 at 23:58
• Another interesting aspect to consider is precision... there's 3+ feet per meter, which means an altitude given in feet can be more precise than one given in meters (assuming no decimal point). Commented Jan 30, 2014 at 14:35
• @JeffBridgman: Why would anyone assume "no decimal point" when there are concerns about a lack of precision? Isn't that the same as saying "Feet cannot be used for measuring anything more precise than 10 feet, assuming that we don't use the last digit."? Commented Feb 10, 2015 at 15:33
• By the way, in English it's "metres". A meter is a device for measuring something, a metre is a unit of length. Commented Feb 24, 2015 at 4:49
• @MarkyMark By the way, "English" usually means American English on SE sites, you might want to be specific and say "British English". ;) Or "Australian", obviously. :D Commented Oct 24, 2016 at 9:28

3 Answers

Unfortunately, countries tend to want to decide themselves how stuff is supposed to work once you're within their borders. That's why we have different rules and laws, and when you go somewhere else in the world, you might need to know some of the differences.

In England they drive on the wrong side of the road, imagine that! :)

In order to have things run smoothly in something so utterly international as aviation, the organization called ICAO (International Civil Aviation Organization) where most countries of the world are members, dictates the rules, (I believe they are) called the ICAO drafts recommendations which the member states should incorporate into their respective laws and regulations. If they choose not to, they need to inform the ICAO of such deviation, don't know exactly what happens then, but I believe the idea is to at least let everybody know there's a discrepancy.

The ICAO Annex 5 specifies units to use, including units for altitude and actually specifying meters as the primary unit, with feet being accepted as non-SI alternative. Why on earth you'd want to go against the majority on this I don't know, perhaps due to the fact that it's an SI unit and they want to be as much SI as they can. So it's basically up to each country how they want to play it. Most countries stuck with feet.

• In England, we drive on the correct side of the road. It's you Americans who copied the French in driving on the wrong side of the road. [Comment intended to illustrate how different countries make their own minds up] Commented Jul 19, 2014 at 16:53
• IIRC, in England people drive on the side of the road which was once considered the "most convenient" side to ride horses from. Right or Wrong, is entirely perspective.
– DDM
Commented Jan 18, 2019 at 10:43

In response to the pros/cons of feet vs. meters, the big advantage that feet have over meters, is that it allows you to use nice human-memorable numbers for cruising altitudes.

In the US, we generally use 500ft for visual separation, and 1000ft for separating IFR traffic from other IFR. This translates to roughly 150m and 300m respectively. This means, we can explain IFR cruising altitudes as odd-thousands going east, and even thousands going west (up to FL410).

If this were in meters, you'd have to try to explain that going east it's 300m multiplied by odd numbers, and west it's 300m multiplied by even numbers. It's certainly not impossible to figure out in your head, it just isn't as quick and easy. Unfortunately 100m is considered too little separation, and 500m would be an inefficient use of airspace (over-separation), otherwise there might be more of a global push to use meters.

Interestingly, although Russia does use meters, they have actually adopted the international "Flight Levels" (i.e. "climb and maintain FL350" which would be equal to 10650m), so now international flights do not have to change altitude when crossing the border into Russian airspace. However, I believe this is not true for other countries which use meters, such as China, where the separation is typically 300m.

• I have heard the use "FL X meters", which I suppose is simply "X meters" of pressure altitude, do you know by any chance? Commented Jan 29, 2014 at 8:11
• @roe I haven't heard that, so I'm not sure. Commented Jan 29, 2014 at 18:33
• @roe That's correct. Commented Jan 30, 2014 at 13:53
• If the system had been set up with metric units from the beginning, isn't it conceivable that 250 m of separation would have been found sufficient, such that one could have xx000 m and xx500 m in one direction and xx250 / xx750 m in the other? Sounds fairly quick and easy to me. Commented Jul 12, 2014 at 17:16
• The only reason imperial units are used at all is because of the US' historically important role in aviation. Changing aviation over to metric at the point would be an interesting challenge. Commented Feb 24, 2015 at 4:56

As to feet being the best measurement for height, what utter nonsense! Prior to WW2, all metric countries flew in meters. That changed after WW2 with America dictating what happened on this planet. Unable to convert to the metric system for one reason, big money that has so far thwarted all efforts to accomplish this important task.America forced the almost 80% metric world to fly in stone-age measurements called feet.Well so much for democracy! Maybe China will eventually do the same and force America to go metric! Sure a drastic step, but a well-deserved one!

• Now if only the metric folks didn't utterly fail their mission in the two most critical pieces: time and angle (which gets used for geocoordinates). Qucik, tell me, what is 1000 ft/min in knots. Now, what is 5 m/s in km/h. Better? Not… Without decimal system for time and angle, much of the benefit of metric is lost here (the examples are the units used for vertical and horizontal speeds in their respective systems). Commented Jan 18, 2019 at 21:37