Choose a board length that matches your deck length or divides into it evenly over joists, so butt joints can be staggered and fall on a joist.
How the deck calculator works
DeckCalc assumes the deck boards run parallel to the house and the joists run out from a ledger on the house to a beam near the outer edge. Rows of boards = depth × 12 ÷ (board width + gap), rounded up. Boards = rows × deck length × (1 + waste) ÷ board length, rounded up. Joists = length × 12 ÷ spacing, rounded up, plus one. Screws = rows × joists × screws per joist.
Deck joist spans (IRC 2021 Table R507.6)
| Species | Size | 12″ o.c. | 16″ o.c. | 24″ o.c. |
|---|---|---|---|---|
| Southern pine | 2×6 | 9′-11″ | 9′-0″ | 7′-7″ |
| 2×8 | 13′-1″ | 11′-10″ | 9′-8″ | |
| 2×10 | 16′-2″ | 14′-0″ | 11′-5″ | |
| 2×12 | 18′-0″ | 16′-6″ | 13′-6″ | |
| Douglas fir-larch, hem-fir, SPF | 2×6 | 9′-6″ | 8′-4″ | 6′-10″ |
| 2×8 | 12′-6″ | 11′-1″ | 9′-1″ | |
| 2×10 | 15′-8″ | 13′-7″ | 11′-1″ | |
| 2×12 | 18′-0″ | 15′-9″ | 12′-10″ | |
| Redwood, western cedars, ponderosa / red pine | 2×6 | 8′-10″ | 8′-0″ | 6′-10″ |
| 2×8 | 11′-8″ | 10′-7″ | 8′-8″ | |
| 2×10 | 14′-11″ | 13′-0″ | 10′-7″ | |
| 2×12 | 17′-5″ | 15′-1″ | 12′-4″ |
No. 2 grade, wet service factor, 40 psf live load, 10 psf dead load, deflection L/360, no cantilever. Higher snow loads need shorter spans.
Maximum joist spacing for decking (IRC Table R507.7)
| Decking | Perpendicular | Diagonal |
|---|---|---|
| 1¼″ (5/4) wood | 16″ | 12″ |
| 2″ wood | 24″ | 16″ |
| Composite / plastic | Per the manufacturer's span rating (ASTM D7032); 16″ and 12″ are typical | |
Results are material estimates. Span values are from the 2021 International Residential Code; your jurisdiction may use another edition or local amendments, and beams, posts, footings, ledger attachment and guards need their own design. Check with your building department before you build.
Frequently asked questions
How wide is a deck board?
A nominal 6-inch wood or composite deck board is 5½ inches wide; check the actual width of your product.
What gap should I leave between deck boards?
Usually 1/8 to 3/16 inch for wood once dry; wet pressure-treated boards are often laid tight because they shrink. Composites list a gap in the instructions.
How much extra decking should I buy?
About 5% for a simple rectangle with full-length boards, 10% normally and 15% for diagonal or picture-frame layouts.
Is DeckCalc free?
Yes, with no sign-up or ads.
Are my measurements saved?
Yes, in this browser only.