Deck Joist Calculator

What size deck joist do I need?

ft
ft
Species
Joist spacing

Minimum table size is

2 × 10joists

From IRC Table R507.6 at 40 psf live and 10 psf dead load, No. 2 grade, wet service. Local code, snow load and decking can change this. Confirm with your building department.

12 ft spanTable max 14 ft 0 in2 × 10 southern pine at 16 in on center13 joists
Table maximum span14 ft 0 in
Your span12 ft
Joists at this spacing13
Joist stock length12 ft

Choose species, spacing and span, and see the smallest size allowed by the IRC deck joist table, plus how many joists to buy.

How it works

Looks up IRC Table R507.6 for your species and spacing at 40 psf live and 10 psf dead load. The smallest size whose listed span covers yours is the minimum.

Before you order

  • Span is measured from the support to the support, so measure after you know where your beam sits.
  • Decking choice can limit joist spacing, so check the product before you space joists at 24 inches.
  • Snow regions may use a heavier table, so call your building department before you buy.

Common questions

How far can a 2 × 10 deck joist span?

At 16 inches on center, about 14 feet in southern pine and 13 feet 7 inches in Douglas fir-larch under the IRC 40 psf table.

What if my span is longer than the table?

Add a beam to shorten the span or get an engineered design.