Log Cabin & Strip Piecing Calculator
The dark half of a log cabin takes exactly 2 x log width x rounds more fabric than the light — 18% more on a typical block.
| Log | Round | Side | Finished length | Cut |
|---|
Strip piecing
The two halves of a log cabin are not equal
A log cabin spirals outward from the centre, each log a little longer than the last. Two logs per round go to the light side and two to the dark — equal numbers — so everyone buys equal amounts of each. They should not. Build a 3″ centre with 1.5″ logs and look at what each side actually receives:
| Round | Light logs | Dark logs | Difference |
|---|---|---|---|
| 1 | 3″, 4.5″ = 7.5″ | 4.5″, 6″ = 10.5″ | +3″ |
| 2 | 6″, 7.5″ = 13.5″ | 7.5″, 9″ = 16.5″ | +3″ |
| 3 | 9″, 10.5″ = 19.5″ | 10.5″, 12″ = 22.5″ | +3″ |
| 4 | 12″, 13.5″ = 25.5″ | 13.5″, 15″ = 28.5″ | +3″ |
| Total | 66″ | 78″ | +12″ |
The same gap every single round, because the dark side always takes the two logs that come after the light pair in the spiral, and each log grows by exactly the log width. So
extra dark = 2 × log width × rounds
That is 18.2% more dark fabric on this block. Buy equal amounts and you run out of the dark first, every time — and it is the dark that carries the design.
And the fabric grows as the area, not the rounds
The strip has to fill the block, so the total strip length times the strip width equals the block area exactly — there is nowhere else for the fabric to go. Since a block is centre + 2 × rounds × log width on a side, adding rounds costs fabric as the square. Doubling the rounds does not double the strip; it roughly quadruples it. Worth knowing before deciding a twelve-inch block would look better than a nine.
Notes
- Alternate the sewing direction on strip sets. Sew every seam from the same end and the set bows into a curve — the strips creep a fraction each time and it compounds. Sew one seam top to bottom, the next bottom to top.
- A strip set finishes at N × (cut − ½), not the sum of the cut widths. Five 2½″ strips finish at exactly 10″, not 12½. Easy to be half an inch out on.
- Cut logs as you go, or cut them all first? Cutting to the measured block as you go is more accurate and much slower. The chart here gives the arithmetic sizes — measure the block every round or two and adjust if it has drifted.
- Press outward, away from the centre. Every seam pressed toward the last log keeps the block flat and stops the bulk piling up in the middle.
How to use
- Enter the centre square, log width and number of rounds.
- Read the finished block size and the per-side strip lengths.
- Buy more dark than light — the tool says how much more.
- Use the strip-set panel for the cross-cut segment arithmetic.
Frequently asked questions
How big will my log cabin block be?
Centre plus twice the rounds times the log width. A 3-inch centre with 1.5-inch logs over four rounds finishes at 15 inches. Every round adds one log width to each of the four sides, so two per dimension.
Do the light and dark sides use the same amount of fabric?
No, and this is the useful finding. The dark side always takes the two logs that come after the light pair in the spiral, and each log is one log width longer than the last — so the dark side needs exactly 2 × log width × rounds more. On a four-round block with 1.5-inch logs that is 12 extra inches, or 18% more dark fabric.
Why do I keep running out of the dark fabric?
Because you bought equal amounts. The log counts are equal — two light and two dark every round — which is why it looks like it should balance. The lengths are not, and the gap is the same every single round. It is the dark that carries the design, so running out there is the worse way round.
How much fabric does a bigger log cabin need?
More than proportionally. The strip has to fill the block, so total strip length times strip width equals the block area exactly — and the block grows in both directions with each round. Doubling the rounds roughly quadruples the fabric rather than doubling it.
How wide does a strip set finish?
Number of strips × (cut width − ½ inch), which is tidier than people expect and easy to be half an inch out on. Five 2½-inch strips finish at exactly 10 inches, not 12½ — each strip loses a quarter inch to each of its seams.
Why does my strip set curve?
Because you sewed every seam from the same end. The strips creep a fraction with each pass and it compounds into a visible bow, sometimes called the rainbow effect. Alternate the direction — one seam top to bottom, the next bottom to top — and it cancels out.
Should I cut all the logs first or as I go?
Cutting as you go, measured against the block you actually have, is more accurate and much slower. The chart here gives the arithmetic lengths, which is the right starting point — measure the block every round or two and adjust if it has drifted, because errors in a log cabin accumulate outward.
Which way should the seams be pressed?
Outward, away from the centre, every time. That keeps the block flat and stops the bulk piling up in the middle where every seam would otherwise meet.
Does this send anything anywhere?
No. Everything is calculated in your browser and nothing is stored or uploaded.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.