This calculator works out how many deck boards you need for a square or rectangular deck. You enter the deck length and width in feet and the board width in inches, and it returns the number of boards required, plus the total board footage in linear feet. It also accounts for a standard 1/8-inch gap between boards.
Formula
The calculator uses two simple formulas. First, it finds how many boards fit across the deck width (in inches):
Boards across = (Deck width in inches) ÷ (Board width + gap)
Then it multiplies that by the deck length to get the linear feet of board needed:
Linear feet = (Boards across) × (Deck length)
Finally, it adds a 5% waste allowance and rounds up to the nearest whole board. The variables are:
- Deck length (l) – the longer side of the deck, in feet.
- Deck width (w) – the shorter side, in feet.
- Board width (bw) – the actual face width of one board, in inches. For a 2×6, that’s 5.5 inches; for a 2×4, it’s 3.5 inches.
- Gap – the spacing between boards, set to 0.125 inches (1/8�).
- Waste – an extra 5% for cuts, defects, and mistakes.
The result is the number of boards you need to buy, rounded up to a whole board.
Worked Example
Let’s use the default values: a 20 ft long deck, 12 ft wide, with 5.5-inch wide boards (a standard 2×6).
- Convert deck width to inches: 12 ft × 12 = 144 inches.
- Add the gap to the board width: 5.5 + 0.125 = 5.625 inches.
- Divide to get boards across: 144 ÷ 5.625 = 25.6. Since you can’t use a partial board, round up to 26 boards across.
- Multiply by deck length: 26 × 20 = 520 linear feet.
- Add 5% waste: 520 × 1.05 = 546 linear feet.
- Divide by board length (if using 12-ft boards): 546 ÷ 12 = 45.5, round up to 46 boards.
The calculator gives 46 boards and 546 linear feet – exactly matching the manual calculation.
How to Choose Your Board Width
The board width you enter should be the actual width of the lumber, not the nominal size. A “2×6â€� is actually 5.5 inches wide; a “2×4â€� is 3.5 inches. If you’re using composite decking, the actual width is usually printed on the product spec – often 5.5 inches or 6 inches. For a more precise count, measure a board yourself, because some products vary slightly.
Wider boards mean fewer pieces to install, but they can cup or warp more than narrow ones. Narrower boards cost less per board but require more fasteners and more cutting. The calculator works for any width – just change the input.
Common Mistakes and How to Avoid Them
- Using nominal instead of actual width – a 2×6 is not 6 inches wide. Always use the real measurement.
- Forgetting the gap – even a 1/8� gap adds up over a 12-ft width. The calculator includes it, but if you plan a larger gap (like 3/16� for wet climates), adjust the formula manually.
- Skipping waste – you will cut boards at the ends, and some boards will have knots or cracks. 5% is a safe minimum; 10% is better if you’re using natural wood with visible defects.
- Mixing units – the calculator expects feet for deck dimensions and inches for board width. If you measure in meters, convert first (1 m = 3.281 ft).
- Rounding down – always round up to the nearest whole board. You can’t buy a fraction of a board.
Cost and Material Considerations
The calculator gives you the board count, but the total cost depends on the material you choose. Pressure-treated pine is the most affordable, but it shrinks and may warp. Cedar and redwood cost more but resist rot naturally. Composite decking is the most expensive upfront but requires little maintenance. For an accurate budget, multiply the number of boards by the price per board at your local supplier. Don’t forget fasteners, joist tape, and stain or sealant – those add 10-20% to the project cost.
Boards are sold in lengths (8, 10, 12, 16 feet). To minimize waste, choose a length that matches your deck’s longest run. For a 20-ft deck, you might use 12-ft boards with a scarf joint, or 10-ft boards with two joints – the calculator’s linear feet total helps you compare.
Safety and Installation Tips
Deck boards are structural – they support people, furniture, and snow loads. Always follow local building codes for joist spacing and board thickness. For residential decks, 2×6 boards on 16-inch centers are common, but check with your local inspector. Wear gloves when handling wood, and a dust mask when cutting pressure-treated lumber. If you’re building a raised deck, have a professional verify the framing; the board count alone doesn’t ensure safety.
When installing, leave a small gap between boards for drainage and expansion. Use a spacer (like a 16d nail) for consistent gaps. Screw or nail each board at every joist, and stagger end joints so they don’t line up in a row – this strengthens the deck.
Frequently Asked Questions
How many deck boards do I need for a 10×10 deck?
Using 5.5-inch boards with a 1/8� gap, you need 10 ft × 12 = 120 inches across. 120 ÷ 5.625 = 21.3, so 22 boards across. Multiply by 10 ft length = 220 linear feet. Add 5% waste = 231 linear feet. If using 10-ft boards, that’s 24 boards (since 231 ÷ 10 = 23.1, round up).
Do I need to account for board length?
Yes. The calculator gives linear feet, but you must decide what board length to buy. If your deck is 20 ft long, you can’t use a single 16-ft board – you’ll need to join two boards. The number of boards changes based on the length you choose. The calculator’s board count assumes you use boards equal to the deck length; if you use shorter boards, you’ll need more of them.
What is the standard gap between deck boards?
For pressure-treated wood, a 1/8-inch gap is typical because the wood shrinks as it dries. For composite decking, follow the manufacturer’s recommendation – often 1/8 to 3/16 inch. For tropical hardwoods, a 3/16-inch gap is common to allow for expansion. The calculator uses 1/8�, but you can adjust the formula if your gap is different.
Should I buy extra boards for mistakes?
Yes, always. The 5% waste allowance covers cutting errors, but if you’re new to decking, buy 10% extra. You’ll also need extra for future repairs – it’s hard to match the color of aged wood later.
Can I use this calculator for a diagonal deck?
No. Diagonal decking requires about 15% more material because you cut each board at an angle. Use the calculator for a straight layout, then multiply the linear feet by 1.15 to estimate diagonal needs.
Does the calculator include the rim joist or fascia boards?
No. It only counts the deck surface boards. Fascia (the boards covering the joist ends) and rim joists are separate – measure the perimeter and add those boards to your total.