A 25 GB Blu-ray holds 5.32 single-layer DVDs, even when software calls it 23.28 GiB
Disc-publisher maker Primera explains the units behind a familiar backup-planning surprise, with a caveat about manufacturer variation.
A 25 GB disc showing 23.28 GiB has not misplaced part of your backup. The units changed while the number on the packaging stayed put.
Disc-publisher maker Primera gives the conversions for both formats. A GB means a billion bytes; a GiB means 1,073,741,824 bytes. Its table calls the first figure “Stated Capacity”.
| Disc | Nominal GB | Listed GiB |
|---|---|---|
| Single-layer DVD | 4.7 | 4.38 |
| Dual-layer DVD | 8.5 | 7.92 |
| Single-layer Blu-ray | 25 | 23.28 |
| Dual-layer Blu-ray | 50 | 46.56 |
The single-layer comparison is 25 ÷ 4.7, or about 5.32 DVDs per Blu-ray. Five DVDs total 23.5 nominal GB, still short of one 25 GB Blu-ray. These are capacity comparisons; Primera notes actual Blu-ray capacity varies with the manufacturer.
The same calculation works before any disc goes into a drive. Dividing the nominal 25 billion bytes by 1,073,741,824 gives about 23.283 GiB. A 4.7 billion-byte DVD comes to about 4.377 GiB. Round those to two decimals and Primera’s table falls into place. Nothing had to be written, formatted or reserved to produce that numerical difference.
The nominal gap between the two single-layer formats is 20.3 billion bytes. Multiplying the DVD figure by five reaches 23.5 billion bytes, leaving 1.5 billion bytes below the Blu-ray figure. That extra fraction explains why saying one Blu-ray equals five DVDs understates the capacity slightly. It remains a comparison of the stated capacities, with the manufacturer’s variation caveat attached.
Layer count changes the answer again. A dual-layer DVD’s 8.5 GB gives it about 1.81 times the nominal space of a 4.7 GB single-layer DVD. The dual-layer Blu-ray figure in this table is exactly twice its single-layer figure. Assuming every second layer doubles the label would therefore produce the wrong DVD estimate before you even reached the burning application.
The unit conversion is separate from the space available to a particular recording job. A folder and a blank disc can use the same measuring unit and still fail a fit check if the job needs additional space. Primera’s figures explain how its software presents capacities; the remaining decision belongs to the actual media and recording setup. A calculator can settle GB versus GiB without certifying a backup.
For an archive spreadsheet, recording bytes alongside the media type would avoid this argument the next time somebody changes software. The person restoring the files can then distinguish a different unit label from a genuinely different quantity of data.
Once the data fits, the next decision is whether the people who will restore it actually have a drive for that disc.