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February 05, 2009

How to Calculate a Video’s File Size

Written by Jeff Sobel

 

A video producer often needs to be able to estimate the size of a video file before that video has been recorded, imported or exported.  Do you need a magic crystal ball to predict how large a video file will be before you hit that Export button?  Nope.  You just need a 5th grader’s grasp of basic math.  Here’s how:

Let’s take the example of exporting a video using Apple’s Compressor which comes standard with Final Cut Studio 2.
The first thing you should know is that digital video is encoded at a certain datarate, commonly called the bitrate.  Higher bitrates generally produce better quality video (less “pixelation” or graininess) but will create larger files.  You need to be sure that you choose a bitrate that’s high enough to achieve satisfactory quality but not so high that the video can’t be streamed on the web, downloaded in a reasonable amount of time, emailed, or however you intend to get it to your audience.  Compressor has presets which are great starting points for making this decision.

The screenshot below shows Compressor’s stock presets for iPod, iPhone, and AppleTV:

You’ll see that there are two different presets for iPod/iPhone.  The 1st is “h.264 video @ 600kbps” and the 2nd is “h.264 video @ 1500kbps”.  Now, it’s safe to assume that the 2nd preset will produce better quality video, but how big will the files be?  Let say we have a 2min long video and we’re hoping to compress it to a small enough filesize to be able to email it.  Will the 600kbps setting do that for us?  Let’s figure it out.

The 1st thing you need to know is that “600kbps” stands for “600 kilobits per second”.  Now, we’re all pretty used to hearing about kilobytes, megabytes, even terabytes.  But what’s a kilobit?  A bit is the smallest piece of data there is.  We represent bit with a lowercase b and byte with an uppercase B.  All you need to know is:
There are 8 bits in a byte. 
There are 1024 bits in a kilobit. 
There are 1024 kilobits in a kilobyte.
There are 1024 kilobytes in a megabyte.

It’s not nearly as complicated as it might seem at first.  It’s just like measurements you make in a kitchen.  You know, 16oz in a pint, 2 pints in a quart, 4 quarts in a gallon, etc…

So let’s figure out how big our 2min video is going to be after we compress it using the 600kbps preset in Compressor:

600kbps / 8 = 75 kilobytes per second
75KB/s * 60 = 4500 kilobytes per minute
4500KB/m / 1024 = 4.4 megabytes per minute

Our 2min video is going to be about 9megabytes when exported with this preset.  Small enough that you might be able to email it.

Now what if we compressed it using the AppleTV preset?  That’s a 5mbps bitrate (5 megabits per second) so:

5mbps * 1024 = 5120 kilobits per second
5120kbps / 8 = 640 kilobytes per second
640KB/s * 60 = 38,400KB per minute
38,400KB / 1024 = 37.5 megabytes per minute

At this setting our 2min video will be about 75 megabytes.  Much larger.  But it’s going to look much better as well, even on an HD TV.

In our next installment we’ll talk about how you can estimate how much disk space you’ll need before capturing or importing your footage from a video camera.

Next entry: The Lennon Bus and Godin Guitars at NAMM!

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