Deck Board & Joist Calculator
Estimate how many deck boards, joists and screws you need to build a deck of a given size.
Unit system
ft
ft
in
ft
in
in
Boards to buy (rounded up)
- 1
Board pitch
5.5 + 0.125 = 5.625Board face width plus drainage gap (both in in) - 2
Boards across deck width
144 ÷ 5.625 = 25.6 - 3
Rows along deck length
ceil(20 ÷ 16) = 2 - 4
Deck boards needed
ceil(25.6 × 2) = 52
How does this calculator work?
Multiply deck length by width to get the area, then divide the width by (board width + gap) and multiply by the number of board rows along the length to get boards. Joists equal length divided by spacing plus one, and screws are about two per joist crossed per board. Add 10% for waste.
Formula
How this is calculated
Enter the deck footprint (length and width) plus the size of one deck board (its face width and its length) and the gap you leave between boards for drainage and expansion. Board face width and gap are entered in the small unit (inches or millimetres) while the deck and board length use the large unit (feet or metres); the calculator converts internally so all comparisons are in one consistent unit.
The number of boards spanning the deck width is the deck width divided by the effective board pitch (board width + gap). Because each board only covers part of the deck length, this is multiplied by ceil(deckLength / boardLength) rows, and the total is rounded up to whole boards. Joists run perpendicular to the boards: their count is ceil(deckLength / joist spacing) + 1, where 16 in (400 mm) on-center is the common residential standard. Fasteners are estimated as two screws at every joist a board crosses, so screws = boards × joistsCrossed × 2.
Results are estimates for purchasing and do not include stairs, railings, picture-frame borders, blocking, or angled/diagonal layouts, all of which increase material. Always round generously and add roughly 10% extra for offcuts, defects and mistakes. Verify on-center spacing and span tables against your local building code before buying lumber.
Frequently asked questions
Add about 10% extra boards for cuts, defective pieces and future repairs. Diagonal or herringbone layouts can need 15% or more because of the longer angled cuts.
Most residential decks use 16 in (400 mm) on-center. Use 12 in for diagonal decking or thin/composite boards, and always confirm the span rating of your specific boards.
No. This estimates only the deck surface boards, joists and surface fasteners. Stairs, railings, posts, beams and blocking must be estimated separately.
Also known as
TG we-Calculate Editorial Team. (2026). Deck Board & Joist Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/deck-board-calculator
TG we-Calculate Editorial Team. "Deck Board & Joist Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/deck-board-calculator.
TG we-Calculate Editorial Team, "Deck Board & Joist Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/deck-board-calculator
@misc{wecalculate_deck_board_calculator, title = {Deck Board & Joist Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/deck-board-calculator}}, year = {2026}, note = {TG we-Calculate} }
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