CalcMaterials

Decking Calculator

Estimate deck boards from your deck's area and board width, then get the joist count and the screws you'll need — gap spacing and waste built in.

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What you'll need to buy

  • Deck boardsPressure-treated, cedar, or compositeShop
  • Joists + ledgerFraming lumber for the structureShop
  • Deck screws / hidden clipsStainless or coated for outdoor useShop
  • Joist hangers + hardwareConnects joists to beams and ledgerShop
  • Joist tape / flashingProtects framing tops from rotShop

Links marked Shop open a retailer search (Home Depot) and are affiliate links — we may earn a commission at no extra cost to you.

How the math works

A 12 ft × 16 ft deck = 192 sq ft. Using 5.5"×12 ft boards (5.5 sq ft each) with 10% waste: 192 × 1.10 ÷ 5.5 ≈ 39 boards. Joists ≈ 192 × 0.75 ≈ 144 linear-ft of joist; screws ≈ 192 × 1.75 ≈ 336.

Formula

boards = (deck area × (1 + waste%)) ÷ coverage per board joists ≈ deck area × 0.75 (16" on-center) screws ≈ deck area × 1.75

Reference values

Decking board & framing reference
ItemValue
5.5" × 12 ft board≈ 5.5 sq ft each
5.5" × 16 ft board≈ 7.3 sq ft each
3.5" × 12 ft board≈ 3.5 sq ft each
Joists16" on-center typical
Screws≈ 1.75 per sq ft

Step by step

  1. Measure the deck length and width for square footage.
  2. Pick your board size; divide deck area by the board's coverage for the board count.
  3. Add 10–15% waste for cuts, angles, and picture-frame borders.
  4. Estimate joists (≈0.75 ×area) and screws (≈1.75 ×area), and round up.

Waste allowance

Always add a waste factor: more for diagonal, herringbone, or irregular layouts and for trenches in rocky soil. It's cheaper to round up than to pay for a second delivery.

Bags vs. bulk

Bagged material is convenient for small jobs, but loose/bulk delivery is almost always cheaper per unit once you pass roughly 1–3 cubic yards. Compare the per-cubic-yard price before deciding.

Frequently asked questions

How many deck boards do I need?

Divide the deck area by the square footage one board covers (including the gap), then add waste. A 192 sq ft deck using 5.5"×12 ft boards (5.5 sq ft each) needs about 39 boards with 10% waste.

How far apart should deck boards be spaced?

Leave about 1/8" to 1/4" between boards for drainage and expansion. This calculator already folds a 1/4" gap into each board's coverage figure.

How many screws do I need for a deck?

Plan on roughly 1.75 screws per square foot — that's about two screws at every joist crossing. A 192 sq ft deck needs around 336 screws; buy a little extra.

How many joists does a deck need?

Joists are usually spaced 16" on-center. As a rough estimate, figure about 0.75 linear feet of joist per square foot of deck; confirm the exact spacing and span with a deck span table for your lumber.

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