onlinecalculator.me

Free online deck calculator

A deck material takeoff counts every decking board, joist, and fastener before you head to the lumber yard. Enter your deck dimensions and spacing to get a full materials list in.

Posts and beam sizing depend on local code and load requirements — consult a structural engineer.

About this calculator

How to use

  1. Enter the deck length and width in feet.
  2. Set the board face width — 5.5 in for standard 2×6 decking, 3.5 in for 2×4.
  3. Choose the board length you plan to purchase (16 ft is the most common).
  4. Select your joist spacing (16 in standard, 12 in for composite).
  5. Set a waste factor — 10% is typical for a rectangular deck.
  6. Read the board count, joist count, and fastener estimate.

Formulas

rows across width   = ceil(deckWidth × 12 / boardWidth)
decking linear ft   = rows × deckLength × (1 + waste% / 100)
decking boards      = ceil(decking linear ft / boardLength)

joists needed       = ceil(deckLength × 12 / joistSpacing) + 1
joist linear ft     = joists × deckWidth

rim joist linear ft = 2 × deckLength + 2 × deckWidth

screws (lbs)        = deckArea / 100 × 350 / 100

Worked example

16 ft × 12 ft deck, 5.5-in boards, 16-ft boards, 16-in joists, 10% waste:

rows across width   = ceil(12 × 12 / 5.5) = ceil(26.18) = 27
raw linear ft       = 27 × 16 = 432 ft
decking linear ft   = 432 × 1.10 = 475.2 ft
decking boards      = ceil(475.2 / 16) = 30 boards

joists needed       = ceil(16 × 12 / 16) + 1 = 12 + 1 = 13
joist linear ft     = 13 × 12 = 156 lin ft

rim joist linear ft = 2 × 16 + 2 × 12 = 56 lin ft

deck area           = 192 sq ft
screws              = 192 / 100 × 3.5 = 6.72 lbs ≈ 7 lbs

Notes

  • The calculator does not size posts, beams, or footings — those depend on local code, load, and soil. Consult a structural engineer or your building department before framing.
  • Add 5–10% to joist count for blocking between joists (required at mid-span for spans over 8 ft).
  • Composite decking brands vary in width and gap requirements — check your manufacturer’s installation guide before ordering.

Frequently asked

How many decking boards do I need?
The calculator divides the deck width by the board face width (including gaps) to find the number of rows, then multiplies by deck length and divides by board length. A 10% waste factor is added by default for cuts and defects.
What is a standard joist spacing for a deck?
16-inch on-center is the most common residential deck joist spacing. It works with 5/4-in or 2-in decking. For composite decking, check the manufacturer spec — some require 12-in spacing to prevent sagging.
What are rim joists?
Rim joists (also called band joists) are the perimeter framing members that cap the ends of the floor joists. They complete the frame, provide lateral support, and give you something to attach deck boards and fascia to. The calculator returns total linear footage for all four sides.
How many screws do I need for a deck?
A common rule of thumb is about 350 screws per 100 sq ft of deck surface. That works out to roughly 3.5 lbs of screws per 100 sq ft (approximately 100 screws per lb). Buy a bit extra — a few will strip or miss.
Does the calculator size posts and beams?
No. Post and beam sizing depends on span, load, wood species, and local code. The calculator notes that a structural engineer or your local building department should determine those members.
How do I share my deck estimate?
Click Share to copy the page link, or Share with my numbers to encode your dimensions in the URL so anyone can reproduce your estimate.