Deck calculator

How to measure it on site

Deck board counts come from rows, not square footage alone. Measure the deck's length and width, then decide the board profile, the gap, and the joist spacing before the calculator can give you a number you can order.

1. 01

   Measure the deck frame, not the finished edge. Board overhang and fascia change the visible size but not the rows. Measure the joist span and the rim-to-rim length, then decide whether the boards will run parallel to the house or out from it.

2. 02

   Use the actual board width. A nominal 5/4 × 6 or 2 × 6 is 5.5 inches wide, not 6. A 2 × 4 is 3.5 inches. The row count depends on that actual figure plus the gap, so nominal sizes overstate coverage.

3. 03

   Add the gap to the board. Wood and composite decking both need about a 1/8 inch gap for drainage and expansion. The effective width is the actual board width plus 0.125 inch, which is what turns a width into a row count.

4. 04

   Note the joist spacing and board length. Joists at 16 inches on center need one more than the simple division suggests. Board length matters too: 8 ft boards on a 12 ft run force a seam, while 12 ft boards do not.

Common mistakes

Deck orders are usually short or long for one of five reasons, and each one is visible before the first board is cut.

• Calculating rows with the nominal board width. A nominal 6 inch board covers 5.5 inches plus a 1/8 inch gap, or 5.625 inches. Using 6 inches undercounts rows on a 12 ft deck by about two rows, which is 24 linear feet of board.
• Forgetting the gap. The gap is small per row but repeats across the deck. Leaving it out of the effective width slightly undercounts every row, and the error compounds across the full width.
• Missing the extra joist. Joists at 16 inches on center across 12 ft divide into nine spaces, which needs ten joists — the division gives nine and the rim needs one more. The calculator adds it for you.
• Ordering the exact linear footage. Cut waste of about 10 percent covers angled cuts, defects, and the board that splits. Ordering to the exact figure usually means a second order, and a second order may not match the first.
• Mixing board lengths on a long run. A 12 ft run with 8 ft boards creates a seam on every row, and seams need blocking and careful layout. Buying 12 ft boards costs more per board but removes a whole class of layout work.

Worked example

What this calculator doesn't cover

This calculator sizes decking, joists, and fasteners. It does not design the structure or cover these cases:

• —Beams, footings, and ledger attachment — these carry the deck and belong with an engineer, a permit office, or the manufacturer's span tables.
• —Stairs, landings, and railings — guard height and stair geometry are code questions, not quantity math.
• —Composite-specific spacing and fasteners — some composites require a wider gap or hidden clips that change the row count.
• —Board layout and seams — the calculator gives a total, not a cut plan or a seam pattern.

Reviewed November 2026. Sources: Span tables and joist spacing guidance (American Wood Council); Lumber dimension standards (nominal versus actual) (ASTM International). Full methodology.

How the math works

Frequently asked questions

What is the actual width of a 5/4 × 6 deck board?

In American lumber terminology, nominal 5/4 × 6 boards actually measure 1 inch thick by 5-1/2 inches wide. Similarly, 2 × 6 boards measure 1-1/2 inches thick by 5-1/2 inches wide. Factoring in a standard 1/8" expansion gap between boards, each board run covers approximately 5-5/8 inches (0.469 feet) of width.

How many screws do I need per deck board?

Standard building code requires 2 fasteners at every joist intersection for 6-inch nominal boards. For a typical 16' board resting across 13 joists spaced 16" on-center, you will use 26 screws per board. Always purchase 10% to 15% extra screws for dropped or stripped fasteners.

Should deck joists be spaced 12" or 16" on-center?

16" on-center is standard for natural wood decking installed perpendicular to the joists. If you are installing composite decking (like Trex or TimberTech), or laying boards diagonally at a 45-degree angle, most manufacturers and building codes require 12" on-center spacing to prevent sagging and bouncy spots.

Which board length should I buy?

Choose board lengths that match or divide evenly into your deck dimensions to minimize butt joints. For example, on a 16-foot wide deck, 16-foot boards allow continuous runs without seams. If you must use joints, stagger them across different joists.