Microsoft has been absolutely pummeled in the press and in reader comments this week by pundits and customers alike. They feel cheated by the amount of free storage space available to them on the new line of Surface Pro devices.

But is that criticism fair or even valid?

If it were valid, then surely Apple would have been scrutinized carefully over its 11.6-inch MacBook Air 128, which has specs that are reasonably similar to the Surface Pro 128. They’re both ultraportable devices that run full-strength operating systems. Both use similar CPUs, offer similar amounts of RAM, and advertise 128 GB of storage. The Surface Pro has touch capabilities and can be used as a tablet, but at its heart it is a full-strength PC designed for extreme mobility.

Unlike the Surface RT, which is a tablet that does a few PC-like things, Surface Pro is a real, no-compromises PC. It can power a 2560x1600 30-inch display, it runs Windows 8 Pro, it supports Hyper-V virtualization, you can run PhotoShop and AutoCAD on it. It deserves to be compared head to head with another full PC like the MacBook Air.

Here’s what you see if you order a MacBook Air from the Apple Store online:

So how much of that storage do you actually get to use? And how does it compare to the Surface Pro?

I’ve done the cold, hard math of looking at disk storage for both devices. Here’s the story, in a single picture:

Wait a minute, I can hear you saying. Those bars look remarkably similar. And in fact, the Surface Pro actually has MORE free disk space for user data in one of those bars than the MacBook Pro.

Yes, that’s true.

Here’s the tl;dr version. The MacBook Air 128 gives you 83.4 percent of the advertised storage space for user data. The Surface Pro 128 gives you 75.2 percent of its advertised capacity for storing data. And with one minor tweak that doesn’t affect the system’s capabilities in any way, you can increase the amount of data storage space on the Surface Pro to 81.8% of the advertised capacity.

Update: I've updated the chart and the tl;dr paragraph to reflect information provided by readers about current MacBook Air models. See details below, in the Update in item 3.

Forget what you’ve read in the past week. The widely reported number that Microsoft mistakenly confirmed (83 GB of free space for the Surface Pro 128) is not accurate. In its Reddit AMA yesterday, the Surface team confirmed that those numbers were wrong:

Initial reports out regarding available disk space were conservative (eg. 23GB available on 64GB and 83GB available on the 128GB system), however our final production units are coming in with ~6-7GB additional free space.

My tests on a production Surface Pro 128 confirm that amount almost exactly. I found that the factory image provided 89.7 GB of free space. (Why the difference? A source inside Microsoft tells me the employee who confirmed the numbers did so using pre-production machines that contained different disk images and debug code that is different from final shipping units that will be on sale beginning this weekend.)

And on top of that, the numbers Microsoft reports in its Windows utilities use a scale that unfairly disadvantages the Surface Pro when you look at supposedly similar numbers for the MacBook Air.

A lot of people have gotten this stuff wrong, so here’s the detailed breakdown.

1. Apple and Microsoft report disk sizes differently.

If you do not take this difference into account, you will not be able to make proper comparisons.

This is not just a technical difference between Apple and Microsoft. It represents a deliberate marketing decision Apple made in August 2009, when it released Snow Leopard. You can read the details in this Apple Support article: How OS X and iOS report storage capacity.

Before that release, Apple reported disk capacity using the same Base 2 system that Windows uses:

When you view the storage capacity of a Mac [running] Mac OS X v10.5 or earlier … the capacity is reported using the the binary system (base 2) of measurement. In binary, 1 GB is calculated as 1,073,741,824 bytes. This difference in how the decimal and binary numeral systems measure a GB is what causes a 32 GB storage device to appear as about 28 GB when detailed by its operating system, even though the storage device still has 32 billion bytes, as reported. [Emphasis added]

But with Snow Leopard, Apple changed the way it reports disk capacity:

In Mac OS X v10.6 Snow Leopard and later, storage capacity is displayed as per product specifications using the decimal system (base 10). A 200 GB drive shows 200 GB capacity…

Here’s what this means in practice. I took a 1 TB Western Digital USB drive and formatted it using a Mac. Here’s what it looks like in the OS X Mountain Lion Disk Utility:

And here’s the exact same disk in the Windows 8 Disk Management console, which continues to use Base 2, as it has since the dawn of Windows NT:

OS X says that disk can hold 999.86 GB. Windows says it can hold 931.51 GB. But it's the exact same disk. The measurements are just expressed differently, in a way that makes Apple look generous and Microsoft look stingy.

The moral of the story: If you want to compare disk sizes between Macs and PCs, you need to convert one measurement so that it is comparable with the other. You can convert the Windows Base 2 measurements to Base 10 (making them appear bigger), or convert Apple’s Base 10 measurements to Base 2 (making them appear smaller). But you can’t compare them directly.

2. At Apple, reported disk sizes are less than advertised. Microsoft reports full disk sizes.

This one shocked me when I looked it up, but it’s true.

In fact, in a separate support document Apple explicitly acknowledges this fact: “The Solid State Drive capacity stated in the product specification may be higher than what is reported by Mac OS X.“

(Try saying that backwards to understand what it really means: The amount of storage you see when you look at your Mac's system drive using OS X will be lower than the number that was advertised.)

Pop quiz: If you buy this new MacBook Air (the same one whose detailed configuration options I showed earlier in this post), how much disk space do you expect to get?

Before you answer, note that there’s a superscript 1 next to the memory listing in the detailed specs below the bold product name, indicating that there's a footnote to that number. The very fine print for footnote 1 at the bottom of the page reads: “1GB=1 billion bytes; actual formatted capacity less.”

Factoring in those adjustments, how much capacity do you actually get?

Yes, that’s only 120.47 billion bytes (remember, a GB in Apple-land doesn't mean the same as a GB in Windows World). If you convert that number to Base 2, so that it’s comparable to the Surface Pro disk, you get 112.2 GB. The same is true of a “256 GB” SSD in a MacBook Pro, which appears in Disk Utility as 250 GB. The 64 GB of flash memory advertised in a MacBook Air shrinks to 60.67 GB after Apple is through with it.

By contrast the Surface Pro 128 disk is 119.12 GB in Base 2, or 127.90 GB in Base 10. That is 7 GB more than the MacBook Air “128 GB” drive (112.2 and 120.47 GB, respectively).

3. Where did the MacBook Air’s missing 7 GB go?

I’m really glad Apple published that support article, because it contains the answer to this very question:

These items may account for the additional space used in your Solid State drive: EFI Partition

Restore Partition

Wear-leveling blocks

Write-buffer area

Metadata

Spare blocks

Grown bad blocks

Factory bad blocks

The parts about wear-leveling blacks and bad blocks are just part of how SSDs work. On a new SSD these numbers should be very small. But hold on... EFI Partition and Restore Partition? The Surface Pro has EFI and Recovery partitions as well. They’re fully documented in the Disk Management console, not hidden in a footnote. Here, see for yourself. This is the partition map, minus the Windows system partition (and not to scale):

That's a 600 MB Recovery Partition, which holds utilities to repair the operating system, followed by a 200 MB EFI System Partition. The other Recovery Partition, at the end of the disk, is a hefty 7.81 GB and contains the Windows Image (WIM) file that restores the out-of-box configuration if you use the Windows 8 Reset feature.

So, more math:

On a MacBook Air 128, the space I labeled "System Reserved" adds up to 7.53 GB. It cannot be recovered. (A Mac includes a hidden EFI partition as well, and a hidden Recovery HD partition, which is roughly 1/10 the size of the Windows version. It is intended for bootstrapping a recovery of OS X from the Internet.)

On the Surface Pro 128, the EFI and Recovery partitions add up to 8.61 GB. Pretty close. But one big difference is that the Windows 8 Recovery partition can be transferred to a USB flash drive and the disk space can be reclaimed.

Finally, there’s the OS and the apps that are included as part of a standard configuration. I was able to confirm the amount of disk space used for Windows and included apps in a base configuration of the Surface Pro 128. I don’t have a MacBook Air to do the same calculation, but I confirmed from several online sources that the available disk space on a new, 2012-model MacBook Air is approximately 99 GB. I originally used that figure as the basis of all calculations here.

Update: There's only one Surface Pro, which makes measurements easy. But Apple has produced many MacBook Airs in the more than four years of the products life. Over time, it has steadily improved the efficiency of builds of OS X. One reader reports that the most recent MacBook Air provides 106.87 billion bytes (99.5 GB in Base 2) of user-available storage out of the box. Other reports from a few months ago show 103-104 billion bytes available. The 99 billion bytes figure I used was from a MacBook Air owner who was reporting details in late 2012. Differences could be due to Lion versus Mountain Lion, updates to iLife, or different flash memory parts. In any case, when expressed in Base 2 terms, the amount is going to be in the mid- to high-90 GB range. In the chart, I have also relabeled the blue blocks as "System reserved" to avoid confusion over their proper purpose.

4. So how much free space is really left on either of these “128 GB” devices?

The picture I published at the top of this post tells this story better than words and numbers. But here’s the detail, with the first number expressed in Windows- and pre-Snow Leopard style Base 2 calculations and the second number in billions of bytes:

MacBook Air 128: 99.5 GB (approx. 106.87 billion bytes)

Surface Pro 128: 89.7 GB (approx. 96.3 billion bytes)

Those numbers are extremely close.

In both cases, you can remove installed software to further free up disk space. The MacBook Air includes iLife ‘11, while the Surface Pro has 18 included apps, some or all of which can be removed.

But there’s one tweak you can make to the Surface Pro that isn’t possible for the MacBook Air.

Using the built-in Recovery Media Creator, you can copy the contents of that large Windows 8 Recovery partition to a USB flash drive.

The final step in the Recovery Drive wizard allows you to delete the recovery partition completely. You can then use the Disk Management console to extend the existing system drive to use the space that had formerly been “devoured” by the Recovery partition,

If you go through those steps (it’s a simple process, really), you end up with 104.7 billion bytes of storage available for user data and new apps. At least that’s how a Mac user would describe it. A Windows user will see 97.5 GB in File Explorer and in Disk Management, but that’s just a difference in terminology.

It’s worth noting, by the way, that that number is roughly equal to the free space available on a MacBook Air fresh out of the box.

Update: A correspondent (and former Apple employee) notes that a utility is available to create an external recovery drive for a Mac: http://support.apple.com/kb/HT4848. As he notes:

The utility does not delete the local hard drive recovery partition, but you can then use the thumb key you created to boot off and then run the disk utility to repartition the internal drive and claim back the missing space. The utility literally copies from the supplied recovery partition on the internal drive, so you are getting an exact copy.

To then remove the Recovery HD utiltity, you have to use a Terminal command to make the hidden partition visible, and enter another Terminal command to delete the partition, followed by yet another operation to resize the system partition. That's a far cry from the built-in Windows 8 wizard, which is very easy to use. Also, my colleagues at The Verge say doing this trick (which requires a separate download and some steps that are not for the faint of heart) will recover about 650 MB of disk space in Mountain Lion. Not a great deal but worth noting.

And before you ask…

No, I haven’t done similar calculations comparing a Surface Pro with 64 GB, because I haven’t seen one. Based on my extrapolations, I expect that the MacBook Air will compare more favorably to a Surface Pro in this configuration. But the size of the operating system doesn’t change, and the smaller overall storage means that the amount of advertised capacity available to the owner of either device is going to be in the 50-64% range for the Surface Pro, starting at right around 32 billion bytes (or 30GB, as expressed in Base 2).

I consider the 64 GB Surface Pro and MacBook Air pure companion devices, designed for people who don't care about carrying around large amounts of personal data. A storage capacity of 30-40 GB is more than adequate for current work, especially when it's supplemented with external devices (SD cards, USB flash drives, external hard drives) and cloud storage.

For anyone who does want to carry around a large number of working files, the 128 GB configuration of either the MacBook Air or the Surface Pro should be more than adequate.

Of course, neither device can compare to a tablet or phone OS in terms of efficiency. Tablet operating systems are designed to be small, and they sacrifice all sorts of capabilities that you expect in a full-strength PC. But if you're going to complain about operating systems using too much of the available storage, you'd better make sure your letter to Redmond is cc'ed to Cupertino.