A log cabin grows by two strip-widths per round, and every log’s length is knowable before anything is cut. Enter your strip width and rounds; the block size, the full cutting list, and the light/dark split follow.
Your Block
Your Block Finishes
14in
3 rounds of 2.5″ strips around a 2.5″ center
For 30 blocks: about 34 light and 43 dark strips at 2.5″ × 40″. A jelly roll holds 40: one roll won’t quite cover it.
| Log | Cut length | Side |
|---|---|---|
| 1 | 2 ½″ × 2.5″ | light |
| 2 | 4 ½″ × 2.5″ | light |
| 3 | 4 ½″ × 2.5″ | dark |
| 4 | 6 ½″ × 2.5″ | dark |
| 5 | 6 ½″ × 2.5″ | light |
| 6 | 8 ½″ × 2.5″ | light |
| 7 | 8 ½″ × 2.5″ | dark |
| 8 | 10 ½″ × 2.5″ | dark |
| 9 | 10 ½″ × 2.5″ | light |
| 10 | 12 ½″ × 2.5″ | light |
| 11 | 12 ½″ × 2.5″ | dark |
| 12 | 14 ½″ × 2.5″ | dark |
The Formulas
finished block = center finished + 2 × rounds × strip finished · each round = 4 logs, two light, two darklog n cut length = the block’s running size when that log goes on, + ½ seam allowanceEach log is as long as the block it joins, which is why the cutting list climbs by one strip-finished-width every two logs. Cut from the list, or do it the strip-pieced way: sew the strip on long, trim flush, repeat.
Worked Example
A 2½ in strip block, 3 rounds, 2½ center:
- Finished: 2 + 2 × 3 × 2 = 14 in.
- The list runs from log 1 at 2½ in to log 12 at 14½, the table writes every one.
- Thirty blocks: about 34 light strips and 43 dark: more than one 40-strip jelly roll holds.
Light Side, Dark Side
- Logs go on in pairs: two light, then two dark, spiraling, the diagonal split that makes barn-raising and straight-furrow layouts work.
- Chain piecing: all thirty blocks take the same log at once; the strip-pieced method skips measuring entirely.
- Courthouse steps variant: same lengths, logs added opposite-opposite instead of spiraling.
Assumptions and Defaults
- Finished sizes = cut − ½ throughout.
- Strip counts assume 40 usable inches and the chain-cut list; add a strip per fabric for insurance.
- The light/dark split is 2 logs each per round; scrappy cabins ignore it freely.
Log Cabin Questions, Answered
What size will my log cabin block be?
A log cabin block finishes at the center’s finished size + 2 × rounds × the strip’s finished width, so 2½ in strips, 2 in finished, around a 2½ in center for three rounds build a 14 in block. That combination is the classic jelly roll log cabin block.
How long do I cut each log in a log cabin block?
Each log matches the block’s current edge plus the seam allowance, and the table above lists every log for your numbers. If you prefer not to measure, you can sew each strip on long and trim it flush, the traditional method.
How many strips do I need for a log cabin quilt?
A log cabin quilt needs the sum of every log’s length across your block count, divided by the usable fabric width, so the 30-block example takes about 34 light and 43 dark strips at 2½ in. That is more than one 40-strip jelly roll holds, which is worth knowing before you shop.
Why is the center of a log cabin block traditionally red?
The center of a log cabin block is traditionally red because it stands for the hearth of the cabin, in the old telling, while a yellow center stands for a lantern in the window. It is a matter of tradition rather than geometry, so the math works the same whatever color you choose.