Deck Calculator
Enter the deck size first, then set the board, layout, fastening, and optional price assumptions.
Deck board formula using actual width and gap
For straight boards, the coverage step is actual board width + gap. The calculator finds the whole number of rows needed across the dimension perpendicular to the boards, then multiplies the rows by the board run length. For example, 5.5 in boards with a 0.125 in gap advance 5.625 in per row.
Straight versus 45-degree deck layouts
A straight layout uses equal-length rows along the chosen deck dimension. For a 45-degree layout, the calculator projects the rectangular deck across the diagonal courses and estimates base linear feet from deck area ÷ effective coverage width. It then adds ordinary waste and the separate diagonal allowance to that same base. The diagonal allowance is not a structural factor or a guaranteed cut schedule.
Theoretical minimum and purchase estimate
The theoretical minimum is ceil(base linear feet ÷ stock board length). It assumes ideal cuts and complete offcut reuse. The adjusted estimate is ceil((base linear feet + ordinary waste linear feet + diagonal allowance linear feet) ÷ stock board length). Board lengths, butt-joint placement, picture framing, damaged material, grain selection, and unusable short offcuts can increase the actual order.
Fastener estimate only
For straight boards, the estimate counts attachment locations along each row from the joist spacing, including a terminal attachment allowance. At 45 degrees, it adjusts the approximate crossing distance for the angle. Face screws use two screws per estimated attachment; hidden fastening uses one clip per estimated attachment. Starter, finish, perimeter, breaker-board, and product-specific fasteners are outside this count.
Optional material cost
Enter a per-board price, a fastener pack size, and a pack price to see a priced subtotal. Either material can be left unpriced, and the result says when the subtotal is partial. Labor, taxes, delivery, tools, substructure, permits, railings, stairs, beams, and footings are not included.
Planning limits
This is a decking-surface estimate for one rectangle, not a framing, span, permit, or code calculator. Confirm actual board dimensions, expansion spacing, allowable joist spacing, fastening pattern, and installation details with the current product instructions and the appropriate project professional or authority.
Frequently asked questions
How many deck boards do I need?
Enter the rectangular deck dimensions, actual board width, gap, stock length, and board direction. The calculator shows a theoretical minimum before allowances and a whole-board count after your ordinary waste and any 45-degree cutting allowance.
Should I enter nominal or actual deck board width?
Enter the actual installed face width. A board sold under a nominal size can have a different actual width, and product dimensions can vary. The result shows actual width plus gap as the coverage step so you can audit the row count.
How much gap should I leave between deck boards?
Use the spacing required for your exact product, material condition, and installation conditions. Manufacturer instructions can account for material movement, drainage, temperature, and product-specific edge shapes; this calculator does not choose a universal gap for you.
Why are waste and the 45-degree allowance separate?
Ordinary waste covers general cuts, defects, and handling. A diagonal layout can create additional short offcuts at the perimeter, so the calculator lists a separate user-set 45-degree allowance instead of hiding both assumptions in one percentage.
Does joist spacing here check whether my framing is safe?
No. Joist spacing is used only to estimate how many board-to-joist attachment points may occur. This page does not design framing, verify spans, interpret building codes, or replace the decking and fastener manufacturers' installation instructions.