Deck Joist Calculator
Span Exceeds Maximum
Your joist span of 16 ft exceeds the maximum allowed span of 13.6 ft for 2×8 at 16" spacing. Choose a larger joist size or reduce the span.
* Calculations based on IRC 2021 and AWC span tables for Southern Pine #2. Add 10% for waste and blocking. Consult local building codes.
Disclaimer: Estimates are for budgeting purposes only. Actual costs vary by location, contractor, material availability, and project complexity. Always get professional quotes for exact pricing.
Deck Joist Span Table (IRC 2021)
| Joist Size | 12" Spacing | 16" Spacing | 24" Spacing |
|---|---|---|---|
| 2×6 | 11'7" | 10'6" | 8'7" |
| 2×8 | 15'0" | 13'7" | 11'1" |
| 2×10 | 18'0" | 16'4" | 13'4" |
| 2×12 | 21'0" | 19'0" | 15'7" |
* Southern Pine #2, 40 psf live load, 10 psf dead load per IRC 2021 Table R507.5
Joists Required by Deck Width
| Deck Width | 12" Spacing | 16" Spacing | 24" Spacing |
|---|---|---|---|
| 8 ft | 9 joists | 7 joists | 5 joists |
| 10 ft | 11 joists | 8 joists | 6 joists |
| 12 ft | 13 joists | 10 joists | 7 joists |
| 16 ft | 17 joists | 13 joists | 9 joists |
| 20 ft | 21 joists | 16 joists | 11 joists |
* Formula: (width in inches ÷ spacing) + 1. Does not include rim joists or blocking.
How We Calculate
Our deck joist calculator uses IRC 2021 span tables to determine joist size, spacing, and quantity. Calculations are based on Southern Pine #2 lumber, 40 psf live load, and 10 psf dead load (standard residential deck).
How the math works: Joist count = (deck width ÷ spacing) + 1. Linear feet = joist count × joist length. Board feet = (thickness × width × linear feet) ÷ 12. The +1 accounts for the rim joist on the far edge.
IRC 2021 Span Tables
Maximum spans are from IRC Table R507.5 for Southern Pine #2, 40 psf live load, 10 psf dead load. These spans assume joists run continuously over a beam. Species and grades vary — Douglas Fir has slightly shorter spans, Hem-Fir has similar spans.
Joist Spacing
12" on center: Stronger, less bounce, required for diagonal decking. Allows maximum span. Costs more (more joists needed).
16" on center: Standard for most residential decks. Works with 5/4" and 2" decking running perpendicular. Best balance of cost and performance.
24" on center: Only for 2" thick deck boards or structural decking. Significantly reduces max span. Not recommended for composite decking.
Blocking Requirements
IRC requires solid blocking or bridging at mid-span for joists longer than 8 ft. Blocking prevents twisting and reduces bounce. Use full-depth blocking (2×8 blocking for 2×8 joists) every 8 ft along joist span. X-bracing is acceptable but solid blocking is stronger.
Cantilevers
Joists can cantilever up to 1/4 of the backspan per IRC. For a 12-ft span from ledger to beam, you can cantilever 3 ft past the beam. Never exceed this ratio — longer cantilevers cause excessive bounce and can pull fasteners from ledger boards.
Data Sources
Span data from IRC 2021 Table R507.5 and American Wood Council (AWC) Span Tables for Joists and Rafters. Always verify with local building codes and engineers for commercial or elevated decks.
Last updated: 2026-02-09
Frequently Asked Questions
What size joist should I use for my deck? +
Deck joist size depends on span and spacing. For 16" spacing: 2×6 joists span up to 10.6 ft, 2×8 up to 13.6 ft, 2×10 up to 16.3 ft, and 2×12 up to 19.0 ft. Most residential decks use 2×8 or 2×10 joists at 16" on center. Check IRC 2021 Table R507.5 for your species and grade.
How far apart should deck joists be? +
Standard joist spacing is 16 inches on center for most deck boards. Use 12" spacing for diagonal decking or thin deck boards (5/4" thickness). Use 24" spacing only for thick boards (2" or thicker) and if joists meet span requirements. Closer spacing provides a sturdier deck and reduces bounce.
How do I calculate number of joists needed? +
Formula: joists needed = (deck width in inches ÷ spacing on center) + 1. For a 12-ft (144") wide deck with 16" spacing: 144 ÷ 16 = 9 spaces, plus 1 = 10 joists. The +1 accounts for the rim joist on the far edge.
What is the maximum span for 2×8 deck joists? +
2×8 deck joists (Southern Pine #2) span up to 13.6 ft at 16" on center per IRC 2021. At 12" spacing, they span 15.0 ft. At 24" spacing, only 11.1 ft. Maximum span depends on wood species, grade, joist spacing, and live load (40 psf residential). Always verify with local codes.
Can I use 2×6 joists for a deck? +
Yes, but 2×6 joists are limited to shorter spans: 10.6 ft at 16" spacing, 11.7 ft at 12" spacing per IRC 2021. 2×6 joists work for small decks, walkways, or when you have a beam mid-span. Most 12×16 ft decks need 2×8 or larger joists to avoid mid-span beams.
What is board feet and why does it matter? +
Board feet (bf) is lumber volume: (thickness × width × length) ÷ 12. A 2×8×12 board = (2 × 8 × 12) ÷ 12 = 16 bf. Lumber is priced per board foot ($0.50–$3/bf depending on species). Knowing total board feet helps get accurate quotes from lumber yards.
Do I need blocking between deck joists? +
Yes, IRC requires blocking or bridging at mid-span for joists over 8 ft long. Blocking prevents joists from twisting, reduces bounce, and transfers loads. Use solid blocking (same dimension as joists) or diagonal X-bracing. Install blocking at 8-ft intervals for joists over 16 ft long.
Can I cantilever deck joists past the beam? +
Yes, joists can cantilever up to 1/4 of the backspan per IRC. If joists span 12 ft from ledger to beam, you can cantilever 3 ft past the beam. Cantilevers create overhang for stairs or aesthetics. Do not exceed 1/4 backspan rule — longer cantilevers cause bounce and structural failure.
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