Waltco Development · Updated October 2026
Box beam sizes can mean the outside face or the inside opening. Mixing them up changes the size and the price. How to measure, and what to send us.
Outside or inside: say which
A 3-sided box beam has a face (the bottom) and two sides, open at the top where it meets the ceiling. Its outside size is what you see: the face width and the side height. Its inside size is the opening that fits over the blocking or an existing beam.
The two differ by the board thickness. The inside width is the outside width minus two boards; the inside height is the outside height minus one. Always write down which one your numbers are.
Measure the length wall to wall
Measure the full length from wall to wall, or from the wall to a post. Write it in feet and inches. Any beam longer than 12 feet needs 16-foot boards; if you want one continuous board per face, tell us, because long stock is not always available.
What to send for an estimate
With these details we can give a ballpark before the site measure:
- Quantity and length of each beam.
- Face width and side height, and whether they are inside or outside sizes.
- Species and surface: oak, white oak, Douglas fir; planed, rough sawn or hand hewn.
- Sides: 3-sided against a flat ceiling, 2-sided in a corner, or 4-sided hanging free.
- Finish: stain, oil or whitewash, and the colour if you have one.
- Ceiling: flat or vaulted, and its height.
- Two or three photos of the room.
No tape measure? Send photos
Photos of the ceiling from each end of the room are enough for a first number. We use the doors, windows and floor tiles in the photos as a guide, then confirm every size at the site measure.
Questions
Frequently asked questions
- Which size should I give, inside or outside?
- Either works, as long as you say which. Most designers give the outside face size.
- How wide is a typical box beam?
- It depends on the room. Beams of about 5 to 8 inches wide are common in homes with standard ceilings; larger rooms take larger beams.
- Do I need to measure the ceiling height?
- It helps. High or vaulted ceilings need scaffolding, which changes the install price.
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