Straight Cut Router Bit Sizes: How to Choose the Right Diameter and Length

A straight cut router bit is sized by matching its cutting diameter to the slot width you need and its working length to the depth you’ll cut.

One wrong pick shows up fast: a bit that’s too wide for the groove, or a shank that won’t seat in the collet, and the cut is ruined before the wood even moves. Getting the straight cut router bit sizes right comes down to two numbers and one compatibility check, and that’s most of the decision.

The core rule: pick the cutting diameter to match the finished width of your groove, mortise, or opening, and choose a working length just past the material thickness or cut depth. Bosch’s straight-bit listings cover cutting diameters from 1/16 inch up to 1 1/2 inches, so the size you need almost certainly exists — the trick is reading the specs in the right order.

Why Cutting Diameter Drives The Whole Choice

Cutting diameter sets the width of the cut, so it’s the first number to lock in. A 1/4-inch bit cuts a nominal 1/4-inch groove; a 1/2-inch bit cuts a nominal 1/2-inch groove. For anything wider, take multiple passes or step up to a larger cutter.

Smaller diameters earn their keep on detail work. Bosch’s US carbide-tipped and solid-carbide ranges list straight-bit diameters at 1/16, 1/8, 3/16, 1/4, 5/16, 3/8, 1/2, 5/8, 3/4, 7/8, 1, 1 1/4, and 1 1/2 inches. Larger bits bring more rigidity, but they also demand appropriate speed limits and a router that can handle the mass.

Plywood gets its own shortcut. Bosch sells dedicated plywood mortising sets in 1/4-, 1/2-, and 3/4-inch cutting diameters — the RBS023XW on a 1/4-inch shank and the RBS024SXW on a 1/2-inch shank.

Working Length, Not Total Length

Working length is the cutting dimension that matters. Choose one at least equal to your required cut depth, with only modest extra projection. Bosch’s US listings run from short miniature sizes to at least 2 1/2 inches, depending on construction and diameter.

Concrete examples across Bosch’s lines:

• Model 85096 — 1/4-inch shank, 1/4-inch diameter, 3/4-inch working length.
• Model 85180 — 1/4-inch shank, 1/4-inch diameter, 1 1/8-inch working length.
• Model 85213M/85213MC — solid carbide, 1/8-inch diameter, 1/2-inch working length.
• Model 85264M — 1/2-inch shank, 1/2-inch diameter, 2-inch working length.

Longer cutting lengths exist for deeper work, but the sound practice is several shallow passes rather than one full-depth plunge. Extra projection means more deflection, more chatter, and harder control.

Straight Cut Router Bit Sizes At A Glance

The table below pairs cutting diameter with shank size so you can spot the common pairings before you open a catalog. Note that the same cutting diameter can appear on different shank sizes.

If you’re stepping into the larger end of the range for heavier stock removal, our breakdown of the best 1 1/4 router bit options covers what to look for in the beefier sizes.

Shank, Collet, And Speed Checks

Shank diameter and cutting diameter are two different numbers, and confusing them is the most common ordering error. Bosch lists many small cutters on 1/4-inch shanks and larger cutters on 1/2-inch shanks — model 85409M is a 3/8-inch cutter on a 1/2-inch shank, while 85410M is a 1/4-inch cutter on a 1/4-inch shank.

Use a matching collet every time. Never force a 1/2-inch shank into a 1/4-inch collet or lean on an unsuitable reducer. Bosch notes reinforced, hardened and tempered shanks on some carbide-tipped double-flute products, and lists balanced cutting and reduced vibration as design goals — but those qualities only help if the bit is seated and rated correctly. Single-flute, double-flute, high-speed-steel, carbide-tipped, and solid-carbide bits are distinct products, so don’t carry one model’s speed assumptions over to another. Check the manufacturer’s maximum RPM and feed guidance for the exact model. The Bosch carbide-tipped double-flute straight bit specifications list the model-level details worth confirming.

Common Sizing Mistakes To Avoid

Most bad cuts trace back to a handful of repeatable errors, and each is cheap to prevent with a scrap-wood test.

• Choosing by shank size instead of cutting diameter.
• Using a cutting length shorter than the depth you need.
• Extending the bit farther than necessary, which adds chatter and deflection.
• Taking full depth in one pass.
• Running a diameter beyond the router’s capacity or an unsuitable RPM rating.
• Assuming nominal plywood thickness is exact — measure the workpiece, since 1/4, 1/2, and 3/4 inch can all differ.
• Ignoring grain direction, weak workholding, dull cutters, or poor chip evacuation.

Install the bit fully into the collet without bottoming out, tighten correctly, verify clearance, and feed against cutter rotation on a conventional handheld cut unless the manufacturer directs otherwise.