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How to Use a Radius Gage
Guide · Updated October 28, 2026
Radius gages, also called fillet gages, are thin leaves cut to exact radii. You hold one against a corner or fillet and look for light. They are quick go/no-go checks, not precise measuring tools.
Trust your leaves
Check suspect leaves against a pin or gage block setup on a comparator. Check it with a mic you've verified against gage blocks; the free Gage Block Calculator builds the stack.
Step by step
- Pick the side: Use the convex end of a leaf for inside fillets and the concave end for outside corners.
- Clean and deburr: Wipe the part and leaf and remove burrs; a burr holds the leaf off the surface.
- Pick a starting size: Start at the drawing size, then try one size up and one down.
- Hold it square: Hold the leaf square to both faces and in line with the corner, not tipped.
- Backlight it: Put a light or white card behind the part and look along the leaf.
- Read the gap: Light at the middle means the radius is wrong one way; light at the ends means it is wrong the other way.
- Record: Note the closest leaf and whether the part is between sizes.
Worked example
Drawing calls an R0.125 in inside fillet. The convex 1/8 in leaf touches both walls but shows light at the middle of the arc, so the fillet is smaller than the leaf. The 7/64 in leaf fits with no light, so the actual fillet is about 0.109 in. The end mill corner radius or the program needs changing.
Reading the light
Convex leaf in an inside fillet: light at the center means the fillet is smaller than the leaf; light at the ends means it is larger. Concave leaf on an outside corner: light at the ends means the corner radius is smaller than the leaf; light at the center means it is larger.
Limits
Most people can see a gap of about 0.0002–0.0005 in with good backlighting, but the leaf steps are often 1/64 in or 0.5 mm. For tight tolerances use an optical comparator or a CMM.
Tips
- Verify leaves: checking radius gages.
- Ball end mill radii: using a ball end mill.
- Edge breaks: deburring and breaking edges.
- Chamfers instead? checking a chamfer.
FAQ
Which side of a radius gage do I use for a fillet?
The convex end checks inside fillets; the concave end checks outside corners.
How do I tell if a radius is too big or small?
Look where light shows: at the ends or at the middle of the leaf.
How accurate are radius gages?
They show fit to the nearest leaf size; use a comparator for tight tolerances.
Fillet gage vs radius gage?
They are the same tool.
Can I check a radius under 1/32 in?
Small leaves exist, but a magnifier or comparator is easier.
Related guides and calculators
- How to Check Radius Gages
- How to Use a Ball End Mill
- How to Deburr and Break Edges
- How to Check a Chamfer or Countersink
- Thread Pitch Gage Basics
- GD&T Basics for Machinists
- How to Use a Corner Rounding End Mill
- How to Cut a Radius on a Lathe
- How to Cut a Radius with a Rotary Table
- How to Cut a Ball on a Lathe
- How to Use a Radius Turning Attachment
- How to Use a Form Tool on a Lathe
- All free machinist calculators
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