What joist spacing should I use?

16″ on-center is the standard for 5/4×6 wood and most composite decking. Use 12″ on-center for diagonal decking, heavy snow loads, or composite brands that require it.

How many deck screws per board?

Two screws per board crossing per joist. For a 12-ft board over a deck with joists at 16″ on-center, that is roughly 18 screws per board. The calculator returns the total.

How long do deck boards last?

Pressure-treated pine: 15–25 years. Cedar: 15–20 years. Composite: 25–50 years (manufacturer warranty). Lifespan drops sharply if boards stay wet without ventilation underneath.

Should I buy 16-ft boards or two 8-ft boards?

16-ft is usually preferable: fewer end joints, cleaner look, and only one cut at each end. Two 8-ft boards is fine if 16-ft is unavailable or hard to transport, but plan butt joints to land on joists.

Does this account for the ledger board, beams, or stairs?

No — it covers decking boards, joists, and fasteners only. Add the ledger, beam, posts, footings, and stairs separately based on your plan.

Is the calculator free?

Yes — free, no signup, runs in your browser.

Deck Calculator

Deck boards, joists, and fasteners from deck dimensions and joist spacing.

Written by Golam Rabbani, Founder & Lead Engineer

This interactive tool requires JavaScript. Read the formula and worked example below; you can compute the result by hand.

How to use this deck calculator

Toggle imperial (ft) or metric (m) for the deck dimensions.
Enter the deck length (the direction the boards run) and width.
Set the board face width in inches — 5.5″ for 2×6 or 5/4×6 decking; 3.5″ for 2×4.
Pick the board length (8, 10, 12, or 16 ft stock) and joist spacing (16″ on-center standard).
Set a waste percentage and press Calculate.

About this deck calculator

The deck calculator estimates the boards, joists, and fasteners needed for a rectangular deck. It computes the deck area, then divides the deck width by the board face width to get the number of board rows. The deck length is divided by the chosen board stock length to get pieces per row. The product is multiplied by 1 plus the waste percentage. Joists run perpendicular to the boards, so joist count is the deck length in inches divided by the on-center spacing plus one for the end joist. Fasteners are estimated at 3.5 screws per square foot — the industry rule of thumb for two screws per board crossing per joist.

Worked example: a 16 ft long × 12 ft wide deck with 5.5″ (5/4×6) boards on 16-inch joist spacing. Deck area = 192 ft². Rows across the width: 144 in / 5.5 in = 27 rows. With 12 ft boards, you need 27 × 2 = 54 pieces (each 12-ft board is split into two 6-ft runs to span a 16-ft length, or use 16-ft boards for one piece per row). Adding 10% waste: 60 boards. Joists at 16″ on-center for a 16-ft span = 13 joists. Fasteners ≈ 192 × 3.5 = 672 deck screws.

Use stainless or coated structural screws made for deck use — drywall screws will snap and rust. For composite decking, follow the manufacturer's exact joist-spacing requirement; some composite boards require 12-inch on-center spacing and hidden clips instead of screws. The calculator is a quick estimate; consult the deck plan and your local code for ledger flashing, joist hangers, and beam sizing.

FAQ

What joist spacing should I use?: 16″ on-center is the standard for 5/4×6 wood and most composite decking. Use 12″ on-center for diagonal decking, heavy snow loads, or composite brands that require it.
How many deck screws per board?: Two screws per board crossing per joist. For a 12-ft board over a deck with joists at 16″ on-center, that is roughly 18 screws per board. The calculator returns the total.
How long do deck boards last?: Pressure-treated pine: 15–25 years. Cedar: 15–20 years. Composite: 25–50 years (manufacturer warranty). Lifespan drops sharply if boards stay wet without ventilation underneath.
Should I buy 16-ft boards or two 8-ft boards?: 16-ft is usually preferable: fewer end joints, cleaner look, and only one cut at each end. Two 8-ft boards is fine if 16-ft is unavailable or hard to transport, but plan butt joints to land on joists.
Does this account for the ledger board, beams, or stairs?: No — it covers decking boards, joists, and fasteners only. Add the ledger, beam, posts, footings, and stairs separately based on your plan.
Is the calculator free?: Yes — free, no signup, runs in your browser.