What load does this assume?

It uses the standard residential deck load of 40 psf live plus 10 psf dead, which is the basis of the IRC and DCA6 deck joist tables. Decks designed for heavier use such as hot tubs need a separate engineered design.

Is span measured center to center or clear?

Span tables use the clear horizontal distance between the faces of supporting beams or the bearing points, not the overall joist length. Measure the unsupported span, which is shorter than the board you cut.

Why does species matter so much?

Different woods have different bending strength and stiffness. Southern Pine and Douglas Fir-Larch span farther than Hem-Fir or SPF at the same size, so the same 2x8 can be code-legal in one species and overspanned in another.

Does this include cantilevers or overhangs?

No. The values are back-span maximums only. Cantilevered ends are limited to one-quarter of the allowed back span under DCA6, and that is checked separately. Stay within the main span first.

Is this an official code approval?

No. The table values follow published IRC and DCA6 figures for No. 2 grade lumber, but your local jurisdiction may amend them. Always confirm with your building department and use it as a planning aid only.

Deck Joist Span Calculator

Choosing a joist size is really a span question: how far can the board reach between supports before it sags or fails? This calculator reads published IRC and DCA6 maximum-span values for common deck lumber and tells you immediately whether your span is within code.

How it works

The tool stores the standard maximum-span table for No. 2 grade lumber under a 40 psf live plus 10 psf dead deck load. For your chosen size, species, and spacing it looks up the allowable span and compares:

allowable = table[size][species][spacing]
verdict   = your span <= allowable  ?  PASS : FAIL

Spans shorten as spacing widens, because each joist at 24 inch on-center carries more tributary load than one at 12 inch on-center. Southern Pine and Douglas Fir-Larch carry farthest; Hem-Fir and SPF are the most conservative.

Example and tips

A 2x8 Southern Pine joist at 16 inch on-center is allowed roughly 13 feet 1 inch. At 24 inch on-center the same board drops to about 10 feet 10 inches. If your span fails, the fastest fixes are tightening the spacing, dropping a beam to shorten the span, or going up a joist size. Always measure the clear span, not the cut length, and confirm the final design with your local building department.