Deck Calculator

1. Enter the deck length in feet. Boards run along this dimension.
2. Enter the deck width in feet. Board rows are counted across this dimension.
3. Choose the board width (5.5 inches is a standard nominal 6 inch deck board).
4. Choose the board length from standard lumber sizes.
5. Select the gap between boards (1/8 or 1/4 inch) for drainage spacing.
6. Pick the joist spacing: 16 inches on center is the most common; use 12 inches for composite decking or heavy loads.
7. Set the post spacing along each beam line (6 or 8 feet typical).
8. Adjust waste (10% is standard; use 15% for diagonal board patterns).

A deck calculator turns your deck dimensions and framing preferences into a materials list. Instead of counting boards and joists by hand, you enter the length, width, board size and spacing, and the tool works out every line item from decking boards to concrete footings.

The result is an estimate for a simple rectangular deck built on posts with beams and joists. Complex shapes like L-decks, multi-level platforms or curved edges need separate calculations for each section. For related material estimates, see the construction calculators hub.

Every output follows basic geometry applied to standard framing dimensions.

1. The width in inches is divided by the board pitch (board width plus gap) and rounded up to get the number of board rows.
2. Total linear feet of decking is rows times the deck length. Dividing by board length and rounding up gives raw board count.
3. The waste factor adds the chosen percentage and rounds up again.
4. Joists are spaced along the deck length. The length in inches divided by the spacing, rounded up, plus one gives the count.
5. Rim joist is the perimeter of the deck frame.
6. Posts are placed along each of two beam lines at the specified spacing.

Take a 12 x 16 foot deck with 5.5 inch boards, 12 foot board lengths, 1/4 inch gap, 16 inch joist spacing, 8 foot post spacing and 10% waste.

Area: 12 x 16 = 192 sq ft.

Board rows across width: 16 ft = 192 inches. Board pitch = 5.5 + 0.25 = 5.75 inches. Rows = ceil(192 / 5.75) = ceil(33.39) = 34 rows.

Linear feet: 34 x 12 = 408 ft. At 12 ft boards: ceil(408 / 12) = 34 boards raw.

With 10% waste: ceil(34 x 1.1) = ceil(37.4) = 38 boards.

Joists: 12 ft = 144 inches. ceil(144 / 16) + 1 = 9 + 1 = 10 joists, each 16 ft long.

Rim joist: 2 x 12 + 2 x 16 = 56 linear ft.

Posts: ceil(12 / 8) + 1 = 3 per beam line, 2 lines = 6 posts.

Footings: 6 footings, each about 0.818 cu ft of concrete.

Screws: ceil(192 x 3.5) = 672 screws. Joist hangers: 10.

Decking boards come in standard widths and lengths. Knowing the coverage per board helps you estimate materials quickly.

Composite decking typically uses the same nominal sizes but may have slightly different actual widths. Check the product data sheet and adjust the board width input if needed.

Joist spacing controls how well the deck supports weight and prevents board deflection between supports.

Your local building code and the decking manufacturer's span rating determine the right spacing for your project.

Posts carry the beams that hold the joists. Footing size and depth depend on soil conditions and frost-line depth, which varies by region.

Post Sizing

4x4 posts work for many low decks; 6x6 posts are needed for taller decks or heavier loads. Your building department specifies the minimum.

Footing Dimensions

This calculator uses a typical 12 inch diameter by 42 inch deep tube footing, which holds about 0.82 cubic feet of concrete. Frost-line depth in your area may require deeper footings.

Concrete Bags

One 80 lb bag of premixed concrete fills about 0.6 cubic feet. A single 12 inch diameter by 42 inch footing needs roughly 1.4 bags. Round up to 2 bags per footing for safety.

Deck Screws

A standard face-screw pattern uses about 350 screws per 100 square feet of decking. The calculator multiplies the area by 3.5 and rounds up. Hidden fastener systems use fewer visible screws but add clips or biscuits.

Joist Hangers

One joist hanger per joist connects each joist to the rim or ledger board. The calculator counts one hanger per joist; if both ends sit in hangers the actual count doubles.

Structural Screws and Bolts

Ledger lag screws, post-to-beam connectors and other structural hardware are not included in this estimate because they vary with design. Your plans or engineer will specify them.

This table shows estimated materials for popular deck sizes using 5.5 inch boards, 12 ft lengths, 1/4 inch gap, 16 inch joist spacing, 8 ft post spacing and 10% waste.

Use the calculator above for your exact dimensions and preferences.

Straight Layout

A straight (perpendicular to joists) board layout wastes the least material. End cuts and defective boards typically account for about 10 percent.

Diagonal Layout

Diagonal patterns create more angled cuts at the deck edges, raising waste to about 15 percent. Some builders use 20 percent for complex angles.

Picture Frame Borders

A picture-frame border adds a perimeter of boards running the opposite direction. Estimate the border separately and add it to the main field.

The calculator lets you set any waste percentage. Start with 10% for a simple straight layout and increase it for more complex designs.

Material costs vary widely by region, lumber species and whether you choose wood or composite decking. A rough budget helps you compare options before buying.

Multiply the board count from the calculator by the per-board price to estimate your decking lumber budget. Add framing lumber, hardware and concrete separately.

Planning a New Deck

Run the calculator before you visit the lumberyard so you know how many boards, joists and posts to order. Over-ordering wastes money; under-ordering means extra trips.

Comparing Layouts

Try different board lengths, joist spacings and waste percentages to see how each choice changes the total material list.

Getting a Contractor Quote

An independent estimate helps you evaluate a contractor's bid. If their board count is very different from yours, ask why.

Replacing Deck Boards

If you are re-decking an existing frame, enter the same dimensions to find how many new boards you need without re-estimating the substructure.

1. Forgetting the Waste Factor

Cutting boards to length always produces offcuts. Skipping the waste allowance leaves you short by several boards.

2. Confusing Nominal and Actual Board Width

A nominal 6 inch board is only 5.5 inches wide. Using 6 inches in the calculation underestimates the number of boards.

3. Ignoring the Gap Between Boards

The 1/8 to 1/4 inch drainage gap adds up across the deck. Leaving it out overestimates how many boards fit.

4. Using the Wrong Joist Spacing

Composite decking often requires 12 inch on center, not 16. Using the wrong spacing may violate the warranty or the building code.

5. Skipping the Building Permit

Most attached or elevated decks require a permit. Building without one can mean fines or having to tear the deck down.

The geometry formulas are exact for a simple rectangle. Real projects add complexity the calculator does not cover.

With 5.5 inch boards, a 1/4 inch gap and 10 percent waste, a 12 by 16 foot deck needs about 38 boards at 12 foot lengths. The calculator above gives the exact count for your board size and waste setting.

Divide the deck length in inches by the joist spacing and add one. A 12 foot deck at 16 inch on center needs 10 joists. Use 12 inch spacing for composite or heavy-load decks.

Sixteen inches on center is standard for most wood decking. Many composite boards require 12 inches on center. Check the decking manufacturer and your local building code.

Posts support the beams. At 8 foot spacing on a 12 foot deck you need 3 posts per beam line. With two beam lines that is 6 posts total.

Footings must reach below the frost line, which varies by region. This calculator uses a typical 42 inch depth, but your local code may require more or less.

A 12 inch diameter by 42 inch deep tube footing holds about 0.82 cubic feet of concrete, which is roughly 1 to 2 bags of 80 lb premixed concrete.

About 350 screws per 100 square feet is typical for a face-screw pattern. A 192 square foot deck needs around 672 screws. Hidden fastener systems differ.

The most common deck board is a nominal 6 inch board, which measures 5.5 inches wide. Nominal 4 inch boards at 3.5 inches are used less often.

Ten percent covers a standard straight layout. Use 15 percent for diagonal patterns and up to 20 percent for complex designs with many angled cuts.

Most jurisdictions require a building permit for an attached or elevated deck. Check with your local building department before you start. This calculator does not replace permit plans.

• IRC Section R507 - Decks (International Residential Code) (General residential deck construction requirements including joist spans and post sizes; local amendments vary).
• American Wood Council Deck Construction Guide (DCA 6: prescriptive deck design and construction guide for wood-frame decks).
• North American Deck and Railing Association (Industry association with deck building resources and best practices).