Floor Joist Calculator
Check floor joist span against the IRC tables by species, spacing and room use, and count the joists.
By Konstantin Iakovlev · Updated September 2026 · Source: International Code Council, 2021 IRC Section R502.3
Max Span
15'-5"
Span Check
PASS
Joists Needed
10
Joist Details
| Max Allowable Span | 15'-5" |
| Your Span | 14'-0" |
| Smallest Size That Passes | 2x10 at 16" OC |
| Design Load | 40 psf live, 10 psf dead |
| Joists Needed | 10 |
| Total Linear Feet | 140.0 LF |
Spans from 2021 IRC Tables R502.3.1(1) and R502.3.1(2) (same values in 2018 and 2024): No. 2 grade, 10 psf dead load, deflection limited to L/360. Tile or stone floors, hot tubs and point loads need larger joists or a design. The joist count adds one for the starting end; add rim joists, doubled joists at openings and waste.
Use the Floor Joist Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.
Disclaimer: This calculator is for informational purposes only and does not constitute tax, financial, or legal advice. Results are estimates based on the information you provide and current rates. Always consult a qualified tax professional or financial advisor for advice specific to your situation.
How It Works
This calculator checks a floor joist against the maximum span allowed by the International Residential Code (IRC) and counts how many joists the floor needs. Choose the lumber size, spacing, species and whether the room is a bedroom or a living area; it looks up the code span, tells you whether your span passes, and suggests the smallest size that works at your spacing.
Spans come from 2021 IRC Table R502.3.1(2) for living areas, designed for a 40 psf live load, and Table R502.3.1(1) for sleeping rooms and attics reached by a fixed stair, designed for 30 psf. Both use a 10 psf dead load, No. 2 grade lumber and a deflection limit of L/360, and the 2018 and 2024 editions print the same spans. Species matters: at 16 inches on center a 2x10 spans 15'-7" in Douglas fir-larch but 14'-0" in Southern pine.
The joist count divides the room width by the spacing, rounds up and adds one joist for the starting end, and total linear feet multiplies that count by the span. Rim joists, doubled joists around stair openings and under parallel walls, and a waste allowance come on top. Tile or stone floors, hot tubs and other heavy loads push the dead load past 10 psf and need the 20 psf columns of the code table or a design.
Example: 13'-6" living room span in spruce-pine-fir
- 1 Input: 2x10 spruce-pine-fir No. 2 joists at 16 inches on center in a living area, a clear span of 13'-6", laid out along a 16-foot wall.
- 2 Span check: Table R502.3.1(2) allows 15'-5" for this size, species and spacing, so 13'-6" passes. A 2x8, allowed 12'-3", would fail.
- 3 Joist count: 16 feet is 192 inches; 192 ÷ 16 = 12, plus 1 for the starting end = 13 joists. Total lumber: 13 × 13.5 feet = 175.5 linear feet.
- 4 Bedroom instead: at the 30 psf design load of Table R502.3.1(1), the same spruce-pine-fir 2x8 spans exactly 13'-6", so it would pass over a bedroom.
Source: International Code Council, 2021 IRC Section R502.3 · Last updated: September 2026
Frequently Asked Questions
What is the standard floor joist spacing?
How far can 2x10 floor joists span?
How many joists do I need for a floor?
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