How many deck boards for a 20x20 deck?
You need 43 deck boards at 20 ft long.
That is 43 rows across the 20 ft span, or 860 linear feet. Buy 947 linear feet to cover the usual 10% for cuts and defects.
Where that number comes from
A standard 5/4x6 or 2x6 deck board is 5.5 in wide once milled, not 6 in. Add the 0.125 in drainage gap between boards and each row covers 5.625 in of deck.
- 20 ft across × 12 = 240 in to cover
- 240 ÷ 5.625 = 42.7 rows, rounded up to 43
- 43 rows × 20 ft long = 860 linear feet
- Add 10% for cuts and culls = 947 linear feet
Boards run the length of the deck here, so the 20 ft dimension sets board length and the 20 ft dimension sets how many rows you need. Running them the other way swaps the two numbers.
Change the numbers
What else the frame needs
- Joists at 16 in on centre: about 16 for a 20 ft run.
- Footings: roughly 16 for a deck this size, depending on beam layout and frost depth.
- Deck screws: around 1896 — two per board at every joist crossing.
Joist and footing counts vary with span tables, beam placement and local frost depth. Treat them as a shopping estimate, not a structural design, and check your local code before you build.
Getting it right
- Measuring the frame, not the decking. Decking usually overhangs the rim joist by an inch or so on open sides. Measure the surface you intend to cover.
- Buying nominal widths. A "6 inch" board is 5.5 in. Sizing off 6 in leaves you roughly 9% short — on this deck that is about 77 linear feet missing.
- Skipping the gap. Boards need the 0.125 in gap to drain and to move with humidity. Butting them tight traps water and cups the boards.
- Ordering exact. Ten percent covers bowed boards, knots and cut-offs. It is cheaper than a second delivery.
Other deck sizes
Deck calculator for any size, or all calculators.