Deck Calculator

What is a deck calculator?

A deck calculator estimates the materials needed for a rectangular deck. Enter the deck dimensions and board and joist details to estimate decking boards, joists and screws. If you enter an installed price per square foot, it can also estimate cost.

The estimate depends on the inputs you choose. It is a planning aid, not a framing plan or a final materials order. A decking calculator cannot account for every layout, cut, obstacle or local building requirement.

Why use a deck calculator?

A materials estimate helps you compare deck sizes, make a preliminary budget and prepare for a conversation with a supplier or contractor. You can see how changing the deck depth or board width affects the board count before you buy.

The estimate can also help you evaluate an installed quote. Enter the quoted installed price per square foot to get an approximate project cost based on the deck area. Confirm what the quote includes, since the calculator does not price every possible labor or site-preparation cost.

How to use this deck calculator

1. Enter the deck length along the house and the deck depth. These dimensions determine the deck area and the number of board rows.
2. Enter the decking board width and length. The board-width calculation includes a 3/16-inch gap between rows.
3. Enter the joist spacing on center. Use the spacing planned for your deck, rather than treating the estimate as a recommendation for structural design.
4. Choose a waste allowance to account for cuts and unusable pieces. The sample uses 10%. Adjust it to fit your layout and plans.
5. Enter an installed price per square foot if you have one. Leave it at 0 if you only want the materials estimate.

Deck calculator example

How to read your deck calculator results

Deck area is the rectangular footprint calculated from your length and depth. Decking boards are an estimate based on board width, the gap and the waste allowance. Joists are estimated from the deck length and the spacing you entered, with an additional joist at the end.

The screw count is approximate and based on the calculator’s stated rate of about 350 screws per 100 square feet. Actual fastener needs depend on the decking system and installation details. Treat each result as a starting quantity, then confirm the order with your supplier or installer.

How to make your deck estimate more useful

1. 01

   Measure the planned footprint

   Measure the length along the house and the deck depth. If the design is not rectangular, divide it into rectangles and estimate each section separately.

2. 02

   Match inputs to the materials you plan to buy

   Use the actual board width, length and planned joist spacing from your design or supplier. If you change board direction or layout, recalculate the board estimate.

3. 03

   Check the estimate before ordering

   Ask your supplier or installer to review the layout, cuts, fasteners and waste allowance. Confirm structural plans and local requirements with a qualified professional.

Common deck calculator mistakes

• Mixing up deck length and depth

  Enter the length along the house as the deck length and the outward dimension as the depth. The board-row formula uses the depth.

• Treating the estimate as a construction plan

  The calculator estimates quantities; it does not establish safe joist sizing, connections, footing details or code compliance. Have the design checked before building.

• Assuming the cost covers everything

  An installed price per square foot is only as complete as the price you enter. Confirm whether materials, labor, demolition, permits and site work are included in any quote.

The deck formula

Key terms

On-center spacing

The distance from the center of one joist to the center of the next.

Waste allowance

An added amount in the estimate for cuts and pieces that cannot be used.

Installed price per square foot

A price for completed deck work expressed for each square foot of deck area. Confirm what work the quoted price includes.