What is a realistic slab thickness for cabochon cutting?

Most lapidary slabs for cabochon cutting are cut 1/4" to 3/8" thick (roughly 6–10 mm). A thickness around 6–7 mm suits most standard cab heights. Thinner slabs risk cracking during grinding; thicker ones waste material and add grinding time. The slab thickness does not affect the number of cabs per slab, only the dome height achievable.

Cabochon Slab Yield Calculator – How Many Cabs From a Slab

Units:

Slab Dimensions

Slab Width (mm)

Slab Length (mm)

Number of Slabs

Slab Cost Each

Enter 0 to skip cost calculations.

Cab Template Size

Shape

Common Sizes (click to fill)

Cab Width (mm)

Cab Length (mm)

Cutting Parameters

Trim Gap Between Cabs (mm)

Recommended: 1.5–3 mm (1/16″–1/8″) for trim saw kerf + grinding clearance.

Edge Buffer Each Side (mm)

Unusable margin around the slab perimeter.

Experience Level

Intended Sale Price per Cab

Optional. Enter 0 to skip revenue projection.

Results

Fill in your slab and cab dimensions to see the yield.

How to Use This Calculator

Enter your slab dimensions — width and length of the usable stone slab (not including cracked or unusable edges).
Choose your cab shape and size — pick from the preset buttons for common calibrated sizes, or enter a custom width and length. For round cabs, width = diameter.
Set trim gap — the space left between each cab outline so the trim saw blade fits and you have grinding clearance. 2 mm is the default.
Add edge buffer — how much of the slab's perimeter is too thin, cracked, or rough to use. 3 mm is a conservative default.
Select experience level — used to estimate session time for the whole batch.
Read results instantly — cab count, cost per stone, session time range, waste percentage, and optional revenue projection.

The Formula Explained

The calculator uses bounding-box grid packing, which reflects real lapidary practice: a trim saw only cuts straight lines, so every cab — oval, teardrop, or marquise — must first be sawn out as a rectangular preform, then ground to shape.

usable_width = slab_width − 2 × edge_buffer usable_length = slab_length − 2 × edge_buffer cols = floor( usable_width / (cab_width + trim_gap) ) rows = floor( usable_length / (cab_length + trim_gap) ) cabs_per_slab = cols × rows total_cabs = cabs_per_slab × num_slabs cab_footprint_area = cab_width × cab_length [× π/4 for ovals/rounds] waste_% = (1 − (total_cabs × cab_footprint_area) / total_usable_area) × 100 cost_per_cab = (slab_cost × num_slabs) / total_cabs

Note for oval/round shapes: the bounding-box column/row count is the conservative real-world count. The displayed waste % uses the actual oval area (π/4 × W × L) for accuracy, since material between each oval preform after grinding is discarded.

Why Bounding-Box Packing?

A trim saw cannot follow curves. To cut an 18×25 mm oval, you saw out an 18×25 mm rectangle, then grind corners away on the cabbing machine. The space you must leave between each rectangle accounts for the saw blade's kerf (about 0.5–1 mm) plus the room needed to avoid undercutting the adjacent preform during the straight cuts. This is why the trim-gap input (default 2 mm) is so important — too small and your saw will nick a neighbouring preform.

Common Mistakes When Laying Out Slabs

Ignoring edge damage. Slab edges are often chipped or uneven from the slab saw. A 3–5 mm edge buffer prevents you from planning cabs that fall in unusable zones.
Too small a trim gap. Leaving only 1 mm works on paper but leaves no room for a 6″ trim saw blade (kerf ≈ 0.5–0.7 mm) plus grinding clearance. Use 2 mm minimum.
Underestimating time. Each grinder stage (coarse shape → medium smooth → fine pre-polish → polish) takes its own time. Beginners often forget to count dopping, undopping, and inspecting after every wheel.
Assuming 100% yield. Inclusions, fracture lines, and patterns that don't work out will remove some cabs from the layout. Build in a 10–20% cushion on valuable material.

Standard Cabochon Sizes Reference

The most commonly used calibrated cab sizes for pre-made jewelry settings (source: industry dealers and Yau Shing Gems):

Oval: 6×8, 8×10, 10×12, 10×14, 13×18, 15×20, 18×25 mm
Round: 6, 8, 10, 12, 14, 18, 20 mm diameter
Rectangle/Cushion: 8×10, 10×12, 12×16 mm
Marquise: 8×20, 8×25, 10×25 mm

Calibrated sizes fit pre-made bezel settings and wire findings — a big advantage for production jewelers. Freeform cuts maximise yield from patterned or precious material but require custom settings.

Frequently Asked Questions

How many cabochons can I cut from a slab?

It depends on your slab dimensions and cab template size. Divide the usable slab width by (cab width + trim gap) and the usable length by (cab length + trim gap), then multiply the two floor values. A 100×150 mm slab with 18×25 mm ovals and a 2 mm gap yields 4 columns × 4 rows = 16 cabs. This calculator does the math instantly for any combination.

What trim gap should I leave between cabs on a slab?

Allow about 1.5–3 mm (1/16″–1/8″) between each cab outline. This space accommodates the trim saw blade's kerf and leaves grinding clearance. The default is 2 mm, which suits most 6″ trim saws. Increase to 3 mm for harder materials like agate or jasper that need more grinding clearance, or when using a wider blade.

How long does it take to cut and polish one cabochon?

A professional cabber on a 6-wheel machine may finish a small stone in 15 minutes or less. Beginners typically spend 45 minutes to 1.5 hours per cab, including trimming, dopping, grinding through all grit stages, and polishing. Each step on each wheel must be completed before moving to the next finer grit, and you must eliminate all scratches from the previous stage.

What is the ideal slab thickness for cabochon cutting?

Most lapidary slabs for cabbing are cut 1/4″ to 3/8″ thick (roughly 6–10 mm). Around 6–7 mm suits standard cab dome heights. Too thin risks cracking during grinding; too thick wastes material and adds time. Thickness doesn't affect how many cabs fit on a slab — only the dome height achievable.

Why does the calculator use bounding-box packing for ovals?

A trim saw only cuts straight lines. To produce an oval preform, you must first saw a rectangle slightly larger than the oval outline, then grind to the final shape on the cabbing machine. The bounding box (width × height of the oval template) is therefore the minimum footprint each cab occupies on the slab during layout. This is standard practice described in lapidary instructional guides including the International Gem Society's cabochon cutting fundamentals.

How do I reduce slab waste?

Use the smallest trim gap that your saw safely allows, orient non-square cabs in alternating rows (rotate 90°) to stagger and reduce lost corner material, cut designer cabs from irregular shapes rather than forcing calibrated sizes, and save all offcuts — small fragments often yield earring-size stones. For very expensive rough, consider a diamond band saw which can follow curves and reduce preform waste significantly.

Method based on standard lapidary bounding-box packing practice as documented by the International Gem Society. Time estimates derived from published ranges (beginners 45–90 min/cab; professionals <15 min). Results are estimates for planning purposes; actual yield depends on material quality, fracture lines, and cutter skill.