Deck Calculator

Enter the deck size, the joist spacing and the decking board width, and get the number of boards, joists, beams and posts, plus screws and concrete for the footings.

Board length options let you see how the layout affects offcuts before you buy.

How to use the deck calculator

Enter the deck dimensions and height.

Set joist spacing and board width.

Read boards, joists, posts, screws and concrete.

Why use the Build Tally deck calculator

Full frame count

Joists, rim, beams and posts together.

Board layout aware

See how board length changes waste.

Footings included

Concrete estimated from the post count.

Fasteners counted

Screws worked out from joist crossings.

Counting the frame

Joists run at 12, 16 or 24 inch centres depending on the decking material and the span — composite boards generally need closer spacing than timber, and diagonal decking needs closer spacing again. The joist count is the deck length divided by the spacing plus one, with rim joists added. Beams and posts follow from the joist span and the beam span, and post count drives the footing count, which drives the concrete. The calculator carries that chain through so a change in spacing updates everything.

Decking boards and offcuts

Board count is deck width divided by board width plus the gap between boards, typically 1/8 to 1/4 inch for timber that will shrink, and per the manufacturer for composite. Board length is where money is saved or wasted: if your deck is 15 ft long, 16 ft boards give a foot of waste each while two 8 ft boards give a butt joint but no waste. Butt joints need to land on a joist and are usually staggered, which affects the layout. Add 5–10% for cuts and for the odd board you reject.

What this tool does not do

It counts material; it does not size it. Joist span, beam size, post spacing, footing depth, ledger attachment and railing height are all structural and code matters, and getting them wrong is a safety failure rather than an ordering error — deck collapses are usually ledger failures. Take spans from your local code's tables or an engineer, and expect to need a permit and an inspection. Use these figures to price the job, not to design it.

Read the guide: Deck Materials List: Working Out Boards, Joists, Posts and Fasteners

Frequently asked questions

How many deck boards do I need?

Divide the deck width by the board width plus the gap, then multiply by the number of rows. Enter your dimensions above and the calculator does it.

How far apart should deck joists be?

Sixteen inches on centre is typical for timber decking, twelve for composite or diagonal layouts. Follow the decking manufacturer's specification and your code.

How many screws per deck board?

Two per joist crossing, so a 12 ft board over joists at 16 in centres takes about 20 screws.

How much concrete for deck footings?

It depends on footing size and frost depth. The calculator estimates from your post count and footing dimensions.

Do I need a permit for a deck?

Usually yes, especially for raised decks. Check with your local building department before starting.

Does this calculator size the joists?

No. It counts material. Joist and beam sizing must come from your local code's span tables or an engineer.