How to Format a 4.7GB DVD to 7.5GB

Digital video discs (DVDs) aren't like hard drives that are available in more or less any capacity you want. There are only two sizes of DVDs: a 4.7GB version and an 8.5GB version. The problem here is that the two types of discs are physically different, so you can't start with a lower-capacity 4.7GB DVD and re-format it to hold 7.5GB. What you can do, though, is change the data to make it fit.

The two types of DVDs look the same to the eye. The discs themselves are the same size, and the drives you use to read and write them look identical. The difference lies in how they work under the hood.

Every DVD has a protective surface on the playable side, and underneath that is a reflective layer. A laser in the drive makes microscopic pits in that layer, with pits and not-pits making up the 0s and 1s of digital data. The size of those pits and the wavelength of the laser that's used, determine how much data the disc can hold.

High-capacity 8.5GB DVDs work around that limitation by making the first layer semi-transparent and putting a second writable layer underneath it. This construction nearly doubles DVD capacity, and it's why they're called dual-layer DVDs. Formatting a single-layer DVD can't physically give it an extra writable layer.

If you need to write files larger than 4.7GB onto a single-layer DVD, you need to change the data instead of the disc.

Think of the box of instant mashed potatoes in your cupboard. It takes six pounds of whole potatoes to make a pound of instant mashed, so a standard 26-ounce box of flakes represents almost 10 pounds of whole spuds. You'd never fit 10 pounds of russets into a box that size, but it's easy when the nonessential part – water – is removed.

With data, it's not a question of removing anything physical. You're just repacking the information more efficiently, so it takes up less space.

The most straightforward way to get around the DVD size limitation is by using one of the many file-compression programs that are available. Paid programs, such as WinZip and WinRAR, and free alternatives, including 7Zip, PeaZip and Zipware, can all pack your files and data into various archival formats. Two of the most common are zip files and RAR files. Files compressed in those formats can take up a lot less space.

You don't necessarily need to use a stand-alone compression program. Windows has file compression built-in, so you can right-click a file, choose Send To from the pop-up menu and then Compressed (zipped) folder. A new compressed folder, with the same name as your original file or folder, is created. You can rename it by right-clicking and renaming it to something more meaningful, like "My personal archive," or go ahead and burn it onto the DVD right away.

Commercial products give you a lot more options, from choosing your archive format to encrypting your compressed files for security purposes. You can also break up your archive over multiple 4.7GB discs, which is handy for large files.

One limitation of this strategy is that sometimes the data you want to shrink is already compressed in one way or another. This applies to most photo, sound and video formats and programs you download.

Compression programs can usually make them a bit smaller, but if they still don't fit on a 4.7GB DVD, you need to get creative. Some compression programs have specific methods for reducing the size of those files without damaging their playback quality. Your original program can help, too. Many of them have a slider that lets you choose "Best quality" or "Lowest file size," or anything in between. Try several settings until you find one that lets your files all fit on the DVD.