Simplified estimate with boards running along the length and joists running across the width. It does not include beams, posts, railings or stairs.
How to use this calculator
Enter the deck size, the width of your deck boards and the board length. Choose the joist spacing and press Calculate to see the number of deck boards, joists and screws.
The formula
Rows of boards = deck width ÷ (board width + 1/8 in gap)
Boards = rows × boards per row × (1 + waste %)
Joists = deck length ÷ joist spacing + 1
Screws = 2 per board at each joist
Example: A 12 ft × 10 ft deck with 5.5 in boards, 12 ft long, and joists 16 in apart needs 22 rows. With 10% waste that is 25 boards, 10 joists and about 440 screws.
Understanding your deck material estimate
The calculator counts the decking boards, joists and screws for the walking surface of a simple rectangular deck. It assumes boards run along the length and joists run across the width. Use it to build a shopping list for the deck surface, then plan the frame, footings and railings separately.
A worked example
A 12 by 10 foot deck with 5.5 inch boards and a 1/8 inch gap needs 22 rows of boards. Each row is one 12 foot board, so with 10% waste you buy 25 boards. Joists at 16 inches on center along the 12 foot length come to 10 joists. Two screws per board at each joist gives about 440 screws.
What changes the numbers
- Board length. Pick a length that matches the deck. A 14 foot deck built with 12 foot boards leaves joints and offcuts. A 16 foot board cut in half suits an 8 foot deck with almost no waste.
- Joist spacing. Closer joists at 12 inches add framing lumber and screws but give a stiffer floor.
- Board width. Wider boards mean fewer rows and fewer screws, but wide wood boards are more likely to cup.
- Layout. Diagonal boards and picture frame borders add cuts. Use 15% waste.
Tips and common mistakes
- Stagger board joints and land every board end on a joist or on solid blocking.
- Use fasteners rated for treated lumber, or the maker’s screws for composite. Ordinary screws can corrode.
- Check that the deck surface slopes slightly away from the house so water drains.
- Leave space under low decks for airflow. Trapped moisture shortens the life of wood.
- Seal cut ends of treated lumber with an end-cut preservative.
Cost and planning
Use your board count with a supplier’s price list to compare wood and composite. Composite boards usually cost more up front but need less upkeep. Add the frame lumber, beams, posts, concrete for footings, joist hangers, flashing, railings and stairs to get a full budget. A deck attached to the house carries real loads, and a failed ledger or footing can cause a collapse. Most areas require a permit and inspection, so check local building codes and consider having a licensed contractor review your plan.
