Deck Joist Calculator
Joists are the backbone of the deck. Enter the run to cover, the spacing on center and the joist size to get the exact joist count, framing linear footage and a cost estimate.
Result
- Joists needed10joists
- Joist linear feet120lin ft
- Rim/ledger framing (2x8)56lin ft
- 8 ft rim boards to buy7boards
- Estimated framing lumber cost162USD
How it works
joists = (width × 12 ÷ spacing) + 1
- Run to cover = 12 ft × 12 = 144 in
- Joists = 144 ÷ 16 = 9, round up + 1 edge joist = 10
- Linear feet = 10 × 12 ft = 120 lin ft
- Estimated cost at $1.35/lin ft = $162
- Rim + ledger = 2 × (16 + 12) = 56 lin ft ≈ 7 boards at 8 ft
How the deck joist calculator works
Joists span across the deck's support direction and are spaced on center — measured from the center of one joist to the center of the next.
16 inches on center is the residential standard for most decking. 12" gives a stiffer floor and is required for diagonal layouts; 24" saves lumber but only works with 2"-thick boards or structural decking.
The final +1 in the formula is the closing joist on the far edge. Without it you'd be one joist short every time.
Formula
joists = (run × 12 ÷ spacing) + 1
Worked example: joists for a 12 ft span at 16" o.c.
- Run to cover = 12 ft × 12 = 144 inches
- Joists = ceil(144 ÷ 16) + 1 = 9 + 1 = 10 joists
- Each joist is 12 ft long → 10 × 12 = 120 linear feet
- Estimated lumber = 120 × $1.35 ≈ $162
- Rim + ledger = 2 × (16 + 12) = 56 linear feet ≈ 7 boards at 8 ft
Joists per run at common spacings
| Run | 12" o.c. | 16" o.c. | 24" o.c. |
|---|---|---|---|
| 8 ft | 9 | 7 | 5 |
| 10 ft | 11 | 9 | 7 |
| 12 ft | 13 | 10 | 7 |
| 14 ft | 15 | 12 | 9 |
| 16 ft | 17 | 13 | 9 |
Joists = (run × 12 ÷ spacing) + 1. The +1 is the far edge joist.
Frequently asked questions
How many joists do I need for a 12×16 deck?
At 16" on center, a 12 ft run needs 10 joists (144 ÷ 16 = 9, plus 1). At 12" spacing it's 13 joists; at 24" only 7. Multiply each joist by its length for total linear feet.
How far apart should deck joists be?
16 inches on center for standard decking. Use 12" for diagonal board layouts or 5/4 boards, and 24" only with 2"-thick boards over short spans. Composite manufacturers usually require a maximum of 16".
What size joist should I use?
For a typical 12 ft span at 16" on center, a 2×10 Southern Pine #2 works (max 16'4"). Shorter spans drop to 2×8. Always confirm against your local code — species, grade and load change the answer.
What is the difference between a joist, a beam and a ledger?
The ledger is the board bolted to the house. Beams carry the joists, sitting on posts or footings. Joists run perpendicular to the beams and directly support the deck boards. All three are included in the rim and framing linear feet this calculator reports.