Bits and Bytes: The Simple Math of Premiere Pro’s Exports

Let me just put it out there - math sucks (for most of us). We all breathed a sigh of relief at the end of college or as soon as our math requirements came to an end. Didn’t we become video editors just to avoid math? And yet to really master the art of the export, we need to sometimes do just a bit of number crunching to hit that perfect balance between quality and file size.

For this article, we’re going to focus on the king of distribution formats, H.264. If you’re looking for an article on master / intermediate file formats like ProRes, Cineform, and DNxHD/HR, this article admittedly won’t be a ton of help. I’m going to use Premiere Pro to demonstrate the settings, but virtually everything in this article is applicable to most other NLEs and compression software. Let’s dig in.

In Premiere’s Export Settings dialog box, after selecting H.264 from the Format dropdown, the next dropdown to select from is Preset. This is one of my least favorite dropdowns in any Adobe program… EVER. It’s a mile long list that confuses editors for 3 reasons: 1. It’s a freakin’ mile-long, 2. Many are outdated, and 3. It might give newer editors the wrong idea that you need to export a different version of your H.264 file for an iPad, Android device, Youtube, etc. This just isn’t true (well, in most scenarios). In most cases, an H.264 file is an H.264 file and will work on almost any modern-ish video playing device. But there MUST be a difference, right?

 Worst. Preset list. ever.

Worst. Preset list. ever.

You’ll notice that many of the presets have numbers at the end, indicating the frame rate and frame size (i.e. ‘Android Tablet 1080p 29.97’). But does that mean that ‘Android Tablet 1080p 29.97’ and ‘Apple iPad 2, 3, 4, Mini; iPhone 4S, 5; Apple TV 3 1080p 29.97’ are identical? (Worst Preset Name Ever, by the way). Maybe yes, maybe no. Take a look near the bottom of the Export Settings window and Premiere gives you an Estimated File Size. This will likely vary a bit from preset to preset, possibly even if the frame rate and size is the same between two presets. What’s going on here? Bitrate.

Let’s say you have a 2-minute video. If you select the ‘Match Source - High Bitrate’ preset (a good catchall preset for many scenarios), you’ll probably see Premiere display an estimated file size of around 150 MB.

 Premiere estimates that a 2 minute video with the ‘Match Source - High Bitrate’ setting will come in at around 150 MB. 

Premiere estimates that a 2 minute video with the ‘Match Source - High Bitrate’ setting will come in at around 150 MB. 

How did it arrive at this number? Scroll down through the Video tab until you see Bitrate Settings. If you haven't touched anything, you should see two sliders, Target Bitrate (set to a value of 10) and Maximum Bitrate (set to 12). These sliders are measured in Mbps, or megabits per second (i.e. how many bits of info are thrown into every second of video). Notice, this is not megabytes.

 Scroll down in the Video tab to find the Bitrate Settings.

Scroll down in the Video tab to find the Bitrate Settings.

So what the hell is a megabit? It’s just a different form of measurement (think miles vs. kilometers). Luckily, the conversion between the two is super easy:

1 megabyte = 8 megabits
or
1 megabit = .125 megabytes
Also 1 megabit = 1,000 kilobits (once in a while you see data measured this way)

So Premiere guessed around 150 megabytes for the 2 minute video because:

1 minute = 60 seconds (duh)
120 seconds (our video length) x 10Mbps (the bitrate) = 1200 megabits
1200 megabits/8 = 150 megabytes

Of course, this estimate doesn't always match the final exported file. Remember there are two bitrate settings, Target and Maximum. Why? It’s because H.264 can be a variable bitrate codec, where you can loosely control how many bits are thrown into every second of video. You have your target, but because H.264 is smart enough to recognize the difference between a simple frame (a talking head, for example) vs. a complex frame (something with a lot of detail, motion, or color), it can allocate different bitrates to different parts of your video. The net effect is that your actual size might be different than the estimate.

As for the Bitrate Encoding drop down, there are usually three options:

CBR (Constant Bitrate)

Use this if you want a fast export, and you’re not worried about being efficient with your compression. Every frame gets the same amount of data applied to it.

VBR (Variable Bitrate), 1-pass

The default in many cases. As mentioned, simple frames gets fewer bits applied to them while complex frames get more bits. Takes a bit longer to render.

VBR (Variable Bitrate), 2-pass

Same as above, except that the renderer does a first pass analyzing the entire file for complexity to create an even more efficient use of the bits before doing a second pass to finalize the file. Takes the longest of the three. Good to use on videos with lots of fast movement or dark scenes.

Side note: One area I see people screw up all the time is properly abbreviating them. Megabytes is usually shortened to MB (both caps), while megabits is usually shortened to Mb (note the lower case b). Yup, it’s subtle, but it’s important. Also, in other applications, bitrate is sometimes referred to as data rate.

So what if you’ve exported this 2 minute file and it was right around 150 MB, but for some reason, the client really needs it to be under 100 MB (for some annoying reason that usually is due to lack of knowledge or some ancient digital asset management system)? We do more fun math, of course! To calculate the needed bitrate, we just work backwards from before:

We need a 100 MB file so we work out how many MBs are in each second of video
100 MB(megabyte) file/120 seconds = .833 MBps
.833 MBps x 8 = 6.66 Mbps

You’re probably safest if you round down a hair and set the Maximum Bitrate to around 6.5 Mbps. As for the Target, most of the time editors will set that to anywhere from 50-90% of the Maximum setting. Yes, that’s a large range, but you’ll have to find out what works best for you. A setting of 5 Mbps would probably be safe here. If in doubt, higher is better, you might just not be able to tell much difference.

So what is the ideal bitrate? There’s no such thing! Sure, higher is better, but it depends on your needs. Don’t just willy nilly crank both Target and Maximum sliders to the far right. You might be wasting space at that point. If the video you are working with was already heavily compressed (most DSLR video, smartphone video, AVCHD, etc.), then you’re not gaining much of anything by over cranking the bitrate.

To put things in some relative perspective, here are some common bitrates used in the video world:

iPhone 1080HD video: around 20 Mbps
iPhone 4K video: around 60 Mbps
DSLR 1080HD video: varies a lot, often 20-60 Mbps depending on the model
Netflix 1080 HD stream: around 6 Mbps (maximum)
Apple iTunes Store 1080 HD movie download: around 5-8 Mbps
YouTube 1080: 8 Mbps (higher bitrates used for 2K and 4K)
Blu-Ray discs: 40 Mbps (maximum, but often a bit lower)

Note: These numbers are pretty accurate as of January 2017, but these things change from time to time so if it’s 2019 and you’re reading this article, don’t come after me on Twitter!).

HUGE SUPER IMPORTANT SECTION ABOUT 4K AND HIGH FRAME RATE VIDEO: Be extremely wary when using ANY H.264 presets with 4K or some high frame video. The majority of the preset bitrates were calculated under the assumption that the video export is HD and 30 fps or below. Even the handy default ‘Match Source - High Bitrate’ is problematic - while it’ll give you a 4K output, it doesn't change the bitrate above 10-12Mbps, and therein lies the problem: you’re spreading your bits across four times as many pixels which will end in a net result of an image with 1/4 the quality. Same with higher frame rates. You might be spreading those bits across 60 frames in one second, rather than 30. So if you’ve mastered bitrate settings for HD video after reading this article, take any number you calculate and multiply it by 4 if you’re creating a 4K export. Don’t forget, if you tweak settings, you can save it as preset.