Deck Joist Calculator
Calculate deck joists, total joist linear feet, and rim joist lumber from deck size and joist spacing. Sizes framing for composite or wood decking.
How this calculator works
Joists are spaced evenly along the deck and each spans the deck’s width. The calculator counts the joists, multiplies by the span for total lumber, and adds the rim joists that close the ends.
Formula: Joists = ⌊(deck length in ÷ spacing)⌋ + 1. Joist linear feet = joists × deck width. Rim joist linear feet = 2 × deck length.
Worked example
A 16 ft long × 12 ft wide deck at 16-inch spacing:
- Number of joists: ⌊(192 ÷ 16)⌋ + 1 = 13 joists
- Joist lumber: 13 × 12 = 156 linear feet
- Rim joists: 2 × 16 = 32 linear feet
Spacing reference
| Deck length | Joists at 12 in OC | Joists at 16 in OC |
|---|---|---|
| 12 ft | 13 | 10 |
| 16 ft | 17 | 13 |
| 20 ft | 21 | 16 |
How to measure
Measure the deck’s length (the way the decking boards will run) and its width (the joist span). Confirm your joist size against a span table for that width, then use the Decking Board Calculator to size the surface boards.
Deck framing is structural. Verify joist size, span, and beam layout against local code before building.
Frequently asked questions
What joist spacing do I need?
16 inches on center suits most wood and perpendicular composite decking. Composite installed diagonally usually requires 12 inches on center, and some wood decks allow 24 inches for lighter boards. Match the spacing to your decking manufacturer's requirement.
Which direction do joists run?
Joists run across the deck's shorter span between the ledger and beam, and the decking runs perpendicular across the joists. In this calculator, enter the deck length as the direction the decking boards run and the width as the joist span.
What is a rim joist?
Rim (or band) joists close off the ends of the joist run, wrapping the deck perimeter along the outer edges. The calculator adds two rim lengths equal to the deck length; a full perimeter frame also caps the two ends.
How far can a deck joist span?
Span depends on lumber size, species, and spacing — for example, a 2×8 southern pine joist at 16 inches on center spans roughly 12 feet. Always check a span table for your lumber and load, and add a beam and posts where the span is exceeded.
Does this include the beam and posts?
No — this sizes the joists and rim. Beam size and post count depend on your span and footing layout, which are structural decisions to confirm against a span table or with a pro.
Estimates only. Verify quantities with your supplier before purchasing.