Corner-to-corner crochet builds a blanket as a grid of small squares, usually double-crochet clusters, worked diagonally from one corner to the opposite corner. Because the piece grows on the diagonal rather than row by row across a fixed width, the underlying math tracks block counts and diagonal rows rather than the straightforward width-times-height math a straight-worked blanket uses.
From finished size to block grid
The starting point is the same as any blanket calculation: divide the target finished width by a single block's width, and the target finished height by a single block's height, rounding up each time, since a partial block isn't practical to work. That gives a grid, blocks wide by blocks tall, and multiplying the two gives the total block count for the whole blanket.
Why the piece grows diagonally
C2C starts at one corner with a single block, then adds a block to each new diagonal row until the work reaches the grid's shorter dimension, at which point it's as wide as the finished piece will get along that diagonal. From there, the work holds at that width for a straight phase, adding a block at one end while dropping one at the other so the diagonal row length stays constant while the piece lengthens, until reaching the corner opposite the start. Then it reverses into a decrease phase, dropping a block each row at both ends, down to a single block at the final corner. This creates the three-phase shape most C2C patterns describe: increase, straight, decrease.
Worked example
A grid of 30 blocks wide by 40 blocks tall (a portrait-oriented blanket) has a shorter side of 30. The increase phase runs 30 diagonal rows, growing from 1 block up to 30. The straight phase runs for the difference between the longer and shorter sides, 40 minus 30 equals 10 rows, holding steady at 30 blocks wide. The decrease phase mirrors the increase phase, another 30 rows shrinking back to 1. Total diagonal rows: 30 + 10 + 30 minus 1 (since the peak row counts once, not twice) works out to a total of 69 diagonal rows for the full blanket, following the general rule that total diagonal rows equal blocks-wide plus blocks-tall minus 1.
Why this matters for planning
Because C2C is worked diagonally, the row count doesn't map onto a simple "rows per inch" the way a straight-worked piece does. A square C2C blanket (equal blocks wide and tall) has no straight phase at all, just a symmetric increase then decrease, while a long, narrow C2C piece spends most of its rows in the straight phase. Knowing which shape you're building tells you what to expect from your row counter well before you're halfway through.