Tool · D-01

Deck board calculator

Boards, joists, screws. Pick your board width and gap, get the exact count, linear feet, and a fastener tally.

Quick answer

How many deck boards, joists, and screws do I need?

Rows = ceil(deck width × 12 ÷ (board width + gap)). Boards = rows × ceil(deck length ÷ board length), plus 10% waste. Joists at 16″ on-center for standard PT decking, plus two screws per board per joist intersection.

Intro

When to use this calculator

Deck math is more than board count. You also need joists, beams, posts, screws, and an honest estimate of board waste. This calculator handles all of it.

Enter the deck dimensions, board width, and joist spacing. The calculator returns linear feet of decking, joist count, beam length, screws by count, and total cost estimate based on current Home Depot and Lowe's pricing.

The waste factor is 10% for rectangular decks, 15% for decks with diagonal patterns or angles. Round up on screws. The box is cheap insurance against a Saturday afternoon trip.

How to use it

How to use this calculator

Enter deck width and depth in feet.
Pick your board width. 5/4 × 6 (5.5 inches actual) is most common; 2×6 (5.5 inches actual) for heavier construction.
Set joist spacing. 16 inches is standard, 12 inches for hot tub or kitchen areas, 24 inches for some applications (check your decking manufacturer's spec).
Set the gap between boards. 1/8 inch for pressure-treated, 3/16 inch for cedar, 1/4 inch for composite (or whatever the manufacturer requires).
The calculator returns linear feet, joists, beam length, posts, and fastener count.

Inputs

Deck length

Deck width

Board material

Board length

Gap

Joist spacing

in OC

Cut waste10%

Reading the output

Understanding your result

The big number is total boards needed at your chosen length. Below it: rows across the deck, total linear feet, joist count, and screw count (with a pound estimate).

What the count assumes: 10% cut waste added to board count, 2 screws per joist crossing, joists at the spacing you set. Pick a board length that minimizes end joints. A 16-foot deck wants 16-foot boards, not two 8-foot pieces per row.

When to bump the waste factor: 15% for diagonal patterns, picture-frame borders, or any deck with non-rectangular shapes. The cuts pile up fast.

When to round up vs round down: always round up, and add one extra board beyond that. A bad board (cup, twist, knot blowout) will surface during install. Better to set it aside than rebuild a row.

Common mistakes (and how to avoid them)

✗ Using 2×8 joists where 2×10 is needed.

Fix: A 12-foot deck with 2×8 joists at 24-inch spacing exceeds the maximum span. The deck will sag and bounce.

✗ Skipping the snow-load adjustment.

Fix: Northern climates need shorter spans than the standard tables show. Subtract 10 to 15% from the southern-pine values.

✗ Forgetting board gap math.

Fix: A 12-foot wide deck with 5.5-inch boards and 1/8-inch gaps needs more boards than a deck with 1/4-inch gaps. The calculator handles this.

✗ Not buying extra screws.

Fix: A box of 100 deck screws at Home Depot is $20. The trip back when you run out at 4pm Saturday costs more in time. Always buy one extra box.

✗ Using the wrong fasteners for composite decking.

Fix: Composite needs hidden fasteners or specific composite-rated screws. Standard wood deck screws will void the warranty.

Frequently asked questions

How far apart should deck joists be?

16 inches on center is the standard. 12 inches for hot tub areas, outdoor kitchens, or where you want extra rigidity. 24 inches works for some lower-load applications but most composite decking warranties require 16 inches max.

How many screws per deck board?

Two screws per joist crossing. So a 16-foot board crossing 13 joists at 16-inch spacing uses 26 screws. The calculator handles the count for you.

What's the difference between 2×6 and 5/4×6 decking?

5/4×6 is roughly 1 inch thick (vs. 1.5 inches for 2×6). 5/4 is the standard for most residential decks, lighter, cheaper, plenty strong for foot loads. 2×6 is for heavier construction or where you want extra rigidity.

How long should I expect a pressure-treated deck to last?

20 to 30 years for the structure if properly built and sealed every 2 to 3 years. The deck boards themselves typically need replacement at 15 to 20 years. The framing underneath usually outlasts the boards.

Can I use composite decking on existing wood framing?

Yes, if the framing meets the composite manufacturer's joist spacing requirements (typically 16 inches max for residential, sometimes 12 inches for diagonal patterns). Check your existing joist sizing too, older decks often have 2×8 joists at 16-inch spacing that work with composite.

Related guides

Deck joist span tables

Maximum joist spans for 2×8, 2×10, and 2×12 lumber by species and spacing.

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Where to buy your materials

We've linked to common products at Home Depot and Lowe's below. These are affiliate links, meaning we earn a small commission if you buy through them, at no cost to you. We've used these specific products on projects ourselves. If your local independent yard has a better price, take it.

Where to buy

×29

16 ft 5/4 × 6 deck boards.

Sized to your run length so you minimize end joints and waste.

Check price atThe Home Depot Check price atLowe's

Affiliate links, same price for you, helps keep the calculators free.

Where to buy

×13

2 × 8 pressure-treated, ground-contact rated.

Spaced to match your board length and standard 16 in OC framing.

Check price atThe Home Depot Check price atLowe's

Affiliate links, same price for you, helps keep the calculators free.

Where to buy

×10

#8 exterior star-drive screws, quantity in pounds.

Use exterior-rated only. Standard wood screws rust through within a season outdoors.

Check price atThe Home Depot Check price atLowe's

Affiliate links, same price for you, helps keep the calculators free.

Want a printable version?

We'll email a one-page PDF summary of this page's math plus a calling script for local supply yards. Everything we know is right here on the page, the PDF just makes it portable.

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