Deck Materials Calculator: Boards, Screws & Cost

I Spent an Entire Saturday Counting Deck Boards Wrong

I'm not proud of this, but a few years back I was building a 12x16 deck for my neighbor — nothing fancy, just a standard platform deck off the back door — and I bought materials based on rough math I did on a napkin at the lumber yard. I figured, hey, 192 square feet, how hard can this be? Turns out I was short by about 14 boards because I completely forgot to account for the gap spacing between them and the fact that I was running them at a slight angle to shed water. Had to make a second trip, and by then the price on pressure-treated 5/4x6 had bumped up a bit, so I ate that cost. It was maybe 85 bucks extra, but the lost time was worse.

That's honestly why I built the calculator we have now. Because even experienced builders mess this up when they're doing it in their heads.

How Deck Board Math Actually Works

The basic idea is pretty simple. You've got a deck area, you know the width of your boards, and you divide one by the other. But the thing is, that only works if you ignore board gaps, waste, and the reality that lumber comes in specific lengths that don't always line up perfectly with your layout.

Here's how I think about it step by step.

First, figure out your deck's total square footage. Length times width. A 14x20 deck is 280 square feet. Easy enough.

Then you need to know the actual coverage width of each board. A standard 5/4x6 deck board is about 5.5 inches wide. But you're also putting a gap between each board — typically 1/8 inch to 1/4 inch, depending on the material and how much expansion you expect. So your effective board width is more like 5.625 to 5.75 inches. That matters when you're multiplying across 20 feet of deck width.

So for that 14x20 deck, if boards run the 20-foot direction and the deck is 14 feet (168 inches) wide:

168 ÷ 5.625 = about 29.9 boards. You'd buy 30, plus I always add 10% for waste — so 33 boards at 20 feet each. And if 20-footers aren't available (they often aren't at regular lumber yards), you're looking at piecing together 12s and 10s, which means even more waste and you need to plan your seams over joists.

This is where people's napkin math falls apart.

Boards Are Only Half the Story — Screws, Joist Tape, Hidden Fasteners

Everyone obsesses over the decking and forgets about fasteners until they're mid-build. Each board gets two screws (or clips) at every joist crossing. Standard joist spacing is 16 inches on center, so a 20-foot board crosses about 16 joists. That's 32 screws per board. With 33 boards, you're at roughly 1,056 screws. A box of deck screws usually holds 350 or so, which means you need 3 boxes — and honestly, grab a 4th because you'll drop some, strip a few, and there's always that one board that splits on you and needs a redo.

Here's a reference table I put together based on common deck sizes. These assume 5/4x6 boards with 3/16" gaps, 16" OC joists, and 10% waste factor:

The cost range is wide because pressure-treated runs about 2–3 per linear foot for 5/4x6, while composite can be 4–8+ per linear foot depending on the brand. Cedar and redwood fall somewhere in the middle but vary wildly by region. I've seen cedar at 3.50 a foot in the Pacific Northwest and nearly double that in the Southeast.

If you're also pricing out the substructure — joists, beams, posts, concrete footings — that's a whole other calculation. Our

The Stuff Nobody Tells You About Estimating Deck Costs

Material cost is maybe 40-50% of a deck project if you're hiring it out. Labor, permits, and hardware (joist hangers, post bases, lag bolts, flashing) add up fast. I had a buddy who budgeted 3,500 for a 300 square foot composite deck and ended up at closer to 7,000 when everything was said and done. He didn't account for the ledger board attachment, the permit fees (which were 400 in his county!), or the fact that his yard sloped and he needed taller posts on one side.

A few things that'll shift your numbers:

• Board direction — running boards at 45 degrees adds about 15% more waste
• Picture framing the edges (that border detail) needs extra boards cut to length
• Stairs — even a simple 3-step set adds materials and time
• Railing systems can cost as much as the decking itself on smaller decks

For railing estimates, try the

One more thing — if you're working with composite decking, the manufacturers usually have their own fastener systems. Hidden clips, starter strips, end caps. Budget an extra 1.50 to 2.50 per square foot just for the fastener system. I know, it's annoying, but that's the trade-off for not having visible screw heads.

For general area math or if your deck has an irregular shape (like an L or a wraparound), the

Frequently Asked Questions

If you're still in the planning phase and trying to figure out how much material to order, just plug your dimensions into our