Floor Joist Calculator — Joist Count & Lumber

How the floor joist calculator works

Joist count = (floor width in inches ÷ spacing) + 1. Total joist lumber = joists × span length. Add rim board (2× floor width) and one row of mid-span blocking.

Frequently asked questions

How far can a 2×10 joist span?

Roughly 15–17 ft at 16 in OC for #2 SPF residential floor loads. Always confirm with your local code and lumber species tables.

What is joist blocking for?

Solid blocking between joists at mid-span prevents rotation and stiffens the floor, especially for spans over 8 ft.

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