Deck Estimator

How to use this calculator

1. Measure the deck's length — the longest side — corner to corner with a tape, not by pacing it off. Pacing is how 16-foot decks become 15-foot-4 decks, and that's a whole extra lumber run you'll wish you knew about at the store.
2. Measure the width from the house wall (or the inner edge of a freestanding deck) outward. If the ground slopes, measure at the deck surface height you're planning, not at the dirt — widths read differently once you're standing on a framed platform.
3. Enter the actual decking board width, not the nominal one. A '1x6' board is really 5.5 inches wide once it's dressed and dried. Typing 6 here is the single most common input error, and it undercounts your courses by a full row on anything over about 10 feet wide.
4. Enter the lumber length you plan to buy — 8, 12, or 16 feet are what yards stock. Longer boards mean fewer butt joints, but every joint needs to land on a joist, so match this to your frame spacing.
5. Set the joist spacing. 16 inches on center is standard for decking run straight across the joists. If you're running boards diagonally or using composite decking, switch to 12 inches — most composite manufacturers require it, and real wood sags on diagonals at 16.
6. Check the screws-per-box number against what you're actually buying. A 5-pound box of #8 deck screws holds roughly 350, but 10-pound boxes and stainless lines differ. Count the box, not the guess.
7. Press Calculate and read the breakdown: courses, boards to buy, joists, posts, total screws, and how many boxes. Then add roughly 10% to the board count before you head to the yard — culling warped and split boards is part of every deck build.

Formula

Worked example

Measurement tips

• Buy your decking from a real lumber yard if you can, not just the big box. Yards let you hand-pick boards, and their pressure-treated stock is usually fresher — Home Depot and Lowe's are fine for screws and hardware, but culling boards matters more than saving a dollar a stick.
• Check the crown on every joist before you fasten it. Sight down the edge like a rifle barrel and set the crown (the slight arch) facing up. A joist installed crown-down becomes a permanent dip in your deck floor.
• Stagger your butt joints across the deck and never let two land on the same joist in a row. A straight line of joints looks amateur and weakens the surface where traffic hits it.
• Use a 16d nail as a spacer for the 1/4-inch board gap instead of eyeballing it. Consistent gaps drain water evenly; wandering gaps make the whole deck look homemade.
• If your deck is more than 30 inches off the ground, most US jurisdictions require a permit and railings — check with your city before you buy lumber, not after the inspector shows up.
• For composite decking, drop to 12-inch joist spacing and budget for hidden fasteners instead of face screws. Composite expands and contracts more than wood, and face-screwed composite shows every seasonal move.
• Order one extra box of screws beyond what the math says. Screws get dropped in grass, driven into knots that eat the heads, and used on the fascia boards you forgot to count.

Common mistakes

• Typing 6 inches for a '1x6' board instead of its actual 5.5-inch width. It happens because the nominal name feels like the truth. The result: your course count comes up one row short on wide decks, and you're 40 feet of decking short at the store.
• Forgetting the 1/4-inch gap between boards. It looks negligible per board, but across 34 courses it adds up to 8.5 inches — a full extra course of lumber you didn't budget for.
• Counting joists without the +1. The math gives you the number of gaps; the frame needs a joist at both ends. Forget it and your order is one joist short, which is exactly one too few to finish the rim.
• Specifying posts for a single beam on a deck that's too wide for one. Anything much past 12 feet of span from the house wants a second support beam with its own posts and footings. One beam on a wide deck sags, and sagging decks bounce.
• Ordering the exact calculated board count with nothing for waste. Between split ends, knots you don't want on a walking surface, and cuts that eat length, plan on culling 5 to 10 percent. The yard is not coming to you.
• Mixing board lengths randomly to 'save money.' Different lengths mean butt joints landing in different places, which means more joists to catch them and a patchwork look. Pick one length and commit.

Factors that affect accuracy

• Board width. A 2x6 (5.5 inches actual) and a 2x4 (3.5 inches actual) produce very different course counts and screw counts on the same deck. Narrower boards mean more courses, more screws, and more labor per square foot.
• Decking pattern. Boards run straight across need the fewest cuts. Diagonal patterns look sharp but waste roughly 15 to 20 percent more lumber and demand tighter 12-inch joist spacing.
• Deck height and beam layout. Low decks can sit on a single beam near the house; high decks need bigger beams, cross-bracing, and footings below the frost line. Tall posts also mean more concrete and more money per post.
• Board direction relative to the house. Boards usually run perpendicular to the house, which decides which dimension is 'length' in this calculator. Flip the direction and every number changes — measure and enter accordingly.
• Stairs, railings, and fascia. This estimator covers the deck platform only. Stairs, railing posts and balusters, and the fascia boards that hide the rim joists are separate shopping lists.
• Lumber availability. Yards stock 8, 12, and 16-foot boards reliably; odd lengths like 10 and 14 exist but vary by region and species. If your plan depends on a length the yard doesn't carry, the whole board count shifts.

Frequently asked questions