Deck Calculator

A 10×8 ft ground-level deck needs 19 twelve-foot decking boards and 9 joists — enter your deck's length, width, and board spacing to get a materials list.

Length (along the house) (ft)

Width (out from the house) (ft)

Board width (in)

Gap between boards (in)

Joist spacing (in o.c.)

Joist stock length (ft)

Waste margin (%)

Deck boards

Joists

Joist boards

Decking boards (12ft)
19 boards
per board for Decking boards (12ft)
Joists (8ft stock)
9 joists total
9 boards
per board for Joists (8ft stock)
Deck screws
306 screws needed
13 box of 25
per box of 25 for Deck screws

Enter your local prices to see an estimated cost.

How we calculate this

Deck boards run parallel to the house, so rows = deck width ÷ (board width + gap), rounded up, and each row takes deck length ÷ board length boards, rounded up (longer rows are butt-joined over a joist). Joists run perpendicular to the boards, spanning the width, spaced along the length at your on-center setting (16 in by default): length ÷ spacing, rounded up, plus one. Each joist must be one piece, so the joist stock has to be at least as long as the width. Screw count assumes 2 screws at every board-joist crossing.

This covers material quantities for a small, ground-level platform only — it's not a structural span or load calculation. Elevated decks, guardrails, and stairs need span tables and often a permit; check with a professional.

Estimates only. Check local codes and product labels. Read our full disclaimer.