H264 profile and level

For
h.264 users, how to specify the profile/level seems to be a fairly common question. Profiles define sets of bit stream features a h.264 stream can use. Levels define restrictions on the video resolution, frame rate and some stuff called VBV (Video Buffer Verifier).

H.264 Profiles
H.264 Profiles are discussed in depth on
Wikipedia, but to simplify the considerations for the average user, I will focus on the Baseline, Main and High Profiles.

Baseline Profile

I/P slices

Multiple reference frames (–refs <int>, >1 in the x264 CLI)

In-loop deblocking

CAVLC entropy coding (–no-cabac in the x264 CLI)

Main Profile

Baseline Profile features mentioned above

B slices

CABAC entropy coding

Interlaced coding – PAFF/MBAFF

Weighted prediction

High Profile

Main Profile features mentioned above

8×8 transform option (–8×8dct in the x264 CLI)

Custom quantisation matrices

H.264 Levels
H.264 Levels are also discussed on Wikipedia, though in my opinion, less explicitly and less successfully
than for the profiles. For practical use, to specify a Level, a number of constraints have to be met.
The resolutions/frame rates in the following table are merely a guideline – the number of macroblocks per second is the actual restriction. To calculate this:
Macroblocks are 16×16 in H.264 and so to code a frame one can calulate the number of macroblocks per frame as:

ceil( width / 16.0 ) * ceil( height / 16.0 )

Note: The ceil() function rounds up its argument to the next integer. For example, ceil( 10.2 ) returns 11.
Then you need to multiply the number of macroblocks per frame by the number of frames per second to get the number of macroblocks per second.

macroblocks per second = ceil( width / 16.0 ) * ceil( height / 16.0 ) * frame rate

Level	VBV maximum bit rate [1000bits/s]	VBV buffer size [1000bits]	Macroblocks/s	Resolution and frame rate
1	64	175	1485	128×96@30 or 176×144@15
1b	128	350	1485	128×96@30 or 176×144@15
1.1	192	500	3000	176×144@30 or 320×240@10
1.2	384	1000	6000	176×144@60 or 320×240@20
1.3	768	2000	11880	352×288@30
2	2000	2000	11880	352×288@30
2.1	4000	4000	19800	352×288@50
2.2	4000	4000	20250	352×288@50 or 640×480@15
3	10000	10000	40500	720×480@30 or 720×576@25
3.1	14000	14000	108000	1280×720@30
3.2	20000	20000	216000	1280×720@60
4	20000	25000	245760	1920×1088@30 or 2Kx1K@30
4.1	50000	62500	245760	1920×1088@30 or 2Kx1K@30
4.2	50000	62500	522240	1920×1088@60 or or 2Kx1K@60
5	135000	135000	589824	2560×1920@30
5.1	240000	240000	983040	4Kx2K@30 or 4