Deck Material Calculator: Boards, Joists, Screws

How many boards will a new backyard deck need? A good takeoff saves extra trips to the lumber yard and keeps leftovers small. The Deck Material Calculator estimates deck boards, linear feet, joists, and screws from a few inputs.

It is for homeowners and small builders planning a simple rectangular deck. Enter the size in feet and pick board and joist options. Results update as you go.

Table of Contents

• What the Calculator Does
• How to Use It, Step by Step
• The Deck Material Formulas
• Worked Example
• How to Read the Results
• Board Length, Spacing, and Planning Tips
• Limitations and Assumptions
• FAQs About the Deck Material Calculator

What the Calculator Does

The tool assumes deck boards run along the deck length, across joists that run the width. It gives five results.

• Decking Surface Boards Required: the number of stock boards to buy, with their length.
• Total Surface Area: deck length times width, in square feet.
• Linear Feet Decking: total board length with 5% added for waste.
• Substructure Joists Needed: the joist count, labeled with the joist length.
• Deck Screws / Fasteners: an approximate screw count for the surface boards.
• Copy Deck Takeoff copies a summary, and Reset restores the sample values.

How to Use It, Step by Step

1. Enter Deck Length (ft). This is the direction the deck boards run.
2. Enter Deck Width (ft). The joists span this direction.
3. Pick a Deck Board Width: 5.5 inches for 5/4 x 6 boards or 3.5 inches for 2 x 4 boards.
4. Pick a Stock Board Length Choice of 12, 16, or 20 feet.
5. Pick a Joist Spacing On-Center (Inches) of 16 inches or 12 inches.
6. Read the board count, then check the four cards below it.
7. Press Copy Deck Takeoff to save the list for your lumber order.

The Deck Material Formulas

The tool adds a 1/8 inch gap to each board. It then counts the rows that fit across the width. Each row runs the full deck length.

Each board row uses as many whole stock boards as fit in the deck length. The leftover piece of each row is cut from shared stock boards. The tool fits as many leftover pieces into one board as it can hold.

• Board rows = (width × 12) ÷ (board width + 0.125), rounded up.
• Whole boards per row = deck length ÷ stock length, rounded down.
• Leftover boards = rows ÷ (leftover pieces that fit in one stock board), rounded up.
• Boards = (rows × whole boards per row + leftover boards) × 1.05, rounded up.
• Linear feet = board rows × deck length × 1.05.
• Joists = (length × 12) ÷ joist spacing, rounded up, plus 1.
• Screws = board rows × joists × 2 × 1.05, rounded up. That is two screws at each crossing.

Take the sample 20 by 16 foot deck with 16 foot stock. It needs 35 rows, and each row takes one whole board plus a 4 foot piece. One 16 foot board yields four of those pieces, so 35 pieces need 9 boards.

That makes 35 whole boards plus 9 leftover boards, or 44 boards in all. Adding 5% gives 46.2, which rounds up to 47 boards.

Worked Example

Marcus is building a 16 by 12 foot deck. He picks 5.5 inch boards, 16 foot stock, and joists at 16 inches on center.

At 12 inch joist spacing, the joist count rises to 17 and screws to about 929. Switching to 3.5 inch boards gives 40 rows, 672 linear feet, and 42 boards.

Stock length can change the count. With 20 foot stock, Marcus still needs 28 boards, since each 4 foot offcut is too short for a row.

How to Read the Results

Boards Required already includes 5% extra for waste. It is the count to order in the stock length you picked.

Linear Feet Decking helps when a store prices boards by the foot. It shows the footage the deck covers, not the total length of boards you buy.

Joists Needed counts joists across the deck length, with one at each end. The label shows the deck width as the joist length.

Deck Screws covers surface boards only. Framing hangers and their nails are not included.

Board Length, Spacing, and Planning Tips

Pick a stock length that matches your deck length or splits into it evenly. A 16 foot deck fits 16 foot boards with no joints. Board ends that meet must land on a joist.

Joist spacing also depends on your decking. Many wood decks use 16 inches on center, while diagonal boards often need 12 inches. Check the board maker's spacing rules.

For a quick board count by area, try the deck board calculator. If the deck sits above grade, the stair stringer calculator helps plan the steps. Footings can be sized with the concrete footing calculator.

To add lumber, hardware, and permit costs, use the project budget calculator. More outdoor tools are in the home and construction hub.

Limitations and Assumptions

• The tool covers decking, joists, and screws only. Beams, posts, ledger boards, rim joists, and railings are not counted.
• The board count plans whole boards per row and cuts leftover pieces from shared boards. It does not check that board ends land on a joist or stagger between rows.
• Joist size is not sized by the tool. Span tables in local code set joist size and spacing.
• Length and width must each be between 1 and 1,000 feet. A blank, negative, or out-of-range size shows a message and N/A in every result.
• The gap between boards is fixed at 1/8 inch, and waste is fixed at 5%.
• Local building codes and an inspector or engineer decide final framing, footings, and fasteners.

FAQs About the Deck Material Calculator

This calculator gives planning estimates only and is not professional engineering, electrical, or building code advice. Reviewed for general accuracy by Prof. Dr. Khalil Mudassar, PhD.