Deck Framing Calculator: Joist & Beam Spans
Enter the deck size, how high it sits and your lumber. The planner picks joists, a beam, posts and footings from the building code's deck tables and draws the framing plan to scale.
- 2021 IRC deck tables (R507)
- To-scale framing plan
- Shopping list
2 Your framing plan
Updates as you type| Lumber | Southern pine, No. 2 or better, treated or naturally durable |
|---|---|
| Joist span (ledger to beam) | 10' 6" (2×8 allows up to 11' 10" at 16") |
| Joist overhang past the beam | 1' 6" (2×8 allows up to 2' 5") |
| Beam | Double 2×10, 16' long (allows 8' 3" between posts) |
| Posts | 3 × 4x4, about 2' 7" tall (table allows up to 11') |
| Load on each inside post | 54 sq ft of deck |
| Footings | 19" round or 17" square, 6" thick, on 1,500 psf soil |
| Ledger fasteners (staggered) | ½" lag screws every 15", or ½" bolts every 29" (for up to ½" sheathing) |
The deck is more than 30 inches above the ground, so it needs guards (railings) at least 36 inches high on the open sides (IRC R312.1).
Footings go at least 12 inches into undisturbed soil and below your local frost depth. Ledgers can't be fastened to brick or stone veneer. This plan follows the code's prescriptive tables; your building department has the final say, and most decks need a permit.
Shopping list
Quick answer
A 16 × 12 ft deck 4 ft high, in southern pine with joists at 16" on center and a 1½ ft overhang, takes 2×8 joists, a double 2×10 beam on 3 4×4 posts 8' 0" apart, and 19" round footings in clay soil, per the 2021 IRC deck tables.
Maximum deck joist spans at 16" on center
| Joist | Southern pine | Douglas fir, hem-fir, SPF | Redwood, cedar, ponderosa |
|---|---|---|---|
| 2×6 | 9' 0" | 8' 4" | 8' 0" |
| 2×8 | 11' 10" | 11' 1" | 10' 7" |
| 2×10 | 14' 0" | 13' 7" | 13' 0" |
| 2×12 | 16' 6" | 15' 9" | 15' 1" |
From 2021 IRC Table R507.6 for 40 psf live load, No. 2 grade, wet service. Closer spacing allows longer spans. Use the calculator above for your exact size.
How the planner sizes your deck
Most US decks are built to the International Residential Code's prescriptive deck tables in Section R507. They size each part for a 40 psf live load plus 10 psf for the deck itself. The planner works through them in the same order an inspector checks a plan:
- Joists (Table R507.6): the smallest 2× joist that spans from the ledger to the beam at your spacing, and that is allowed the overhang you want past the beam.
- Beam (Table R507.5): how far each beam size can span between posts. The table is read at an "effective" joist span, which goes up as the joist overhang grows.
- Posts (Table R507.4): the smallest post that's allowed at your post height for the deck area each post carries.
- Footings (Table R507.3.1): the pad size for that area on your soil.
- Ledger (Table R507.9.1.3(1)): how far apart the lag screws or bolts go along the house.
Worked example: a 16 × 12 ft deck, 4 ft high
- With joists running 1½ ft past the beam, the joists span 10' 6" from the ledger to the beam. Southern pine 2×8 joists at 16" are good for 11' 10".
- The effective joist span for the beam table is 9.45 ft, and a double 2×10 beam can span 8' 3" between posts, so 3 posts 8' 0" apart carry it.
- Each inside post carries about 54 sq ft of deck. At about 2' 7" tall, a 4×4 post is allowed, on a 19" round footing 6" thick in clay soil.
- The ledger gets ½" lag screws every 15", staggered.
Rules the tables assume
- No. 2 grade or better lumber, pressure-treated or naturally durable, with joists in hangers or on top of the beam.
- Joist ends and bearing points blocked or held by a rim joist so they can't roll over (R507.6.2).
- Footings at least 12 inches into undisturbed soil and below the frost line where you live (R507.3).
- A ledger of at least 2×8 fastened to the house's band joist with flashing, never to brick or stone veneer (R507.9.1).
- Guards at least 36 inches high where the deck is more than 30 inches above the ground (R312.1).
Local codes can change these numbers, especially in snow country, where the 2021 IRC has separate tables for 50, 60 and 70 psf snow loads. A building permit and inspection are required for most decks, and the plan an inspector approves is the one to build.
Frequently asked questions
How far can a 2×8 deck joist span?
At 16 inches on center, 11' 10" in southern pine and 11' 1" in Douglas fir, hem-fir or SPF, under 2021 IRC Table R507.6. At 12 inches on center it rises to 13' 1" in southern pine.
How far apart should deck posts be?
It depends on the beam and the joist span it carries. A bigger beam spans farther between posts. Enter your deck above; the planner picks the beam and the fewest posts the code tables allow.
How big should deck footings be?
Table R507.3.1 sizes them by the deck area each post carries and the soil. On clay soil a post carrying 60 sq ft needs about a 19 inch round footing; firmer soils allow smaller ones. They must also go below your frost line.
Can I use 4×4 posts for a deck?
The 2021 IRC allows 4×4 posts for short posts with small loads, as Table R507.4 shows. Taller decks and posts carrying more deck area need 4×6 or 6×6 posts.
Last reviewed 2026-10-23. See how we calculate for our sources and assumptions.