Design / Structural
Floor Joist Span Calculator
Check allowable spans for floor joists by species, grade, spacing, and deflection limit.
Calculator
2x6 / 2x8 / 2x10 / 2x12
12, 16, or 24
L/360 / L/480 / L/240
What makes this different
Span tables are usually static PDFs. This one reproduces the IRC span table logic for the most common lumber species (DF-L, Hem-Fir, SPF, Southern Pine) and lets you pick between L/360 (live load), L/240, or stricter limits used for tile or stone.
Who it's for & when to use it
Designing a deck floor, a shed floor, or a renovation with new joists; sanity-checking a contractor's plan.
How to use
- Enter the dimensions of your material or the parameters of your project.
- Adjust the optional knobs (waste factor, kerf, planer allowance) to match your shop practice.
- Click Calculate and read the output. Use the per-item breakdown for ordering, the totals for budgeting.
- Compare the result to your lumber yard's quote — the effective $/BF column is the apples-to-apples number.
Formula: Allowable span = function of F_b, E, spacing, and deflection criterion. Values from IRC Table R502.3.1(1)/(2).
Notes & tips from the shop
- Span tables assume simple span (one continuous member over two supports). Multi-span rules differ.
- Joist depth matters more than width — a 2×10 spans much further than a 2×8.
- Always check both bending and deflection; the smaller of the two governs.
Frequently asked questions
Can I increase the span by sistering joists?
Yes — sistered joists roughly double capacity if properly connected. Stagger the joints.
What's the difference between L/360 and L/240?
L/360 is the standard for floor live load (people, furniture). L/240 is allowed for roof loads. L/480 is used for stone/tile floors.
Does the table I have at home match yours?
Tables are updated with each code cycle. Confirm with your jurisdiction's adopted code and a licensed engineer for anything outside a simple shed.
Limitations
Single span, dry service condition, normal duration of load. Not for industrial loads, heavy equipment, or wet conditions.
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