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A mortise gauge marks two parallel lines for the sides of a mortise. Set correctly, it puts the slot on the centerline of a board without measuring each side separately. The key is to set the gauge from the actual chisel you’ll use, then keep the stock flat against the same reference face as you mark.
What you need
Use a mortise gauge with two pins or cutters, a ruler or combination square, a sharp pencil, and the mortise chisel that will cut the slot. A combination square makes it easier to check the board’s thickness and find its center. A marking knife gives a cleaner, more precise line than a pencil, but a sharp gauge point is usually enough for the parallel lines.
A basic wooden gauge is adequate for occasional work if its fence is square and its locking screw holds the beam firmly. A metal gauge or a better-made wooden one can be worthwhile for frequent layout because it is less likely to shift or wear at the fence. Compare mortise gauges by fence fit and pin adjustment, not appearance alone.
Choose the chisel first
The chisel determines the spacing between the gauge’s two cutters. Use the actual chisel intended for the mortise, not a nominal size from a label: a chisel marked 1/4 inch may measure slightly more or less across its cutting edge. Set the cutters so the gap matches the chisel’s width at the cutting edge. That leaves enough room for the chisel to work without making the mortise unnecessarily wide.
| Tool | Best for | Trade-off |
|---|---|---|
| Basic wooden mortise gauge | Occasional joinery and simple layout | Low cost; pins and fence can wear or shift |
| Adjustable mortise gauge | Changing between chisel widths | Convenient, but more parts can loosen if poorly made |
| Separate marking gauge | Marking one line or a shoulder from an edge | Not a direct substitute for two fixed, parallel mortise lines |
Find the center of the board
Measure the board’s thickness where the mortise will go. If the board is 3/4 inch thick, its centerline is 3/8 inch from either face. For a 1-inch-thick board, use 1/2 inch. Check the thickness in more than one place if the board is rough or tapered; a changing thickness means there is no single centerline unless you first choose which faces to reference.
For a square, consistent board, set a combination square to half the thickness and mark a short center tick on the end or face. If the mortise runs along the board’s length, choose one face as the reference face and keep using it. This matters when the two faces are not parallel: measuring from one face in one place and the opposite face elsewhere can shift the slot off center.
If the board’s thickness varies, decide whether the slot should be centered on the local thickness or aligned from a chosen face. Centering locally may make the mortise wander relative to other joinery. Referencing one face is often the better choice for fitting a joint to an adjoining part.
Set the gauge for the chisel
Loosen the gauge’s adjusting mechanism and set the two cutters to match the chisel’s width. On a gauge with one fixed cutter and one sliding cutter, adjust the sliding cutter while holding the chisel alongside the cutters as a visual reference. Tighten the lock and check the spacing again; some gauges shift slightly as they’re tightened.
Next, set the distance from the fence to the midpoint between the cutters. That distance should equal half the board thickness: 3/8 inch for a 3/4-inch board. A ruler works, but placing the gauge against the board and comparing the cutter pair with your center tick is often more reliable. The two cutters must sit equally on either side of the centerline. If the gap is 1/4 inch, each cutter should be 1/8 inch from the centerline.
Test the setting on scrap of similar thickness. Run the gauge once from the reference face, then check whether the two lines are evenly spaced around the center tick. If one line falls on the tick or both lines sit noticeably to one side, reset the fence distance before marking the workpiece.
Mark the slot
Hold the gauge so its fence stays flat against the chosen reference face. Keep the beam square to the board’s length and make a light first pass. Then make one or two firmer passes to deepen the lines. Pressing hard on the first pass can make a cutter skate, especially on hard, figured wood, and produce a line that is difficult to correct.
Mark both ends of the mortise with a square or knife, using your chosen length measurement. The gauge establishes the two long sides; it does not set the slot’s length or guarantee that the ends are square. If the mortise is near an edge, check the fence is still riding on a clean, flat reference face rather than rocking on a bevel or damaged corner.
For crisp layout in hard wood, a woodworking marking knife can define the ends and help prevent fibers from tearing when you chop. A sharp pencil is fine for rough work, but its line is wider and less useful as a cutting boundary.
Check before chopping
Before cutting, compare the marked lines with the chisel. The chisel should fit between them with little or no sideways play. If it is wider than the marks, forcing it into the mortise will enlarge the slot or split the edge. If it is much narrower, the mortise will be wider than needed and may leave a loose joint.
Also check the layout at both ends and along the full length. A line that gradually moves toward one face usually means the fence lifted, the board edge is uneven, or the gauge was referenced from different faces. Erase or plane away a bad layout if necessary, then remark from the same reference face. Taking a minute to reset the gauge is cheaper than trying to disguise an off-center mortise with a loose tenon.