An 80-minute data CD has the sector capacity of 600 PC/AT floppy disks
Comparing exact bytes gives a cleaner result than dividing the rounded 700 MB and 1.2 MB labels, with filesystem overhead kept separate.
Dividing 700 by 1.2 gives about 583.3. The labels make the answer look easy, then quietly change the units underneath it.
Philips labels its disc “700MB/80min.” Wikipedia’s CD-R entry gives 737,280,000 bytes for the 80-minute format. Its floppy-disk record identifies the PC/AT’s 1.2 MB disk as 1,228,800 bytes. With both quantities in bytes, the division works without asking the packaging what a megabyte meant.
| Capacity measure | PC/AT 5¼-inch floppy | 80-minute Mode 1 data CD |
|---|---|---|
| Familiar label | 1.2 MB (Wikipedia) | 700 MB (Philips) |
| Nominal data-sector bytes | 1,228,800 (Wikipedia) | 737,280,000 (Wikipedia) |
| Number of those floppies | 1 | 600, calculated |
The CD calculation starts with time because the format inherited that organization from audio. Eighty minutes supplies 4,800 seconds; at 75 sectors per second, that produces 360,000 sectors. Giving each of those sectors 2,048 data bytes reaches the nominal 737,280,000-byte result. The full multiplication makes each assumption visible instead of letting the familiar 700 MB label do all the explaining.
A different floppy changes the result, even when it comes from the same old storage box. The comparison here names the high-density PC/AT format. It doesn’t stand for every disk in a flexible jacket, and it doesn’t stand for the later 3½-inch disks people may picture first. A capacity claim needs the format as well as the object, just as a stack of envelopes says very little about the amount of paper inside.
The distinction between a sector image and a folder of extracted files also changes a migration estimate. The sector-capacity ratio compares the space belonging to the formats. Copying files can leave behind empty sectors and disk-level structures, while preserving whole images keeps those parts in the job. Six hundred partly filled floppies and six hundred full images therefore ask different things of the destination.
The CD-ROM format description also distinguishes Mode 1 and Mode 2 sector layouts. That is why the Mode 1 qualifier stays attached to this number. Using the total bytes associated with audio sectors would produce a larger figure, but would change the thing being measured halfway through the comparison.
An inventory that records format, image size and readable-file size could answer the practical question without pretending all three are interchangeable. The exact 600-to-one relationship is a satisfying historical comparison. Finding out what survives inside the actual disks still requires reading them.
The Mode 1 sector explanation reserves 2,048 bytes per sector for data and runs at 75 sectors per second at 1× (Wikipedia). The arithmetic is 80 × 60 × 75 × 2,048 = 737,280,000; divide that by 1,228,800 to get 600. The entry calls this “1× speed,” a rate distinct from the total capacity.
For a real box of old floppies, the next number worth discovering is how many still yield readable files.