Decks planning tool

Deck Joist Spacing Calculator

Turn a measured clear run and independently verified maximum spacing into equalized joist positions.

Joist layout arithmetic

Enter the verified layout conditions

This tool only equalizes positions. It does not recommend span, spacing, joist size, load, materials, waste, or code compliance.

Plan responsibly

Before You Build

This project may be subject to local permit, zoning, site, or structural requirements.

Confirm permit scopeConfirm permits, drawings, inspections, and required structural review.
Verify project loadsVerify live, snow, wind, and other loads before entering maximum spacing.
Verify joist designConfirm local species, grade, size, span, spacing, bearing, blocking, connection, and decking limits.

User-verified input: The calculator equalizes positions; it does not choose or approve maximum spacing.

No structural determination: Layout arithmetic is not structural design or a compliance finding.

View code references

Model-code reference

2012 OTHER, Section Table 2

Maximum joist spans: DCA 6 Table 2 relates species, grade, span, size, and spacing. It is not locally adopted law, and this calculator chooses none of those inputs.

Source: American Wood Council — DCA 6 Prescriptive Residential Wood Deck Construction Guide · Reviewed 2026-08-24

Local requirements may differ. A model-code reference does not confirm the code edition, amendments, zoning rules, or permit requirements adopted where the project is located.

Model-code references

2012 OTHER, Section Table 2: Maximum joist spans. Source: American Wood Council — DCA 6 Prescriptive Residential Wood Deck Construction Guide. Reviewed 2026-08-24.

Local requirements may differ. Verify the adopted code, amendments, zoning rules, and permit requirements with the local authority.

Methodology

How we calculate this

The calculator uses bounded scaled arithmetic to round the equal-bay count up, then includes or excludes each edge position exactly as selected.

bays = ceil(run width ÷ verified maximum spacing); spacing = run width ÷ bays

Layout arithmetic only

This tool does not recommend span, spacing, joist size, load, materials, waste, or code compliance.

Verify before entry

Measure the clear run and obtain maximum spacing from the complete independently verified design.

Choose edge conditions

State explicitly whether the layout includes a joist on the start edge, end edge, both, or neither.

Worked examples