A few people have asked me about a mortise and tenon calculator.
I'd always thought it was a bit too basic for a dedicated calculator, but have had a go at one anyway.

It might help in visualizing the layout of the joint, with scaled (and full scale) animated diagrams.
You can select between a single centred tenon, 2 tenons horizontally and 2 tenons vertically, and set tenon thickness, shoulder width etc, with sliders to animate the diagrams and see the results with possible tenon and mortise spacing.

Please let me know if it needs editing, additions or fixing in any way.

It lives here: Mortise and Tenon Joint Calculator - Metric