Why Are File Sizes Different On Macs Than Windows and Other Places?

Hi, this is Gary with macMost.com. Let me show you why file sizes are different between Windows and Mac and also other systems like Goggle Drive and DropBox. 

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So if you use both Windows and Mac, or perhaps you store your files in a Cloud service like DropBox or Goggle Drive, you may have noticed that the file sizes show up as different when they are on your Mac versus when they are on Windows or one of these other services. For instance, if I select this file on my Desktop and I use Command i, I can see it says 5.8 MB. Now I have that same file stored here in Goggle Drive, it's exactly the same file, I uploaded it and if I select it it shows that it is only 5.6 MB. If I go to DropBox where I also have that same file it shows it is 5.55 MB. So why does the file get bigger when you bring it onto your Mac or conversely smaller when you upload it to one of these services? The same thing would happen if I would transfer the file between Windows and Mac. So the file would appear to be smaller on Windows than it is on Mac. 

The truth is the file is exactly the same size. It's just that the systems measure file sizes a little differently. So in this case it's all about the definition of terms that have to do with file size. Like kilobyte or megabyte. So, what is a kilobyte or megabyte? Well, a byte is a single piece of data. In a text file it would represent a single letter. In an image file it would represent a single pixel or actually just either the red, green, and blue value of a single pixel. So it actually takes three bytes per pixel. So if you have a photo that's one million pixels it may actually be three million bytes. Three bytes for each pixel to represent red, green, and blue. 

Now it's cumbersome to represent all file sizes as bytes. We'll be talking about billions of different bytes for file sizes, trillions and even more for the size of hard drives. So, we have other terms for larger amounts of bytes. Just like you may measure things in millimeters or inches when they are small but when they get larger you use feet and meters or miles and kilometers. So when you have a lot of bytes you would say you've got a kilobyte or KB. When you have lots more you may go to a bigger size, megabyte MB, or even Gigabytes GB. Now the problem is there are different definitions for what a kilobyte is. Kilo is a metric term. We see it in things like kilograms, kilometers, and such. Its got Greek and Latin roots and all of that. But basically it stands for one thousand. When you have 1000 meters you have a kilometer. 

Now computers don't use decimal numbers. Like 9, 10, 11, 12 and all of that. They use Binary. Just zeros and ones. So it was useful, especially when computers were just starting, to have programs written and shift designed around binary numbers. The binary number 2 to the power or 10 or the equivalent to 1 followed by ten zeros is 1024, which is really close to 1000. So a convention came to be where a kilobyte was defined as 1024 bytes, not 1000 bytes. Likewise a megabyte, which would be a million in metric terms, would actually be defined as 2 to the ten times 2 to the ten or actually 1,048,576 bytes. So it starts to get a little bit further away. You've got almost 5% more than a million here but the convention still kind of held and this was the definition for megabyte for a long time. 

So we have a single unit which is a byte which really doesn't matter if you're talking about binary or metric here. A byte is a byte. But if you go to kilobyte a binary measurement of that would have been 1024 B whereas the metric measurement of that would have been 1000 B. Megabyte would have been 1024 KB and it would have been 1000 KB metric. The same thing for gigabyte. You just go up by multiplying everything either by 1024 or 1000. So you've got 1000 bytes is a kilobyte in metric and 1,000,000 bytes is a megabyte using metric. But if you're doing binary then you can see that you actually have a lot more bytes inside of a kilobyte and a megabyte. This gets confusing because you're using the same terms. When you say kilobyte you want to be precise. Is it 1000 bytes or is it 1024 bytes. 

Well, it turns out there is another term used to describe the binary measurement. That is to replace the last two letters of the four letter prefix with bi for binary. So Kibibyte is actually the correct term to use when measuring 1024 bytes. Instead of megabyte you would use Mebibyte. It is the same thing with Gibibyte and tebibyte.

So if we go back to our file here you could see it says 5.8 MB on the Mac. If we look at it it will tell us the exact size is 5,821,027 bytes. This is going to be the exact same number of bytes no matter where you put it. Whether you put it on Windows. Whether you put it on Goggle Drive or whether you put it on DropBox or somewhere else. So Goggle Drive shows this as a 5.6 MB. Let's see if the math works out. Let's start out here with the 5,821,027 B of file. Now to convert that to megabytes first we want to convert it to kilobytes. So we'll divide by 1024 or 1000. You can see in other systems like Windows and Goggle Drive and such we divide by 1024 we get 5,685 KB. But on Mac it is a simple division by 1000. So we get 5,821 KB. Now to get megabytes we have to divide again. So we divide by 1024 and we get 5.55 MB. That's 5,685 divided by 1024 to get that answer. On the Mac side it's fairly easy again. Just divide by 1000. So move the decimal point over and we have 5.8. So we can see we've got 5.8 MB on the Mac. We've got 5.55 or rounding to one decimal place 5.6 that we see with Goggle Drive and the 5.55 that we see with DropBox since DropBox actually uses two decimal places to give us the file size. 

So the file, in either case, is still 5,821,027 bytes in size but by using the 1024 divider you get kilobytes and megabytes you get 5.55 MB, or mebibyte. On the Mac you get the simpler 5.5 MB. Now it's important to point out a couple of things about the differences here. First of all Apple also used 1024 prior to Snow Leopard and iOS 10.10. That is when Apple switched over to using 1000 instead of 1024 for the sizes. Now one part of the industry that used 1000 for much longer than that was actually hard drive manufacturers. For a long time now hard drives have measured megabytes, gigabytes, and terabytes using 1000 rather than 1024. So Apple was actually coming inline with how hard drives are measured and sold by using 1000. Of course hard drive manufacturers still like to round a lot. So .97 in terabyte drive may actually be marketed as a one terabyte drive. Of course there's also rounding involved in all these file sizes as well. You saw that in 5.6 versus 5.55 between Goggle Drive and DropBox. This doesn't change the actual size of the file. It's just rounding it to a more convenient number. 

So if you've ever been interested in why file sizes show as a little bit different between Mac and Windows and Mac and other things, this is the reason. Hope you found this useful. Thanks for watching.