Estimate floor or deck joist counts from user-supplied dimensions and on-center spacing, with continuous stock pieces, rim boards, blocking, waste, linear footage, and cost.
The joist run is the direction each joist travels; framing width is the dimension across which joists are spaced. The quantity layout starts at one edge, treats the entered spacing as a user-supplied limit, and adds an ending edge joist.
This is a material takeoff only. The entered run, spacing, and member dimensions must come from the project design or applicable requirements; the calculator does not select a structurally adequate joist.
ceiling(framing width ÷ user-entered spacing) + 1ceiling(base joists × (1 + waste rate))2 × framing width(joists per area − 1) × blocking rows × area quantityA 20 × 12 ft platform with joists running 20 ft at a user-entered 16 in OC spacing has 9 spaces and 10 base joists. With 10% extra, buy eleven 20-ft joists.
A 13-ft framing width at a user-entered 16 in OC spacing needs 10 spaces and 11 joists; nine full 16-in bays are followed by a 12-in final bay.
A 6 m framing width at a user-entered 400 mm OC spacing needs 15 spaces and 16 base joists. With 10% extra, buy 18 joists.
Running along project length makes length the joist run and width the framing width; switching direction reverses those roles.
Each joist consumes one full-length stock member. Offcuts are not pooled to create other joists.
Blocking length is spacing minus entered actual joist thickness and remains an approximation.
The calculator layout keeps the full user-entered spacing from the first edge and shortens only the final bay when needed. This arithmetic does not establish allowable spacing.
Stock shorter than the required run is rejected because automatic splicing would be a structural assumption.
Two-side and full-perimeter options estimate material length only; they are not framing prescriptions.
Published joist span guidance depends on factors such as loads, species, grade, member size, spacing, bearing, and installation assumptions. This calculator does not evaluate those factors or determine allowable span, structural capacity, or code compliance.
Divide framing width by the user-entered spacing, round spaces upward, and add one joist so both edges are included.
Using 16 inches as the project-supplied spacing, there are nine spaces and ten edge-to-edge joists.
Direction determines which project dimension is the joist run and which is the framing width used for spacing.
The final bay is shorter than the user-entered spacing.
Choose two sides, full perimeter, or a custom material length, then provide rim stock length.
Each row has approximately one block between adjacent joists. Optional length uses spacing minus actual joist thickness.
No. Those are structural-design decisions that depend on project-specific criteria.
No. Verify the project with applicable requirements or a qualified professional.