# How do I determine odd or even when working with altitudes?

I am always confused about how to calculate safety altitude, especially when it comes to even heights and odd heights, e.g. is 7500ft even or odd?

• these is called the "hemispheric rule"
– rbp
Commented Apr 16, 2015 at 22:39
• It would be very helpful if you would mention which country you're asking about; these rules can be different in different places Commented Apr 17, 2015 at 12:24
• Commented Apr 17, 2015 at 16:12

See below for the wording of 14CFR91.159 which is the reg you need to answer this question. Note it says "Odd [or Even] thousand feet plus 500". 7000 is an odd thousand even though it is an even number.

§ 91.159 - VFR cruising altitude or flight level.

Except while holding in a holding pattern of 2 minutes or less, or while turning, each person operating an aircraft under VFR in level cruising flight more than 3,000 feet above the surface shall maintain the appropriate altitude or flight level prescribed below, unless otherwise authorized by >ATC:

(a) When operating below 18,000 feet MSL and—

(1) On a magnetic course of zero degrees through 179 degrees, any odd thousand foot MSL altitude 500 feet (such as 3,500, 5,500, or 7,500); or

(2) On a magnetic course of 180 degrees through 359 degrees, any even thousand foot MSL altitude 500 feet (such as 4,500, 6,500, or 8,500).

• Do we know if the OP is in the US? Some countries have a north/south rule, not an east/west one. Commented Apr 17, 2015 at 12:23

Look at the thousands of feet only.
Is 7(thousand) even or odd?

I'm sure a mathematician views 7,500 as equally close to 8,000 as 7,000.

But in aviation terms, it is 7,000 plus another 500 feet, and therefore an odd altitude.

• A mathematician would also see 7500 as an obviously even number Commented Apr 17, 2015 at 4:05
• @raptortech97 This is why mathematician are not allowed to fly :P Commented Apr 17, 2015 at 5:41
• Look at the first (most significant) digit. ahem, then 12000 feet is odd? Commented Apr 17, 2015 at 7:58
• Sounds like that should be restated to be "Look at the number of thousands of feet to determine if even or odd." Commented Apr 17, 2015 at 12:28
• @SentryRaven I hope that rule doesn't apply in the U.S. because my CFI is a math professor with a Ph.D in math. :) Commented Apr 17, 2015 at 15:19

In the US there is a simple rhyme memory trick that you can use to figure out what altitudes would be valid for you to fly both VFR and IFR.

EAST IS LEAST - WEST IS BEST

What that means is that you fly odd number altitudes when flying East (Magnetic) and even number altitudes when flying in a magnetic Westbound direction. For VFR just add 500 ft.

So valid West bound flights would be: 4000,6000,8000 IFR : 4500,6500,6500 VFR

Same with East Bound: 5000,7000,9000 IFR : 5500,7500,9500 VFR

A simple rhyme that works pretty well for me. Also BTW, if you are filing then chose the initial leg as your filing altitude. If you are under ATC control, you'll see that ATC will eventually ask you to climb or descend if your course leg changes your direction. IF VFR, and you make a bend that takes you to the other side, then climb or descend even if you are not talking to ATC. If talking to ATC, let them know. They can either tell you to stay put or pilots discretion to the new altitude.

The reason for this is that head on's are pretty hard to do if everyone is flying the same direction. You might get chewed up in the tail, but that's the job of the pilot in the plane overtaking you or ATC.

• Welcome to the Aviation Stack Exchange! The rhyme you mention is great for converting true to magnetic headings, but why does "least" mean "odd"? Commented Apr 19, 2015 at 21:16
• I remember it from my first flying days about 28 years ago when my flight instructor explained both magnetic correction and well as flying directions. He said there were many mnemonics to remember but remember that it also applies to correcting magnetic headings +/- as well then you would only have to remember one of them. As a beginning student pilot he knew that too much information could overload a newbie and get you confused. Commented Apr 19, 2015 at 21:36
• “Least” is 5 letters, so it’s odd. “Best” is 4 letters, so it’s even. Commented Feb 2, 2020 at 21:38
• @Kolom comment: Finally! I've started to think I'm the only one poor soul studying IFR rules and consistently confusing when to use odd and when even. No CFI (CRI) around me was kind enough to spell any useful mnemonic or at least any useful reasoning (even some foolish/childish/obscene one) helping to recall the rule correctly each time after couple of weeks of yet another futile attempt of memorizing it. I've never had such challenge even during my fights with PPL exams but this rule somehow got outstanding in its complete and illogical inexplicability. Commented Mar 29, 2023 at 23:41
• @VanJone Yeah this is frustrating at times, but after you ingrained it in your brain, you just forget how stupid it is when you first hear it. And you become a proponent. It's even funnier how we do it in my native language; the word "east" starts with D, and the word "west" starts with B. D has one hole, B has two holes, and that's your odd and even. :) Commented Mar 30, 2023 at 5:49

I don't find any of the other answers to provide a useful mnemonic, which is what you need for rote memorization of arbitrary rules like this. The one I learned (and still remember decades later) is this:

ONE = Odd for North and East

In other words, if you're on a North or East course (360 - 179), then use Odd thousands.

By elimination, if you're on a South or West course (180 - 359), then use Even thousands.

I just remember that East is Odd, because East is Even woud make too much sense.

Like Nav lights - Red on the the left, because Red on the Right would make too much sense.

• Red on the right would require rewriting the right of way rules. Commented Feb 21, 2023 at 18:38

NEodd, SWeven, North to East “odd” altitude plus 500. SWeven, South to West “even” altitude plus 500. East is least and West is best is for magnetic variation when figuring true course ves magnetic course.

• Hi and welcome to ASE, what country/countries does this apply to? And what about sectors E-S and W-N? Commented Feb 2, 2020 at 20:17
• Your answer is very jumbled... Commented Feb 3, 2020 at 5:49
• AFAIK, different countries aligned along N-S axis use different odd / even assignments, it seems your examples relate to New Zealand, don't they? E-W seems to be way more consistent assignment but finding a good mnemonic on it was rather challenging. Commented Mar 29, 2023 at 23:48

“East is Odd, West is Even Odder” is a pilots mnemonic to remember what altitudes to fly at under visual and instrument flight rules. First, east and west are defined by the magnetic headings of zero to 179 degrees (East) and 180-359 degrees (West.)