Segmented Woodturning Calculator

How It Works

A segmented turning is a ring assembled from wedge-shaped pieces of wood glued together, then turned on a lathe as a single unit. The geometry that makes it work is simple: a ring with N segments has a miter angle of 180°/N on each cut. For 12 segments the miter is 15° per cut; for 8 segments it's 22.5°; for 24 segments it's 7.5°.

To calculate segment dimensions, the tool uses these relationships from Malcolm Tibbetts' The Art of Segmented Woodturning, Ch. 3: Segment Geometry:

• Miter angle = 180° / N (angle set on the saw)
• Segment length (long side) = OD × sin(180° / N) — the outer edge length of each segment
• Segment length (short side) = ID × sin(180° / N) — the inner edge length
• Segment width = (OD − ID) / 2 — how much stock width you need per segment
• Stock waste per segment ≈ 15-25% depending on cutting method

A ring with 250 mm outside diameter and 200 mm inside diameter, in 12 segments, has segments that are 25 mm wide, with outer edges 64.7 mm long and inner edges 51.8 mm long. Each miter is 15°.

The tool also handles contrasting-species rings — layers alternating between two wood species for visual effect. Because different species have different tangential shrinkage coefficients, mixing them in the same ring can open joints seasonally. Walnut (tangential ~7.8%) and maple (tangential ~9.9%) in the same ring shift by 0.7 mm per 100 mm of ring width at a 4% MC swing — visible to the eye, and enough to open a hairline gap over years of use. The tool flags high-differential pairs from the USDA Wood Handbook tangential shrinkage tables.

How to Use This Calculator

1. Enter outside diameter (OD) and inside diameter (ID) — the finished ring dimensions in millimetres.
2. Enter number of segments — commonly 8, 12, 16, or 24 (higher segment counts give smoother rings but need tighter miter tolerance).
3. Enter stock thickness — how tall each segment (and therefore the ring) will be.
4. Choose primary wood species — from the built-in database with tangential shrinkage coefficient.
5. Optional: add contrasting species — the tool flags if the tangential shrinkage difference exceeds 2%, which risks seasonal joint opening.
6. Read the results — miter angle, segment length (outer and inner), segment width, total stock length needed with a 20% waste allowance, and any species-compatibility warnings.

Real-World Use Cases

12-segment ring

A woodturner makes a 250 mm outside diameter bowl ring in 12 segments (each 30° at the saw, since the miter angle is 180°/12 = 15° per cut and both sides of the segment get cut). At 25 mm segment width (200 mm ID), the segment outer edges are 64.7 mm long. Total stock length required: 12 × 64.7 = 776 mm plus 20% waste = 932 mm of 25 mm × stock-thickness board.

Contrasting-species vase

A turner layers rings of 16 walnut segments and 16 maple segments to create a black-and-white striped vase. Walnut tangential is ~7.8%, maple ~9.9% — a 2.1% differential. The tool warns that this pair will open joints by roughly 0.15 mm per 100 mm of ring width per 4% MC swing. For a 150 mm ring, that's about 0.22 mm of visible joint movement seasonally. The turner accepts the trade-off for the contrast.

Simple 8-segment cup

A beginner makes a small 80 mm outside diameter drinking cup ring in 8 segments. Miter angle is 180°/8 = 22.5°. Segment width is (80 − 60) / 2 = 10 mm at a 60 mm ID. Outer edge is 80 × sin(22.5°) = 30.6 mm. Total stock: 8 × 30.6 = 245 mm plus 20% waste = 294 mm from a scrap board. The tool confirms the numbers before the first cut.

Tips & Safety Notes

• A 0.25° miter error is visible after 12 joints. A properly-tuned miter saw is required; check the saw with a machinist's square before starting.
• Cut all segments in one setup without adjusting the saw between them. Cumulative drift from remounting the workpiece is the leading cause of gaps in a ring.
• Use a dedicated segment sled for repeatable cuts on a table saw or miter saw. Store-bought sleds work; shop-built jigs work better once tuned.
• Test-fit dry before gluing. Twelve joints go together dry-fitted around the ring; if the last joint doesn't close cleanly, the saw is off and you need to re-cut before glue-up.
• Titebond III is the standard for open-air rings. Epoxy handles the biggest thermal shifts but is harder to work; CA cures too fast for a 12-piece ring.

Limitations & What This Tool Cannot Do

• Species-shrinkage warning is heuristic. If two species with very different tangential shrinkage are mixed in the same ring, seasonal humidity change will open some joints. The tool warns when the differential exceeds 2%, but does not simulate the physical joint movement or predict which joint will open first.
• Assumes an exact miter cut. Actual miter accuracy depends on saw setup. A 15° cut off by 0.25° yields a visible gap after 12 joints (cumulative 3° error). The tool does not model tolerance stack; test-fit the segments dry-glued before committing to full assembly.
• Only handles flat-topped rings. Sloped or angled ring geometries (compound-mitered vessels, tapered rings for chalices) need trigonometry not covered here. For those, consult Tibbetts' book or the AAW segmented turning primer.

FAQ

What is a segmented turning and why not just turn from solid stock?

A segmented turning is a ring built up from wedge-shaped wood pieces glued together, then turned on a lathe. Compared to turning from solid stock, segmented turning gives you much larger bowls (single boards are limited by tree width), lets you combine contrasting species for visual effect, and uses off-cut stock instead of expensive slabs.

How do I calculate the miter angle for a segment count?

The miter angle at the saw is 180° / N, where N is the number of segments in the ring. For 12 segments, 15° per cut. Both sides of the segment get cut at this angle, so the "included" cut angle on the saw is often labeled as 15° in one direction and 15° in the other, or 30° total between the two cuts.

What is the relationship between segment count, ring diameter, and segment length?

For a given ring diameter, more segments means shorter (and narrower) each. Segment outer-edge length = OD × sin(180°/N). Doubling the segment count roughly halves the segment length. Higher counts (24+) give smoother rings but need tighter miter tolerance and more setup time.

How much stock waste should I plan for on each ring?

Plan for 20-25% waste on each ring — the miter cuts themselves waste kerf material, and any test-fit segments that are off get discarded. A ring with 12 segments needing 65 mm each has a raw stock requirement of about 12 × 65 × 1.25 = 975 mm total board length.

Can I mix wood species between rings without accounting for shrinkage differences?

Yes, generally — mixing species across rings (each ring is a single species) is safe because the rings can shift independently at their glue joints. What causes problems is mixing species within a single ring: alternating walnut and maple segments in the same ring will open the walnut-maple joints seasonally. The tool flags high-differential pairs when you use them within a ring.

What glue works best for segmented rings — Titebond, epoxy, or CA?

Titebond III is the standard for most segmented work. Long enough open time (10 minutes at 20 °C) to align 8-12 segments; strong enough to survive the lathe forces of turning; food-safe once cured. Epoxy for maximum thermal stability on rings that will sit in a temperature-swinging environment. CA is too fast — you can't align a full ring before it sets.

Related Tools

• Wood Shrinkage Calculator — Compute shrinkage for the same species used in ring
• Sheet Metal Bend Calculator — Materials-lab sister tool for a different geometry problem
• Wax-to-Metal Casting Calculator — Materials-lab sister tool

References

• Malcolm Tibbetts, The Art of Segmented Woodturning, Ch. 3: Segment Geometry — miter angle and segment length formulas
• Curtis Turner segmented turning technical papers (segmentedturners.org) — advanced ring geometries and species compatibility
• American Association of Woodturners (AAW) segmented turning primer — beginner-friendly overview and glue selection

Reviewed by the Craft Calc Lab team on August 4, 2026.