What is the Actual Storage Capacity of a DVD Disc?

By · Tuesday, November 9th, 2010

What is the Storage Capacity of a DVD?If you have ever purchased a recordable DVD disc, you have probably noticed that the packaging says 4.7GB of storage space. However, when you go to burn the disc, the available free space is just 4,400 MB. This may leave a lot of folks wondering what the actual capacity of a DVD is. Let’s take a moment to discuss this discrepancy and what it means for optical media users.

The debate over storage capacity goes back far beyond optical media to the early days of computing. Many of the first computer companies were started by engineers, mathematicians, and other people from the scientific community. These people were used to reporting the capacity of their products according to the International System of Units, also known as the metric system.

The International System of Units (SI prefix) is also known as the decimal system because it is a “base 10″ system. This means that all measurements use a value of ten as their “base.” So, a disc with 4.7 gigabytes of storage space is equal to 4.7 billion bytes, right? Well, not exactly.

The majority of computer operating systems do not use a “base 10″ system for counting numerical values. Instead, they use the binary system for numerical values which is a “base 2″ system. This means that values for things such as disc space are calculated differently in binary than they are in decimal. A 4.7 GB blank DVD-R disc is actually 4.38 GiB (gibibytes) as reported by a binary operating system.

A metaphor might help explain the situation. Imagine that you are taking a road trip from the United States to Canada. The gas tank in your car holds 15 gallons of fuel, but when filling up at a Canadian gas station, the receipt reads 56.78 litres. The amount of fuel your tank holds does not change, it is only the scale of measurement that makes it seem larger or smaller.

So disc storage capacity is measured in decimal form by manufacturers and in binary form by the operating system. This is actually a widespread practice in the computer industry. Everything from optical discs to floppy diskettes, memory modules, and hard drives are all measured using a “base 2″ system. But doesn’t this discrepancy make things confusing for users? You bet it does.

Back in 2005, a lawyer from New Mexico filed a class-action lawsuit against Western Digital, Seagate, and other manufacturers of computer hard drives. His claim was that these companies were intentionally misleading consumers by advertising their hard drives in the (larger) decimal system rather than the binary system. A computer hard drive advertised as 80GB by the manufacturer would be reported as 74.4 GB by the operating system.

It’s hard to pin the blame for this on hard drive manufacturers, as they had been following an industry-standard practice for decades. However, the lawyer and the hard drive companies eventually reached an agreement. Hard drive manufacturers agreed to be clearer about their advertising claims in the future. As part of the settlement, Western Digital gave away free copies of their software to customers who had purchased the hard drives.

Until a similar case is brought against optical disc manufacturers, they will likely continue the long-standing practice of using SI units for measuring the capacity of blank recordable media. Here is a table to help illustrate the differences between decimal and binary measurements for some common types of blank discs.

Disc Type Decimal Binary
CD-R 700 MB 700 MB
DVD-R 4.7 GB 4.3 GB
DVD-R DL 8.5 GB 7.95 GB

So let’s take a look at our original question: what is the actual storage capacity of a DVD disc? Well, it is 4.7GB if measured in the decimal form and 4.3GB if measured in binary form. Either way, it is enough space to store 2 hours of DVD quality video and audio. I hope this article has been useful to you!

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