Calculate layout dimensions for a centered rectangular tenon and a blind mortise. Enter the tenon-bearing stock’s thickness and width, the receiving member’s available depth along the insertion direction, three dimension ratios and an end clearance.
These are geometric layout dimensions, not structural design recommendations. Default ratios are worked-example choices, not recommended joint proportions. The calculator does not assess loads, timber species, grain direction, adhesive, tooling or joint strength.
All length inputs and outputs use metres. Divide millimetres by 1,000 before entering them: 24 mm is 0.024 m. Multiply a result in metres by 1,000 to read it in millimetres. Ratios are dimensionless fractions: enter 0.3 for 30%.
Stock thickness accepts 0.005–0.300 m; stock width accepts 0.005–0.500 m; receiving depth accepts 0.010–0.500 m. Thickness and width ratios accept 0.100–0.900; the length ratio accepts 0.100–0.800. End clearance accepts 0–0.001 m. Length input steps are 0.0001 m; ratio steps are 0.001.
These bounds define this calculator’s supported layout range, not safe design limits. Together they keep the calculated shoulders positive and the mortise shallower than the receiving member. They do not establish adequate remaining material or joint strength.
Measure receiving depth from the mortise entry face toward the opposite face, along the tenon’s insertion axis. End clearance is added once, beyond the tenon tip. It does not change tenon thickness, width or shoulders. Mortise width, side-fit allowances, receiving-member side walls and cutter geometry are not calculated. Six-decimal output is numerical display precision, not a machining tolerance. An unavailable result must not be interpreted as zero.
The formulas are derived here from the stated geometric definitions; they are not empirical woodworking rules or externally sourced strength formulas. A chosen dimension ratio gives t/T = r_t, w/W = r_w and L/D = r_L. Multiplying each ratio by its corresponding stock dimension gives the tenon dimensions.
Centering the tenon leaves equal shoulders on opposite sides, so T = s_T + t + s_T and W = s_W + w + s_W. Rearranging gives s_T = (T − t)/2 and s_W = (W − w)/2. Defining end clearance as the gap beyond the tenon tip gives M = L + c. Remaining depth follows from D = M + B.
No external source is claimed for the example ratios or clearances.
| Quantity | Example 1: defaults | Example 2 |
|---|---|---|
| Stock thickness T | 24 mm; enter 0.024 m | 30 mm; enter 0.030 m |
| Stock width W | 60 mm; enter 0.060 m | 80 mm; enter 0.080 m |
| Receiving depth D | 50 mm; enter 0.050 m | 60 mm; enter 0.060 m |
| Thickness ratio r_t | 0.300 | 0.400 |
| Width ratio r_w | 0.700 | 0.750 |
| Length ratio r_L | 0.600 | 0.500 |
| End clearance c | 1 mm; enter 0.001 m | 0.5 mm; enter 0.0005 m |
| Tenon thickness t | 24 × 0.3 = 7.2 mm | 30 × 0.4 = 12 mm |
| Each thickness-direction shoulder s_T | (24 − 7.2) / 2 = 8.4 mm | (30 − 12) / 2 = 9 mm |
| Tenon width w | 60 × 0.7 = 42 mm | 80 × 0.75 = 60 mm |
| Each width-direction shoulder s_W | (60 − 42) / 2 = 9 mm | (80 − 60) / 2 = 10 mm |
| Tenon length L | 50 × 0.6 = 30 mm | 60 × 0.5 = 30 mm |
| Mortise depth M | 30 + 1 = 31 mm | 30 + 0.5 = 30.5 mm |
| Remaining depth B | 50 − 31 = 19 mm | 60 − 30.5 = 29.5 mm |
Are the shoulder results per side or combined?
Each result is for one shoulder. The two thickness-direction shoulders are equal, and the two width-direction shoulders are equal. This assumes a centered tenon.
Does end clearance specify the fit between the tenon cheeks and mortise walls?
No. End clearance only adds depth beyond the tenon tip. Side fit requires separate decisions and measurements; this calculator does not prescribe it.
Can I use this for offset, haunched or through tenons?
The equations describe a centered rectangular tenon in a blind mortise. Offset shoulders, haunches, through tenons and multiple tenons need a different layout model.
Does a positive remaining depth mean the joint is strong enough?
No. It only means the calculated mortise bottom lies before the opposite face along the modeled insertion direction. Structural adequacy and remaining side-wall dimensions are outside this calculator.
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