How this calculator works
The number of spaces across the floor is rounded up, then one edge joist is added. Two conceptual rim pieces close the ends. Floor pressure times on-centre spacing gives the interior-joist line load. Span mode uses actual dimensions and final adjusted design values. Openings, blocking, bridging, rim splices, bearing and connections are not taken off.
Formula
Joists = ceil(floor width/spacing)+1; line load = floor pressure x spacing + additional self-weight; I=bh^3/12. Limiting span is the minimum from bending, shear and delta<=L/r. Cost = purchased joists x joist price + 2 x rim piece price.
Worked example
A floor 4 m in the span direction and 5 m across at 400 mm spacing needs ceil(5/0.4)+1 = 14 span joists. With no extra allowance, $35 per joist and two $40 rim pieces, material cost is $570.
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Frequently asked questions
Are the two edge joists counted?
Yes. The span joist count includes an edge joist at each side. Do not add them again.
Does one rim piece have to be the entire floor width?
The takeoff treats each end as one conceptual piece. Divide it into purchasable lengths and design any splices separately.
Can I use the span directly for construction?
No. Results are estimates and are not a substitute for a licensed engineer. Bearing, connections, stability, local code requirements and all applicable load combinations need separate checks.