The volume header gives the block where the bitmap starts (6 on every volume ProDOS 8 formats) and the number of blocks on the volume. There is one bit for each block, in order: bit 7 of the first byte is block 0, bit 6 is block 1, and so on, with a set bit meaning the block is free. A 5.25-inch disk of 280 blocks needs 35 bytes, a 1,600-block disk 200 bytes; one bitmap block (512 bytes) covers 4,096 blocks and larger volumes use as many consecutive blocks as they need (a 32 MB volume uses 16). The unused bits after the last block are clear.
The explorer counts the set bits and shows the free blocks; the picture of the volume colours a block as free, or as in use. A block in use that is not a boot block, a directory block, a bitmap block or one of a file's blocks is drawn red as "in use but not part of any file". On the real utilities disk the bitmap says 1,324 blocks are free, and the 276 in use are exactly the two boot blocks, the four directory blocks, the bitmap block and the blocks of the ten files (index blocks included), counted separately from the bitmap.