Deck framing calculator
Estimate the structural lumber needed for your deck frame. Calculate joists and rim joists for both attached and freestanding decks.
Note: These results are a planning aid to help you get started. Framing requirements vary by local building code, so please double-check joist spans and lumber sizes in your area before you buy materials or start construction.
How to calculate deck framing
- Enter your deck’s length and width. The frame is two end pieces, called rim joists, with a series of joists running between them. Joists run parallel to the deck width, so deck length determines how many you need.
- Pick a lumber thickness and length from the dropdowns, or type custom values that match what you have available.
- Set the joist spacing on center. 400 mm (16 inches) is a good default for most projects.
- The result shows your total joist count and the actual spacing between joists. Adjust the waste factor under Advanced options if needed.
Advanced options
Waste factor covers off-cuts and the occasional warped or damaged board you set aside. 10% is a sensible starting point for a straight framing job.
Common joist sizes
Joist depth is what gives a deck its stiffness. Most residential decks use 2×8 or 2×10 (roughly 45×195 mm or 45×220 mm in metric markets), with deeper joists for longer spans or heavier loads. Lumber is sold by nominal size, and the actual milled dimensions are slightly smaller, which your lumber yard will know. Always check a span table or your local code to confirm the size is right for your deck width and load.
Frequently asked questions
What is a rim joist and a regular joist?
The joists are the parallel boards that run across the deck and carry the load of the deck boards above. The rim joists are the perimeter boards along the ends of the joists. They tie everything together into a rigid box, define the outer edge of the deck, and give the regular joists something solid to attach to.
What is the standard joist spacing?
Most residential decks use 16 inches (400 mm) on center, which works for typical timber and composite decking. Tighter spacing of 12 inches (300 mm) is common for thinner boards, composite decking with manufacturer requirements, or heavier loads like a hot tub. Wider than 16 inches usually means noticeable flex underfoot.
How many joists do I need?
Divide your deck length by your chosen spacing and add one. For example, a 4.8 m deck with 400 mm spacing needs 13 joists. The calculator handles this automatically, including the perimeter boards.
What is the maximum span for deck joists?
It depends on the lumber size and species. As a rough guide, 2×8 joists at 16 inches on center can typically span around 12 feet (3.6 m). Wider spans need deeper lumber. Confirm against a span table for your specific lumber grade and species.
Can I use 2×6 for deck joists?
For very small decks or low-profile landings, yes. Most modern decks use at least 2×8 to allow for longer spans and better stiffness. Check your local span tables to be sure.