A span table is a conditional lookup, not a universal statement that a board can span a certain distance. The result is valid only when the project matches the table’s code edition, member size, spacing, species, grade, loading, and stated limitations.
Use the deck joist size calculator
Measure clear span correctly
Clear span is the horizontal distance between the inside faces of the joist supports. Do not include joist overhang beyond a beam, beam width, decorative projection, or the unsupported distance to an assumed centerline.
Support faces: Measure from the bearing face at one support to the bearing face at the next support.
Match spacing, species, and grade
Joist spacing is measured center to center. Species grouping and grade come from the lumber stamp or approved documentation; they cannot be inferred reliably from appearance. Preservative treatment and incising may introduce additional design considerations.
- Actual nominal member size
- Center-to-center spacing
- Species or species group
- Lumber grade
- Treatment and wet-service conditions
Confirm the design-load assumption
A prescriptive row assumes particular live, dead, and sometimes snow-load conditions. A span that appears in one row cannot be carried into a different load case. Hot tubs, roofs, concentrated loads, unusual guards, and heavy finishes can fall outside a simple deck table.
Keep cantilevers separate
Joist backspan between supports and joist cantilever beyond a beam are different checks. The maximum allowed overhang may depend on the backspan, member properties, loads, and the adopted provisions. Do not treat unused table span as permission to add the same amount of cantilever.
Read the result as a limit
The applicable table value is the maximum prescriptive clear span for the exact selected row. A joist option is usable only when its listed limit equals or exceeds the required clear span. The smallest passing option is not automatically the best choice for stiffness, availability, connections, or the complete structure.
Verify the complete deck
Joist span is only one part of deck design. Beams, posts, footings, ledgers, lateral connections, fasteners, blocking, bearing, guards, stairs, flashing, corrosion exposure, and local permits require separate review.
Local adoption: Selecting an IRC edition does not prove that your jurisdiction uses that edition without amendments.
Sources and review
Last reviewed July 26, 2026. The associated calculator includes tested fixtures, explicit assumptions, and input validation.
Field conditions, product instructions, approved plans, and applicable local requirements control the actual project. Review the full measurement disclaimer.