Layout marks

Size four rails for a mitered frame

Convert the intended inside opening and rail face width into short-point and long-point dimensions for four 45-degree corners.

A frame rail has two useful lengths: the short edge facing the opening and the long outside edge. Ordering or marking the wrong one changes the opening by twice the rail width. This worksheet treats the frame as flat, equal-width rectangular stock with four exact 45-degree corners. It supplies marking dimensions, not tool-angle or workholding instructions.

How to use the result

1. Decide the clear opening required. If you are designing a rebate for glass, a backing or an object, work out that section separately before treating the opening as an object size.
2. Measure the rail face across the flat front, perpendicular to its length. All four rails must use that same width for this formula.
3. Label two horizontal and two vertical rails. Mark both the short and long edges, keeping the intended visible face consistent.

Worked example

A 200 by 300 mm opening with 30 mm wide rails needs two rails with 260 mm long points and two with 360 mm long points. Their short points remain 200 and 300 mm. The sum of long-point lengths is 1,240 mm, before blade kerfs, end trimming or defects.

Before making

• Bring the four pieces together without glue and confirm the opening and outside dimensions.
• Check both diagonals and the corners; a length calculation cannot correct an inaccurate angle.
• Do not use these lengths for unequal-width rails, compound miters or a structural frame.

Continue your plan

• diagonal check
• strip yield