Deck Joist Calculator — Span, Size & Joist Count
Enter joist span, species, spacing, and deck width to get the smallest joist size that spans, the allowed cantilever, and how many joists to buy.
No Joist Spans This
No 2x6 through 2x12 joist in IRC 2021 Table R507.6 spans this far at this spacing. Shorten the joist span with an intermediate beam, or tighten the joist spacing, and calculate again.
Joist Size
2x10
Smallest size that spans 12 ft · table allows 14 ft 0 in at 16" o.c. (Southern pine)
Max Cantilever
3 ft 0 in
For a 2x10 with a 12 ft back span (Table R507.6)
Joists Needed
13
16 ft ÷ 16" o.c., rounded up, plus one end joist
Joist Length to Buy
12 ft
Cut length 12 ft 0 in (span + cantilever), rounded up to stock
How deck joist spans are sized
The International Residential Code sizes deck joists prescriptively: Section R507.6 of the 2021 IRC says maximum joist spans shall be in accordance with Table R507.6. That table lists, for 3 species groups and 4 joist sizes (2x6 through 2x12), the allowable span at 12, 16, and 24 inches on center, and the maximum cantilever past the beam by joist back span.
The table's conditions are printed in its notes: 40 psf live load and 10 psf dead load (the residential deck design load of Table R301.5), No. 2 grade lumber with the wet service factor included, deflection limited to L/360 at the main span and L/180 at the cantilever with a 220-pound point load at its end. The Douglas fir-larch, hem-fir, and spruce-pine-fir values include the incising factor for pressure treatment; the redwood, cedar, and pine group does not. The table also has rows for 50, 60, and 70 psf ground snow loads, which this calculator does not reproduce.
This calculator looks up that table and returns the smallest joist size whose allowable span meets yours, so the answer is the code minimum, not a recommendation to go bigger. Confirm against your locally adopted code, and involve a designer for anything the table doesn't cover: other loads, other grades, or engineered lumber.
Deck joist span table (IRC 2021 Table R507.6, 40 psf live load)
Allowable joist span by spacing, and maximum cantilever by joist back span. NP = not permitted. Interpolation between back-span columns is allowed by the table; extrapolation is not.
| Species | Size | Allowable span, joist spacing (in o.c.) | Max cantilever, joist back span (ft) | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 12 | 16 | 24 | 4 | 6 | 8 | 10 | 12 | 14 | 16 | 18 | ||
| Southern pine | 2x6 | 9'-11" | 9'-0" | 7'-7" | 1'-0" | 1'-6" | 1'-5" | NP | NP | NP | NP | NP |
| 2x8 | 13'-1" | 11'-10" | 9'-8" | 1'-0" | 1'-6" | 2'-0" | 2'-6" | 2'-3" | NP | NP | NP | |
| 2x10 | 16'-2" | 14'-0" | 11'-5" | 1'-0" | 1'-6" | 2'-0" | 2'-6" | 3'-0" | 3'-4" | 3'-4" | NP | |
| 2x12 | 18'-0" | 16'-6" | 13'-6" | 1'-0" | 1'-6" | 2'-0" | 2'-6" | 3'-0" | 3'-6" | 4'-0" | 4'-1" | |
| Douglas fir-larch, hem-fir, spruce-pine-fir | 2x6 | 9'-6" | 8'-4" | 6'-10" | 1'-0" | 1'-6" | 1'-4" | NP | NP | NP | NP | NP |
| 2x8 | 12'-6" | 11'-1" | 9'-1" | 1'-0" | 1'-6" | 2'-0" | 2'-3" | 2'-0" | NP | NP | NP | |
| 2x10 | 15'-8" | 13'-7" | 11'-1" | 1'-0" | 1'-6" | 2'-0" | 2'-6" | 3'-0" | 3'-3" | NP | NP | |
| 2x12 | 18'-0" | 15'-9" | 12'-10" | 1'-0" | 1'-6" | 2'-0" | 2'-6" | 3'-0" | 3'-6" | 3'-11" | 3'-11" | |
| Redwood, western cedars, ponderosa pine, red pine | 2x6 | 8'-10" | 8'-0" | 6'-10" | 1'-0" | 1'-4" | 1'-1" | NP | NP | NP | NP | NP |
| 2x8 | 11'-8" | 10'-7" | 8'-8" | 1'-0" | 1'-6" | 2'-0" | 1'-11" | NP | NP | NP | NP | |
| 2x10 | 14'-11" | 13'-0" | 10'-7" | 1'-0" | 1'-6" | 2'-0" | 2'-6" | 3'-0" | 2'-9" | NP | NP | |
| 2x12 | 17'-5" | 15'-1" | 12'-4" | 1'-0" | 1'-6" | 2'-0" | 2'-6" | 3'-0" | 3'-6" | 3'-8" | NP | |
Source: 2021 International Residential Code, Table R507.6 "Maximum Deck Joist Spans", 40 psf live load row. Values in feet-inches as printed.
Cantilevers
A cantilever is the part of the joist that continues past the beam. The 2021 table gives the maximum by joist back span (the span behind the beam) in 2 ft columns from 4 to 18 ft. Every value in the table is at most 1/4 of the back span; the earlier 2018 edition stated that quarter-span limit in the text of R507.6, and the 2021 edition folds it into the table's columns. The calculator interpolates between columns for in-between spans, rounds the result down to a whole inch, and treats any interval that ends in an NP cell as not permitted.
If the cantilever you enter is longer than the selected size allows, the result is flagged and the calculator names the smallest size that would carry it, or tells you to shorten the cantilever when no size in the table does. The joist count and length still use the cantilever you entered.
Joist count and length
Joists are spaced across the deck width along the ledger: count = width ÷ spacing, rounded up, plus one for the joist at the far end. The calculator uses the deck width exactly as entered; if you frame with a doubled rim joist or blocking layout, add those pieces separately.
Each joist's cut length is span plus cantilever. Dimensional lumber is sold in fixed lengths (8, 10, 12, 14, 16, 20 ft), so the length to buy is the next stock length up. Beyond 20 ft the calculator rounds up to the next even foot and flags it as a special-order length.
Frequently asked questions
- How far can a 2x8 deck joist span?
- It depends on species and spacing. At 16 inches on center, IRC 2021 Table R507.6 (40 psf live load, 10 psf dead load, No. 2 grade, wet service) allows a 2x8 to span 11 ft 10 in (Southern pine); 11 ft 1 in (Douglas fir-larch, hem-fir, spruce-pine-fir); 10 ft 7 in (Redwood, western cedars, ponderosa pine, red pine). The full table above covers 2x6 through 2x12 at 12, 16, and 24 inches on center.
- How many deck joists do I need?
- Divide the deck width along the ledger by the joist spacing, round up, and add one for the joist at the far end. A 16 ft wide deck framed at 16 inches on center needs 13 joists. Buy each one at the stock length that covers span plus cantilever — 12 ft boards for a 12 ft span with no cantilever.
- Can deck joists cantilever past the beam?
- Yes, within the limits of IRC 2021 Table R507.6, which lists the maximum cantilever by joist size and back span (the span behind the beam). No value in the table exceeds 1/4 of the back span, and the deeper the joist the more it may cantilever. A 2x10 Southern pine joist with a 12 ft back span may cantilever 3 ft 0 in.
- What size deck joist do I need?
- The calculator returns the smallest size in the table that meets your span. For a 12 ft span in Southern pine at 16 inches on center, that is a 2x10, which the table allows up to 14 ft 0 in. Going one size deeper than required is a common choice for a stiffer deck, but the table value is the code minimum.