Deck Calculator

How to Use the Deck Calculator

Planning a deck project requires knowing how much material to buy. Enter the overall deck length and width in feet, along with the individual board length and width. The calculator determines how many decking boards you need by calculating how many boards fit across the width and how many runs are required to cover the length. It also computes the number of joists spaced at 16 inches on center, the number of support posts, and the total screws needed.

If you enter a price per board, the calculator provides a cost estimate for the decking material alone. This does not include joists, posts, concrete footings, hardware, or labor, but it gives you a solid starting point for budgeting the largest single material expense in most deck projects.

Deck Framing Basics

A typical deck consists of concrete footings, support posts, beams, joists, and decking boards. Footings must extend below the frost line in your area. Posts sit on top of footings using post anchors and support the beams. Beams run perpendicular to the joists and transfer the load to the posts. Joists span between the ledger board attached to the house and the outer beam, typically at 16-inch intervals. The decking boards run perpendicular to the joists.

Choosing Decking Material

Pressure-treated pine is the most affordable decking option at approximately $2 to $5 per linear foot. Cedar and redwood cost more but offer natural rot resistance and attractive grain patterns. Composite decking ranges from $5 to $13 per linear foot and provides excellent durability with minimal maintenance. PVC decking is the most expensive at $8 to $15 per linear foot but resists moisture, insects, and fading. Consider your budget, maintenance preferences, and local climate when choosing materials.

Permits and Structural Requirements

Most municipalities require a building permit for any deck attached to a house or elevated more than 30 inches above grade. The permit process typically requires a site plan, structural drawings, and an inspection at various stages of construction. Deck ledger boards must be properly flashed and fastened to the house rim joist with lag screws or through-bolts to prevent the deck from pulling away from the structure. Failure to properly attach the ledger is the leading cause of deck collapses.

Frequently Asked Questions

How do I calculate how many deck boards I need?

Divide the deck width by the board width to find the number of boards across. If the deck length exceeds the board length, multiply by the number of runs needed. For a 16x12 foot deck using 12-foot, 5.5-inch-wide boards, you need about 27 boards across in 2 runs, totaling 54 boards.

How far apart should deck joists be spaced?

Standard deck joist spacing is 16 inches on center for most decking materials. Composite decking may require 12-inch spacing at a 45-degree angle or 16-inch spacing for perpendicular installation. Always check the decking manufacturer specifications.

How many posts does a deck need?

Posts are typically spaced every 6 to 8 feet along each beam line. A standard deck has two beam lines, so a 16-foot-long deck needs about 3 posts on each side for 6 posts total.

How many screws do I need for a deck?

Plan for approximately 350 deck screws per 100 square feet of decking, or about 20 screws per board. A standard 5-pound box of deck screws contains approximately 350 screws.

What size lumber do I need for a deck?

Decking boards are typically 5/4x6 with an actual dimension of 1 inch by 5.5 inches. Joists are usually 2x8 or 2x10 depending on span. Posts are 4x4 for decks under 8 feet high or 6x6 for taller decks.