Mortise & Tenon Size Calculator

Enter your stock thickness, mating part width, and shoulder depth to calculate tenon thickness, tenon length, mortise dimensions, and glue surface area.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter the Stock Thickness
Input the thickness of the workpiece that will receive the tenon in inches (e.g., 1.5 for 1-1/2 inches).
2
Specify the Mating Part Width
Provide the width of the part the tenon will enter in inches. This influences tenon length.
3
Input Shoulder Depth (each side)
Enter the depth of the shoulder on each side of the tenon in inches (e.g., 0.25 for 1/4 inch).
4
Review your results
The calculator will instantly display the ideal tenon thickness, length, mortise depth, and glue surface area.

Example Calculation

A woodworker needs to size a mortise and tenon joint for a table leg made from 1.5-inch thick stock, joining a 3-inch wide rail.

Stock Thickness (in)

1.5

Mating Part Width (in)

Shoulder Depth (each side) (in)

0.25

Results

0.500 in

Tips

Measure Accurately

Precision is paramount in joinery. Use digital calipers or a marking gauge to ensure your stock thickness and shoulder depths are measured and marked with extreme accuracy to achieve a tight, strong joint.

Account for Wood Movement

While this calculator provides ideal dimensions, consider wood movement. For wider mating parts, a haunched tenon or multiple smaller tenons can prevent the joint from splitting due to seasonal expansion and contraction.

Test Fit Before Glue-Up

Always dry-fit your mortise and tenon joint before applying glue. The fit should be snug but not require excessive force, which could split the wood. Adjustments are much easier before the glue is applied.

Precision Woodworking: Sizing Mortise & Tenon Joints

The Mortise & Tenon Size Calculator is an indispensable tool for woodworkers aiming for strong, precise joinery. Based on the classic 1/3 rule, this calculator instantly provides optimal dimensions for tenon thickness, length, mortise depth, and glue surface area, ensuring structural integrity for any stock size. For example, using 1.5-inch thick stock and a 3-inch wide mating part, the calculator recommends a 0.500-inch tenon thickness and a 2.000-inch tenon length, vital for crafting durable furniture or cabinetry in 2025.

Achieving Strong Woodworking Joints

The mortise and tenon joint is a cornerstone of traditional woodworking, renowned for its exceptional strength and durability. Its effectiveness comes from the mechanical interlocking of parts combined with a large glue surface area. A well-executed joint, adhering to principles like the 1/3 rule for tenon thickness, can withstand significant racking and shear forces, often proving stronger than the surrounding wood itself. This robust construction is why it remains a preferred method for building high-quality furniture, doors, and timber frames, where longevity and structural integrity are paramount.

The 1/3 Rule for Mortise and Tenon Joints

The Mortise & Tenon Size Calculator applies the fundamental "1/3 rule" to determine the optimal dimensions for your joint. This rule dictates that the tenon thickness should be approximately one-third of the stock thickness. The tenon length is typically set at two-thirds of the mating part's width, ensuring adequate glue surface area and mechanical strength. Mortise depth includes a small clearance to prevent bottoming out.

Tenon Thickness = Stock Thickness / 3
Tenon Length = Mating Part Width × (2 / 3)
Mortise Depth = Tenon Length + 0.0625  (for 1/16 inch relief)
Tenon Width = Stock Thickness - (Shoulder Depth × 2)
Mortise Width = Tenon Thickness
Mortise Length = Tenon Width
Glue Surface Area = 2 × (Tenon Thickness × Tenon Length) + 2 × (Tenon Width × Tenon Length)

By following these proportions, the calculator ensures a balanced joint that maximizes both mechanical interlocking and glue bond strength.

💡 For other structural calculations in your woodworking or construction projects, our Deck Beam Size Calculator can help ensure your designs meet load requirements.

Designing a Mortise and Tenon Joint

Let's design a mortise and tenon joint for a workpiece with a Stock Thickness of 1.5 inches, mating with a part that has a Mating Part Width of 3 inches, and Shoulder Depth on each side of 0.25 inches.

Calculate Tenon Thickness: 1.5 inches / 3 = 0.500 inches.
Calculate Tenon Length: 3 inches × (2 / 3) = 2.000 inches.
Calculate Mortise Depth: 2.000 inches (tenon length) + 0.0625 inches (clearance) = 2.063 inches.
Calculate Tenon Width: 1.5 inches (stock thickness) - (0.25 inches × 2) = 1.000 inch.
Calculate Mortise Width: This matches the tenon thickness, so 0.500 inches.
Calculate Mortise Length: This matches the tenon width, so 1.000 inch.
Calculate Glue Surface Area: 2 × (0.500 × 2.000) + 2 × (1.000 × 2.000) = 2 + 4 = 6.00 in².

These dimensions provide a balanced and strong joint adhering to established woodworking principles.

💡 When planning the foundation for larger structures, our Deck Footing Concrete Calculator can assist with material estimates.

Master Woodworkers' Approach to Joinery

Experienced woodworkers approach joinery with an emphasis on both precision and an intuitive understanding of wood mechanics. While the "1/3 rule" provides a solid foundation for mortise and tenon sizing, master craftsmen often adjust these ratios subtly based on the species of wood (e.g., a slightly thicker tenon for softer woods), the specific load the joint will bear, and aesthetic considerations. They also pay close attention to grain direction, ensuring the tenon's long grain runs parallel to the mating part's long grain for maximum strength. The goal is not just to create a strong joint, but one that accommodates seasonal wood movement without stressing the surrounding material, ensuring the piece's longevity.

Frequently Asked Questions

What is the 1/3 rule for mortise and tenon joints?

The 1/3 rule is a widely accepted guideline in woodworking for sizing mortise and tenon joints, stating that the tenon's thickness should be approximately one-third of the stock thickness. This ratio ensures optimal strength by leaving sufficient wood on either side of the mortise to prevent splitting, while also providing adequate glue surface area for a robust connection, balancing mechanical strength with material integrity.

Why is tenon length important for joint strength?

Tenon length is crucial for the mechanical strength and glue surface area of a mortise and tenon joint. A longer tenon provides more surface area for glue adhesion, significantly increasing the joint's resistance to pull-out and racking forces. It also distributes stress over a greater area within the mating part, reducing the likelihood of failure, with a common guideline being two-thirds of the mating part's width.

What is the difference between tenon width and tenon thickness?

Tenon thickness refers to the dimension that runs parallel to the thickness of the stock, typically following the 1/3 rule. Tenon width, on the other hand, is the dimension that runs perpendicular to the stock thickness, often determined by the available space between the shoulders of the joint. Both dimensions are critical for a strong joint, with thickness affecting mortise integrity and width contributing to overall glue surface and resistance to twisting.

How much glue surface area is ideal for a strong joint?

For a strong mortise and tenon joint, maximizing glue surface area is key, with a general goal of at least 2-3 square inches per joint for typical furniture applications. This calculator helps determine the theoretical surface area, which should be sufficient to create a bond stronger than the wood itself, assuming proper glue application and clamping. For critical structural elements, exceeding this can provide additional security.