Deck Calculator
Deck calculator: count boards, joists, posts and screws for any deck size Free, no sign-up, and it runs in your browser.
Dedicated page: /en/deck-calculator
Default checked 10 Nov 2026: $9 per sq ft (pressure-treated pine materials, $9.20 per sq ft). Leave it, or enter your exact price if you know it. Check the current value here.
Quick answer
A deck calculator counts the materials for a new deck. Enter the length and width and it works out the decking boards, joists, posts and footings, and screws, with waste included, for pressure-treated pine, cedar or composite, plus railing and stairs.
About this Deck Calculator
Deck calculator: count boards, joists, posts and screws for any deck size It is one of the Home and DIY on ToolUstad, and like every tool here it runs entirely in your browser: nothing you type, paste or open is uploaded to a server, and there is no sign-up or usage limit.
What is a deck calculator?
A deck is a platform of boards on joists on posts, and the materials are counted in that order. The boards cover the area, and their count depends on the board width, the gap between boards, and the board length you buy. The joists run underneath at 12 or 16 inch spacing, and their count depends on the deck length. The posts and footings sit about 6 feet apart and hold the whole thing up. The calculator counts all of it from the deck size, so the shopping list is complete before you reach the store.
The material choice is the biggest cost decision. Pressure-treated pine is the cheapest and the standard choice, but it needs staining and can warp as it dries. Composite costs the most up front, never needs staining, and lasts the longest, but it needs closer joist spacing, which adds joists. The calculator handles all three, because the board counts are the same and only the price and the joist spacing change.
How does deck maths work?
A 16 by 12 foot deck is 192 square feet. Boards run the 16 foot length, and across the 12 foot width, 5.5 inch boards with a 1/8 inch gap give 26 rows. That is 416 linear feet of decking, which is 35 boards of 12 feet, plus 10% waste, which is 39 boards. Joists every 16 inches across 16 feet need 13 joists. Posts about 6 feet apart need 4 along the length and 3 across the width, which is 12 posts and footings. Screws are 39 boards times 13 joists times 2, which is about 1,014. At $9 a square foot for materials, the total is about $1,806.
When and why to use a deck calculator
- Before buying lumber: boards come in 8, 12 and 16 foot lengths, and the right length cuts waste and joins.
- Comparing materials: the same deck costs very different amounts in pine, cedar and composite, and the calculator shows the gap.
- Budgeting a build: lumber is the biggest cost, and a correct count sets the whole budget.
- Checking a contractor's quote: knowing the board and joist counts yourself shows whether the quote has padded the materials.
Best practices
- Use 16 inch joist spacing for wood decking and 12 inch for composite or diagonal layouts, because composite bends more between joists.
- Leave a 1/8 inch gap between dry boards and 1/4 inch between wet boards, so water drains and the boards can expand.
Common mistakes to avoid
- Buying boards exactly the deck length with no waste: cut ends, damaged boards and wrong cuts need the 10% allowance.
- Spacing joists by eye instead of measuring: a joist an inch out of place leaves a board unsupported at its end.
- Skipping the permit: most attached decks and any deck over 30 inches high need one, and an unpermitted deck can block a house sale.
Who is the deck calculator for?
Homeowners building a deck themselves, small deck builders pricing a job, and anyone getting quotes who wants to check the materials. The tool is an estimator, not a structural design: joist spans, footing depths and railing heights are set by local building codes, so check them before you build.
Current value, checked 10 Nov 2026: the default material price is $9 per sq ft (pressure-treated pine materials, $9.20 per sq ft). The price box is an optional override: leave the default, or enter your exact price and the cost uses it.
Learn: how to use the Deck Calculator
- Enter the deck length and width in feet. Boards run along the length you enter first.
- Pick the board width, the gap between boards, the board length you will buy, and the joist spacing.
- Read the board, joist, post and screw counts, add railing and stairs if you have them, and check the materials cost.
Features
- Pressure-treated pine, cedar and composite decking
- Board counts from width, gap and length, with 10% or 15% waste
- Joists at 12 or 16 inch spacing, posts and footings about 6 feet apart
- Screw estimate at 2 per board per joist crossing
- Railing and stairs priced in, with the materials cost totalled
Worked example
A 16 by 12 foot pressure-treated deck, 5.5 inch boards with a 1/8 inch gap, 12 foot boards, joists 16 inches apart: 192 sq ft, 26 rows of boards, about 39 boards after 10% waste, 13 joists, 12 posts and footings, about 1,014 screws, and roughly $1,806 of materials at $9 a sq ft.
Frequently asked questions
How much should I budget per square foot for a deck?
Materials only: about $8 to $15 a square foot for pressure-treated pine, $12 to $20 for cedar, and $15 to $30 for composite. With labour, most US decks cost $25 to $50 a square foot installed.
What gap should I leave between deck boards?
1/8 inch for dry pressure-treated lumber, 1/4 inch for wet lumber that will shrink as it dries, and whatever the maker specifies for composite, usually about 3/16 inch. Too tight a gap traps water and leaves.
How far apart should deck joists be?
16 inches on centre is standard for wood decking laid straight. Composite decking and diagonal layouts need 12 inches, because composite bends more and diagonal boards span further between joists.
Do I need a permit for a deck?
Usually yes if the deck is attached to the house or more than 30 inches above the ground. Freestanding low decks often do not need one, but the rules vary by town, so check before you buy materials.
How much does it cost to build a deck?
Materials only run about $8 to $15 a square foot for pressure-treated pine, $12 to $20 for cedar, and $15 to $30 for composite. With labour, most US decks cost $25 to $50 a square foot installed.
What size deck should I build?
Big enough for the furniture plus walking space: a 12 by 12 foot deck fits a table and four chairs, and a 16 by 12 foot deck fits a grill and a seating area as well. Bigger is not always better, because the cost rises with every square foot.
How long does a deck last?
Pressure-treated pine lasts 15 to 20 years with staining, cedar lasts 20 to 25, and composite lasts 25 to 30 with almost no maintenance. The structure underneath usually outlasts the boards.
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