How the deck calculator works
The calculator lays out a simple, common deck: a ledger board on the house, joists running out from the house at your spacing, a rim joist and a doubled or tripled beam at the outside edge, and posts on concrete footings under the beam. It then counts every piece in the stock lengths stores sell.
Board rows = depth ÷ (board width + gap), rounded up
Boards = rows × pieces per row, + waste %
Joists = width ÷ spacing + 1 · joist hangers = joists
Posts = width ÷ allowed beam span + 1
Deck screws = 2 × every board-to-joist crossing
Footing concrete = posts × π × (diameter ÷ 2)² × depth, + 10%
Example
A 16 × 12 ft deck with 5.5 in boards and 1/8 in gaps has 144 ÷ 5.625 = 25.6, so 26 rows of 16 ft boards. Joists at 16 in on center: 192 ÷ 16 + 1 = 13 joists.
Joist spans (Southern Pine No. 2)
Maximum joist span from AWC DCA 6 Table 2 for a 40 psf live load. The calculator's Auto setting uses these to pick the joist size.
| Joist | 12 in o.c. | 16 in o.c. | 24 in o.c. |
|---|---|---|---|
| 2x6 | 9'-11" | 9'-0" | 7'-7" |
| 2x8 | 13'-1" | 11'-10" | 9'-8" |
| 2x10 | 16'-2" | 14'-0" | 11'-5" |
| 2x12 | 18'-0" | 16'-6" | 13'-6" |
Beam spans between posts
Maximum distance between posts for a Southern Pine beam, by joist span, from AWC DCA 6 Table 3A. The calculator adds posts until no beam span is longer than this.
| Beam | 6 ft joists | 8 ft joists | 10 ft joists | 12 ft joists | 14 ft joists | 16 ft joists | 18 ft joists |
|---|---|---|---|---|---|---|---|
| 2-2x10 | 10'-1" | 8'-9" | 7'-9" | 7'-1" | 6'-6" | 6'-1" | 5'-9" |
| 3-2x10 | 12'-9" | 11'-0" | 9'-9" | 8'-9" | 8'-3" | 7'-8" | 7'-3" |
What's not included
Stairs, railings, flashing, ledger bolts, post bases, beam hardware, hanger nails and cantilevers are not counted, and decks deeper than 16 ft need a second beam. Those details — and ledger attachment in particular — are where local codes differ most, so take your plan to the building department before you buy. For what a finished deck costs installed, see deck cost per square foot and DIY vs hiring.
Frequently asked questions
How many deck boards do I need for a 12×16 deck?
For a deck 16 ft along the house and 12 ft deep, with 5.5 in boards and a 1/8 in gap running parallel to the house, you need 26 rows of 16 ft boards. With Trex's suggested 5% extra that is 28 boards. Boards laid diagonally need more — the calculator works it out from the board length and angle.
How far apart should deck joists be?
16 inches on center is standard for most decking laid square to the joists. Trex requires joists no more than 16 in on center for standard installs and 12 in on center when boards run diagonally. Wider 24 in spacing is used with thicker 2× wood decking. Always follow the spacing printed in your decking manufacturer's instructions.
What size joists do I need for my deck?
It depends on the joist span and spacing. Under the American Wood Council's DCA 6 guide, Southern Pine No. 2 joists at 16 in on center can span up to 9'-0" as 2×6, 11'-10" as 2×8 and 14'-0" as 2×10. The calculator's Auto setting picks the smallest joist that fits your deck depth. Your local building department has the final say.
How many screws do I need for decking?
Plan on two screws where each board crosses each joist, as both Trex and the AWC DCA 6 guide call for. A 16 × 12 ft deck with joists at 16 in on center has 13 joists and 26 rows of boards, which is about 680 screws — roughly 10 lb of #10 × 3 in deck screws. For composite boards, FastenMaster rates a 350-screw pack at about 100 sq ft.
How much concrete do I need for deck footings?
Multiply the area of the hole by its depth for each post: a 16.5 in diameter hole 24 in deep holds about 3 cubic feet, or 5–6 bags of 80 lb mix with a little extra. DCA 6 requires footings to bear at least 12 in below undisturbed ground or below the frost line, whichever is deeper, and footing diameter depends on the load — ask your building department for the size and depth required where you live.
Sources
Prices are national shelf prices and cost-guide ranges read on September 17, 2026. The calculator uses the typical price (the median of the shelf prices we saw) and shows the low–high range; type your own local price to update the total.
All sources for this page (29)
Coverage, yield and building specs
- Trex Decking Installation Guide (2023) accessed 2026-09-16
- American Wood Council — DCA 6: Prescriptive Residential Wood Deck Construction Guide (2015) accessed 2026-09-16
- Trex — How many deck boards do I need? accessed 2026-09-16
- DecksDirect — FastenMaster TrapEase 3 composite deck screws accessed 2026-09-16
- Trex Academy — How to install deck joists accessed 2026-09-16
- QUIKRETE Concrete Mix No. 1101 product data sheet accessed 2026-09-17
- Sakrete High-Strength Concrete Mix product page accessed 2026-09-16
- QUIKRETE — Setting posts (project sheet) accessed 2026-09-16
- Sakrete — Setting posts & poles accessed 2026-09-16
Prices
- The Home Depot — Pressure-treated deck boards accessed 2026-09-16
- The Home Depot — Pressure-treated lumber, 12 ft accessed 2026-09-16
- The Home Depot — 5/4 in. x 6 in. wood decking accessed 2026-09-16
- The Home Depot — Trex composite decking boards accessed 2026-09-16
- The Home Depot — Fiberon composite decking boards accessed 2026-09-16
- The Home Depot — Composite decking boards accessed 2026-09-16
- The Home Depot — Pressure-treated lumber accessed 2026-09-16
- The Home Depot — Pressure-treated 2 in. x 8 in. accessed 2026-09-16
- The Home Depot — Pressure-treated 2 in. x 10 in. accessed 2026-09-16
- The Home Depot — Pressure-treated 6 in. posts accessed 2026-09-16
- The Home Depot — Simpson Strong-Tie 2x8 joist hangers accessed 2026-09-16
- The Home Depot — Concrete Mix category listing accessed 2026-09-16
- Schillings — Sakrete Concrete Mix 80 lb accessed 2026-09-16
- HomeGuide — Concrete prices accessed 2026-09-17 · page dated 2026-01-06
- Cox Hardware — Sakrete Hi-Strength Concrete Mix 80 lb accessed 2026-09-16
- Ace Hardware — QUIKRETE Concrete Mix 80 lb accessed 2026-09-16
- Denver True Value — QUIKRETE Concrete Mix 80 lb accessed 2026-09-16
- The Home Depot — #10 x 3 in. deck screws accessed 2026-09-16
- The Home Depot — Wood deck screws accessed 2026-09-16
- The Home Depot — Trex composite deck screws 350-count accessed 2026-09-16
Deck framing is structural: check local building codes and get a permit where required before you build. Estimates for planning purposes only, not professional construction advice. Check local building codes and your product's instructions before you build. Prices are national averages collected from public retail listings and cost guides and will differ in your area. BuildHowMuch is independent and not affiliated with, endorsed by or paid by any retailer or manufacturer mentioned on this page. Nothing you enter is stored or sent anywhere.