Mortise and tenon joints are fundamental to traditional woodworking, providing exceptional strength for furniture frames, doors, and structural applications. This guide covers sizing rules, cutting techniques, and assembly methods for creating perfect-fitting joints. Use our mortise and tenon calculator for precise dimensions.
Joint Sizing & Proportions
The Rule of Thirds
Traditional mortise and tenon proportions follow the "rule of thirds" for optimal strength. Note: These are general guidelines based on traditional woodworking practices. Adjust based on specific wood species, application, and structural requirements.
Mortise Width:
1/3 of stock thickness
Tenon Thickness:
1/3 of stock thickness (matches mortise width)
Tenon Width:
2/3 of stock width (typical)
Mortise Depth:
Tenon length + 1/8" clearance
This ratio provides sufficient material around the mortise to prevent splitting while maximizing glue surface area. For example, 1.5" thick stock uses a 1/2" wide mortise.
Tenon Length Guidelines by Application
| Application Type |
Tenon Length |
Stock Thickness |
Typical Projects |
| Light-Duty |
1" - 1.5" |
3/4" - 1" |
Picture frames, small boxes |
| Standard Furniture |
2" - 2.5" |
1.5" - 2" |
Chair rails, table aprons |
| Heavy-Duty |
3" - 4" |
2" - 3" |
Bed frames, workbenches |
| Structural |
4"+ |
3"+ |
Timber framing, beams |
Note: These are general guidelines. Always consider the specific load requirements and grain orientation of your project.
Fit Tolerances & Wood Movement
Proper fit tolerances ensure strong joints without splitting. Note: These are general reference ranges. Actual tolerances depend on wood species, moisture content, cutting precision, and assembly conditions.
Tight Fit (Structural):
0.002"-0.005" clearance
Standard Fit (General):
0.005"-0.010" clearance
Loose Fit (Movement):
0.010"-0.015" clearance
Wood Species Selection Guide
| Wood Type |
Common Species |
Recommended Tolerance |
Key Characteristics |
| Hardwood |
Oak, Maple, Cherry, Ash |
0.005" - 0.010" |
Dense, stable, excellent strength |
| Softwood |
Pine, Fir, Cedar, Spruce |
0.008" - 0.015" |
Softer, more prone to compression |
| Exotic |
Teak, Mahogany, Walnut |
0.005" - 0.012" |
Variable density, oil content |
| Engineered |
Plywood, MDF |
0.010" - 0.020" |
Dimensionally stable, less glue adhesion |
Grain Orientation Best Practices
- Parallel Grain: Align grain direction parallel to tenon length for maximum strength
- Avoid Short Grain: Grain should run full length of tenon cheeks to prevent breaking
- Cross-Grain Considerations: Allow looser fit (add 0.003"-0.005") for cross-grain joints
- Quarter-Sawn Advantage: More stable than plain-sawn, tighter tolerances possible
Testing Fit Quality
A properly fitted joint should slide together with firm hand pressure. Too tight risks splitting during assembly; too loose reduces glue contact and joint strength. Always test fit on scrap wood before cutting final pieces.
Cutting Techniques
Mortise Cutting Sequence
- Mark Layout: Use marking gauge to scribe mortise location
- Drill Waste: Remove bulk of waste with drill bits
- Square Walls: Pare to final dimensions with sharp chisels
- Test Depth: Verify depth exceeds tenon length by 1/8"
Tool Selection Guide
| Tool Type |
Best For |
Advantages |
Skill Level |
Hand Tools
(Chisels, Saws) |
1-5 joints, custom work |
Quiet, precise control, traditional |
Intermediate+ |
| Drill Press + Chisels |
5-20 joints, mixed sizes |
Faster waste removal, accurate depth |
Beginner+ |
| Router Table |
10+ identical tenons |
Consistent results, efficient |
Intermediate |
| Mortising Machine |
Production work, 20+ joints |
Fast, square mortises, repeatable |
Beginner+ |
| Table Saw + Jig |
Tenons, production work |
Accurate, repeatable cuts |
Intermediate |
Tenon Cutting Methods Comparison
- Table Saw Method: Use tenoning jig or multiple passes with dado stack. Most efficient for production work, excellent repeatability.
- Hand Tool Method: Mark with marking gauge, saw cheeks with tenon saw, remove waste with coping saw, pare to fit. Best for custom work and fine adjustments.
- Router Method: Use router table with straight bit and fence. Consistent results, good for multiple identical tenons, requires setup time.
Pro Tip: Cutting Sequence
Always cut the mortise first, then fit the tenon to match. Mortises are harder to adjust than tenons. Use the mortise as your reference and carefully pare the tenon for a perfect fit. This approach reduces errors and ensures tight-fitting joints.
Assembly & Gluing
Pre-Assembly Checklist
- Dry fit all joints to verify fit and alignment
- Mark joint orientation to prevent assembly errors
- Have all clamps ready and adjusted to size
- Prepare glue, brushes, and cleanup materials
Glue Application
Apply glue to mortise walls, not the tenon. This prevents glue from being scraped off during assembly and ensures even distribution. Use a small brush or glue syringe to coat all four mortise walls.
Clamping Guidelines
- Apply moderate clamp pressure - enough to close joints, not crush wood
- Check for square immediately after clamping
- Wipe away squeeze-out with damp cloth before it dries
- Leave clamps on for minimum 1 hour (PVA glue)
- Allow 24 hours cure time before stressing joints