Order Once, Build Without a Second Lumberyard Run
Running out of decking boards mid-project is one of the most expensive mistakes in deck building. A second trip means a second delivery fee, and boards from a different batch can differ in color and moisture content. Guessing high is no better — quality decking is too expensive to leave stacked behind the garage.
This free deck board calculator removes the guesswork. Enter your deck’s length and width, pick your board width and the gap between boards, choose the board length you want to buy, and it tells you exactly how many boards to order — plus board feet, total linear feet, and a realistic deck screw estimate. The 10% waste allowance contractors rely on is built in.
How this was calculated
Screw estimate assumes 2 fasteners per board at every joist, with joists spaced 16 in on center. A 5 lb box of #8 × 2½ in deck screws holds roughly 350 screws — divide the screw total by 350 and round up to get boxes.
Frame note: this calculator sizes the decking only. Budget rim joists of about 2 × (length + width) feet and joists every 16 in along the board run — your lumberyard can size the beam and posts from the same dimensions.
How to Use This Calculator
- Measure the deck length. Measure in the direction the boards will run (usually the longer dimension), in feet.
- Measure the deck width. Measure across the boards — perpendicular to the run — in feet. This is the dimension that determines how many courses (rows) of boards you need.
- Pick your board width. Standard decking is a nominal 1×6, which actually measures 5½ inches wide. Narrower 1×4 (3½ in) and wide 1×8 (7¼ in) options are included.
- Choose the gap. The ⅛-inch gap is the most common; ¼ inch drains better but lets more debris through. Wet lumber shrinks as it dries, so install it tighter than your target gap.
- Select the board length to buy. Longer boards mean fewer end joints. A 16-foot deck built with 16-foot boards needs one board per course; with 8-foot boards it needs two per course plus staggered joints.
- Click Calculate Deck Boards. You get the boards to order (with 10% waste), board feet, linear feet, and screw count — plus a full step-by-step breakdown.
What Does a Deck Board Calculator Actually Figure Out?
The heart of the calculation is the board course: one row of boards running the length of the deck. To find how many courses fit, divide the deck’s width (in inches) by the effective width of one board — the board’s actual width plus the gap. A 12-foot-wide deck (144 inches) with 5½-inch boards and a ¼-inch gap fits 144 ÷ 5.75 = 25.04, which rounds up to 26 courses.
Then each course is filled with boards end to end: course count × (deck length ÷ board length), rounded up. Finally, a waste allowance — 10% is the trade standard — covers cuts, split boards, and the pieces you reject at the lumberyard. Two more numbers fall out naturally: board feet, the lumberyard’s unit for pricing (linear feet × board width ÷ 12, at 1-inch thickness), and fasteners, estimated at two screws per board at every joist with joists 16 inches on center.
One thing the calculator deliberately does not do: size your frame. Decking is the skin; the joists, beams, and posts underneath need their own structural plan. The results include a quick framing note so you can hand the same dimensions to your lumberyard for the rest.
The Formula (With a Worked Example)
Boards = courses × (deck length ÷ board length), rounded up
Order = boards × 1.10 (10% waste), rounded up
Board feet = total linear feet × board width in inches ÷ 12
Worked example — a 16 ft × 12 ft deck, 5½ in boards, ¼ in gap, 16 ft boards.
- Effective board width: 5.5 + 0.25 = 5.75 inches.
- Courses: 144 ÷ 5.75 = 25.04 → 26 courses.
- Boards per course: 16 ÷ 16 = 1 → 26 boards before waste.
- With 10% waste: 26 × 1.10 = 28.6 → 29 boards to order.
- Board feet: 29 × 16 = 464 linear ft; 464 × 5.5 ÷ 12 = 213 board feet.
- Screws: a 16 ft run has 13 joist lines (16 in on center); 13 × 2 screws × 29 boards = 754 screws, about three 5 lb boxes.
Quick Reference: Boards for Common Deck Sizes
Assuming 5½-inch boards, a ¼-inch gap, 16-foot boards running along the longer side, and 10% waste:
| Deck size | Deck area | Boards to order |
|---|---|---|
| 10 × 12 ft | 120 sq ft | 24 |
| 12 × 12 ft | 144 sq ft | 29 |
| 12 × 16 ft | 192 sq ft | 29 |
| 14 × 20 ft | 280 sq ft | 66 |
| 16 × 20 ft | 320 sq ft | 75 |
| 20 × 24 ft | 480 sq ft | 93 |
Tips for Buying and Laying Deck Boards
- Keep the 10% waste allowance. Boards get rejected for knots and warping, ends get trimmed square, and stair stringers eat extra material. For picture-frame borders or diagonal patterns, raise it to 15%.
- Buy boards longer than you need when it reduces joints. Fewer butt joints mean a cleaner look and fewer places for water to sit. Stagger any joints you cannot avoid so no two land on the same joist in adjacent courses.
- Set the gap for your lumber’s moisture. Pressure-treated boards arrive wet and shrink as they dry — install them nearly tight. Kiln-dried or composite boards go down at the final gap from day one.
- Crown every board up. Sight down each board before fastening; the growth rings should arc upward (bark side up) so the board sheds water instead of cupping.
- Count screws before you shop. Two screws per joist crossing is the standard; a 16-foot deck needs roughly 750 screws, and running out of matching screws halfway through shows in the finished surface.
- Let boards acclimate. Stack the decking on stickers (spacer strips) at the job site for a few days so it adjusts to local humidity before you fasten it down.
Joist Spacing: The Limit Most People Miss
Your board count comes from the deck’s size, but whether those boards feel solid or spongy is decided underneath, by the joists. The standard layout is 16 inches on-center, which standard 5/4 and 2x deck boards span comfortably when they run perpendicular to the joists. Two layouts need the tighter 12-inch on-center spacing: boards laid at a 45-degree diagonal (each board effectively spans further between joists), and most composite boards. Wider 24-inch spacing is only suitable with thicker 2x stock and is never a good idea for diagonal or composite decking.
Joist spacing also drives your fastener count. At 16 inches on-center, the joists under a 16-foot run number 13, so every full-length board takes 2 screws at 13 crossings — 26 screws per board. Tightening the spacing to 12 inches raises that to 17 joists and 34 screws per board.
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Frequently Asked Questions
How do I calculate how many deck boards I need?
Divide the deck width in inches by the board width plus the gap (both in inches) and round up to get the number of courses. Multiply courses by the deck length divided by the board length, round up, then add 10% for waste. The calculator above does all of this for you.
How many deck boards do I need for a 12×16 deck?
With standard 5.5-inch boards, a 1/4-inch gap, and 16-foot boards, a 12×16 deck needs 26 boards before waste — 29 boards with the standard 10% allowance. Using 8-foot boards instead, you would need 58 boards because each course takes two.
What gap should I leave between deck boards?
A 1/8-inch gap is the most common choice — it drains water while keeping debris and heels out. Use 1/4 inch in very wet climates. If your lumber is wet (typical pressure-treated), install boards nearly tight; they shrink to the final gap as they dry.
How many screws do I need per deck board?
Two screws at every joist crossing. With joists 16 inches on center, a 16-foot board crosses 13 joists and needs 26 screws. For the whole deck, multiply 26 screws by the number of boards — the calculator estimates this for you and converts it to 5 lb boxes (about 350 screws each).
What is the standard deck board size?
A nominal 1×6, which actually measures 5.5 inches wide and 1 inch thick (5/4 decking). It is the default in the calculator because it is the most widely stocked. Narrower 1×4 (3.5 in) and wide 1×8 (7.25 in) boards are also available.
Should deck boards run parallel or perpendicular to the house?
Either works structurally, but running boards perpendicular to the house (outward from the ledger) usually looks better and uses longer, uninterrupted boards. Diagonal or picture-frame patterns need about 5% more material on top of the standard waste allowance.
How many board feet are in a deck board?
One board foot equals 144 cubic inches of lumber. A 16-foot 1×6 deck board (5.5 in wide, 1 in thick) is 16 x 5.5 / 12 = 7.33 board feet. Multiply by your board count for the total — the calculator shows this as “Board feet” in the results.
Can I space deck joists 24 inches apart?
Not with standard deck boards. Five-quarter and standard 2x boards laid perpendicular can span 16 inches on-center; 24-inch spacing feels springy underfoot and most composite manufacturers void it entirely. Diagonal layouts and composites need joists 12 inches on-center. If your frame is already built at 24 inches, use thicker 2x decking and expect more flex.
How many screws do I need for a 12×16 deck?
Using this page’s worked example — 26 boards with joists 16 inches on-center — each board crosses 13 joists and takes 2 screws per crossing: 26 × 13 × 2 = 676 screws. That is roughly 7.5 lb of standard deck screws, so two 5 lb boxes cover the job with spares for the inevitable dropped or stripped screws.