Use the deck calculator for a material estimate
Start with the dimensions of one rectangular deck. Enter Deck length, Deck width, Decking board width, Gap between boards, Board length (stock size), Waste / overage %, and the framing values. The calculator returns estimated counts, deck area, converted inputs, assumptions, intermediate values, and a text result you can copy or download.
- Enter Deck length and Deck width, then choose the unit beside each value.
- Enter Decking board width, Gap between boards, Board length (stock size), and Waste / overage %.
- Enter Joist spacing (on-center), Number of beams, and Post spacing (on-center).
- Select Calculate to show the counts, deck area, formula, assumptions, and intermediate values.
- After a valid calculation, select Copy result or Download result (.txt). Select Reset to clear the inputs and hide the result.
How the calculator counts boards, joists, and posts
Decking boards run along Deck length, with rows placed across Deck width. The calculator rounds the row count up, applies Waste / overage % to the decking linear footage, then rounds the stock board count up. Joists span Deck width and are spaced along Deck length. Beams run along Deck length, and posts are spaced along each beam using the entered Number of beams.
Decking rows = ceil(Deck width x 12 / (Decking board width + Gap between boards))
Boards needed = ceil(rows x Deck length x (1 + Waste % / 100) / Board length)
Joists = ceil(Deck length x 12 / Joist spacing) + 1
Posts = Number of beams x (ceil(Deck length / Post spacing) + 1)For example, 20 ft by 12 ft with a 5.5 in board, 0.25 in gap, 12 ft Board length (stock size), 10% waste, 16 in Joist spacing (on-center), 2 beams, and 6 ft Post spacing (on-center) produces 48 decking boards, 16 joists, 10 posts, and 240 square feet of deck area.
Units and input limits
- Deck length and Deck width accept ft, in, yd, m, or cm.
- Decking board width and Gap between boards accept in or cm. Board length (stock size) and Post spacing (on-center) accept ft or m.
- Joist spacing (on-center) accepts in or cm. The script checks the entered numbers against these limits: Deck length and Deck width, 0.1-500; Decking board width, 0.5-24; Gap between boards, 0-2; Board length (stock size), 1-100; Waste / overage %, 0-50; Joist spacing (on-center), 4-48; and Post spacing (on-center), 0.5-20.
- Number of beams must be a whole number from 2 to 20. A blank, nonnumeric, nonfinite, or out-of-range value stops the calculation and shows an error.
Assumptions in the estimate
The model covers one rectangular deck. Decking runs the full Deck length, joists span the full Deck width, and beams run the full Deck length. Your Joist spacing (on-center), Post spacing (on-center), and Number of beams control the framing counts. The calculator does not look up span tables, building codes, lumber grades, supplier data, or material prices.
The estimate leaves out stairs, railings, picture-frame borders, cantilevers, ledger attachment, footing depth or size, and non-rectangular layouts. It is a quantity estimate, not certified structural advice.
When to use the estimate
Use a sketch's length and width to compare board, joist, beam, and post counts before checking prices.
A carpenter can enter framing values alongside the decking count.
Choose a construction calculator
Use Construction Calculators for another construction job. Choose Concrete Calculator for slab or footing volume, Lumber Calculator for board feet and cost, or Fence Calculator for fence length, posts, panels, and gates.
Frequently Asked Questions
How many decking boards does the calculator count?
The calculator rounds the decking rows across Deck width up, applies Waste / overage % to the total linear footage, then rounds the board count up to whole stock boards. A 20 ft by 12 ft example with 5.5 in boards, a 0.25 in gap, 12 ft stock boards, and 10% waste produces 48 boards.
How does the deck calculator count joists and posts?
It calculates Joists as ceil(Deck length x 12 / Joist spacing) + 1 and posts per beam as ceil(Deck length / Post spacing) + 1, then multiplies by Number of beams. The entered Number of beams is used as supplied.
What units can I use in the deck calculator?
Deck length and Deck width accept ft, in, yd, m, or cm. Decking board width, Gap between boards, and Joist spacing (on-center) accept in or cm, while Board length (stock size) and Post spacing (on-center) accept ft or m.
Does the deck calculator use building-code span tables?
No. Joist spacing (on-center), Post spacing (on-center), and Number of beams are values you enter, and the calculation uses those values without code, span-table, lumber-grade, supplier, or price lookups.