What Is Kerf?

RailChop Team · April 2026 · 5 min read

The Short Definition

Kerf is the width of the slot a saw blade cuts into the material. Every cut you make removes a thin strip of moulding equal to the blade's thickness. That strip becomes sawdust. For a standard picture framing miter saw, kerf is typically 0.125 inches — one-eighth of an inch. It's small enough that nobody thinks about it on a single cut. Large enough that it matters over a day of cutting.

Why Kerf Is Different from Allowance

This is the confusion that trips up new framers. Kerf and allowance both show up in cut calculations, but they're different things with different purposes.

Kerf is physical. The blade is a physical object with thickness; it removes material as it passes through the wood. You can't get rid of kerf — only account for it.

Allowance is deliberate. It's the extra 1/8" (or 1/16") you add to each cut so the frame opening is slightly larger than the artwork, giving room for the glass and mat package to sit without binding. Allowance is a design choice.

Both appear in the cut formula (cut length = artwork dimension + 2 × moulding width + allowance, with kerf added between cuts on the same stick), but they serve different roles. Allowance makes the frame fit correctly. Kerf accounts for material you'll never see again.

Where Kerf Applies (And Where It Doesn't)

The rule that saves the most mistakes: kerf is consumed between cuts on the same stick, not before the first cut.

Picture a 120-inch stick. You make five cuts, producing six pieces (the last being the leftover remnant). The first cut doesn't consume kerf — the blade enters fresh material that wasn't yet part of any piece. Each subsequent cut, though, removes 0.125" of material that would otherwise have been part of the previous piece's usable length.

So for five cuts on a single stick: 4 × 0.125" = 0.5 inches of kerf total. If you budgeted for "5 × kerf", you over-budgeted by 0.125". If you budgeted for "0 × kerf", you'd come up short by half an inch — exactly enough to ruin the last cut.

The Math, Worked Out

Say you're cutting four 20-inch pieces from a 120-inch stick (ignoring moulding width and allowance for simplicity). Without kerf:

• 4 pieces × 20" = 80" used
• 120" − 80" = 40" left over

With kerf accounted for:

• 4 pieces × 20" = 80" in pieces
• 3 kerfs between cuts × 0.125" = 0.375" in sawdust
• 120" − 80" − 0.375" = 39.625" left over

Small difference on one stick. Over 20 sticks per day across a year, the kerf you didn't budget for becomes 2-3 short cuts. Each short cut is a ruined piece of moulding. At $5-20/ft, those add up fast.

The Annual Kerf Bill

A rough estimate for a working frame shop: 40 saw cuts per day, 250 working days per year = 10,000 cuts annually. At 0.125" per cut, that's 1,250 inches of kerf — just over 104 feet of moulding turned to dust. On $4/ft moulding, that's $416 per year in kerf. On gilded gallery moulding at $20/ft, that's over $2,000.

This isn't waste you can recover. It's the cost of using a saw blade with thickness. But two things are true: (1) knowing about kerf prevents the much larger waste of miscuts, and (2) batching cuts by profile reduces the number of kerfs by reducing the number of cuts — which is why optimization saves material.

Thin-Kerf vs. Standard-Kerf Blades

Saw blades come in different thicknesses. Standard-kerf blades are typically 0.125" (1/8"). Thin-kerf blades are typically 0.094" (3/32") or even 0.080". Thin-kerf blades remove less material per cut — which over 10,000 cuts a year means real savings.

The trade-off: thin-kerf blades flex more. On delicate gilded mouldings, that flex can cause chatter marks or imperfect miters. For most shops cutting standard hardwood and softwood mouldings, thin-kerf works fine and saves material. For high-end gilded or decorative work, the extra rigidity of a standard blade is worth the extra kerf.

If you switch blades, update your kerf setting. In RailChop that's one field in your moulding profile; in a spreadsheet workflow it's a find-and-replace exercise easy to miss. Using the wrong kerf value in a cut plan means every frame comes out slightly wrong — not enough to obviously fail, but enough to build in play that shouldn't be there.

Common Kerf Mistakes

Budgeting kerf before the first cut. A 120-inch stick cut into four 20-inch pieces with three kerfs in between should leave a 39.625" remnant. If you added a fourth kerf "before the first cut," you'd calculate 39.5" — which doesn't matter for the remnant but throws off multi-stick optimization where every inch counts.

Ignoring kerf entirely. The most common error in manual cut planning. Shops cut stock "to dimension" without accounting for the material removed by each cut, then wonder why the last piece on each stick comes out short. It's always kerf.

Using the wrong kerf value. Shops that upgrade to a thin-kerf blade but leave their cut calculations set to 0.125" lose the savings and overestimate material needed. Shops that downgrade to a thicker blade without updating kerf cut short pieces.

Applying kerf to pieces from different sticks. Kerf only applies to cuts made on the same stick. If pieces A and B come from different sticks, there's no kerf between them. This is subtle and shouldn't matter to end users of optimization software — the algorithm handles it — but it's a common error in manual spreadsheets where people apply kerf to every piece in the cut list.

How RailChop Handles Kerf

Kerf is configured per moulding profile, defaulted to 0.125". The optimizer applies it correctly — between cuts on the same stick, not before the first cut, not between cuts from different sticks. You don't have to think about it. See the moulding calculator for the formula, and see how the optimizer handles multi-stick planning where kerf accounting becomes complex enough that spreadsheets start producing errors.