# 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 Apr 16 '15 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 – Pondlife Apr 17 '15 at 12:24
• – Federico Apr 17 '15 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. – Pondlife Apr 17 '15 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 – raptortech97 Apr 17 '15 at 4:05
• @raptortech97 This is why mathematician are not allowed to fly :P – SentryRaven Apr 17 '15 at 5:41
• Look at the first (most significant) digit. ahem, then 12000 feet is odd? – Federico Apr 17 '15 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." – FreeMan Apr 17 '15 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. :) – reirab Apr 17 '15 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"? – NathanG Apr 19 '15 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. – TB Flyer Apr 19 '15 at 21:36

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 heading (360 - 179), then use Odd thousands.

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

“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.)

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.