Floor joist span calculator: how the span is worked out
A joist's allowable span is the shortest of three limits: bending stress, deflection and shear. This is the method behind the IRC floor joist tables and the American Wood Council span tables[5]:
- Bending uses the live plus dead load and the adjusted bending value Fb′ = Fb × size factor × 1.15 for repetitive members[1][2].
- Deflection uses the live load only, limited to span ÷ 360 for floors[3]. It governs some joists, such as the 2x8 spruce-pine-fir below; bending governs the other three.
- Shear rarely controls sawn joists at normal spans.
Spans are clear spans, face of support to face of support[5]. Reference design values come from the 2018 NDS Supplement, Tables 4A and 4B; the 2024 Supplement has the same values for these grades[1].
Check against the IRC tables
The calculator reproduces IRC Table R502.3.1 entries to the inch[3][4]:
| Joist | Spacing | Live load | IRC table | Calculator |
|---|---|---|---|---|
| 2x10 Douglas fir-larch No.2 | 16 in | 40 psf | 15-7 | 15 ft 7 in |
| 2x8 Spruce-pine-fir No.2 | 16 in | 40 psf | 12-3 | 12 ft 3 in |
| 2x12 Southern pine No.2 | 16 in | 40 psf | 16-6 | 16 ft 6 in |
| 2x10 Hem-fir No.2 | 24 in | 30 psf | 13-10 | 13 ft 10 in |
Loads and deflection limits
Use 40 psf live load for living areas and decks and 30 psf for sleeping rooms, from IRC Table R301.5; floors are limited to L/360 deflection[3]. Use 20 psf dead load under tile or other heavy finishes, and choose L/480 if the floor will carry large-format tile and you want it stiffer.
| Species and grade | Fb | Fv | E |
|---|---|---|---|
| Douglas fir-larch, Select Structural | 1500 (× size factor) | 180 | 1.9 M |
| Douglas fir-larch, No.1 | 1000 (× size factor) | 180 | 1.7 M |
| Douglas fir-larch, No.2 | 900 (× size factor) | 180 | 1.6 M |
| Hem-fir, Select Structural | 1400 (× size factor) | 150 | 1.6 M |
| Hem-fir, No.1 | 975 (× size factor) | 150 | 1.5 M |
| Hem-fir, No.2 | 850 (× size factor) | 150 | 1.3 M |
| Spruce-pine-fir, Select Structural | 1250 (× size factor) | 135 | 1.5 M |
| Spruce-pine-fir, No.1/No.2 | 875 (× size factor) | 135 | 1.4 M |
| Southern pine, Select Structural | 1950 / 1700 / 1600 (2x8 / 2x10 / 2x12) | 175 | 1.8 M |
| Southern pine, No.1 | 1250 / 1050 / 1000 (2x8 / 2x10 / 2x12) | 175 | 1.6 M |
| Southern pine, No.2 | 925 / 800 / 750 (2x8 / 2x10 / 2x12) | 175 | 1.4 M |
Joist calculator: how many joists
The number of joists is the floor width divided by the spacing, rounded up, plus one: a 20 ft floor at 16 in centres takes 240 ÷ 16 = 15, + 1 = 16 joists, plus a rim joist across each end of the joists. Each joist is as long as the span plus its bearing at both ends. The deck framing calculator counts joists, hangers and blocking for a deck.
Deck joists and wet service
Deck joists stay wet, so their design values are reduced: tick Wet service to apply the NDS wet-service factors (0.85 on Fb above 1,150 psi, 0.97 on Fv, 0.9 on E)[2]. AWC's deck guide, DCA 6, is built on 40 psf live plus 10 psf dead load and limits joist overhangs to a quarter of the back span[9]. The deck framing calculator lays out the joists, beam and posts.
I-joist span calculator: use the maker's tables
I-joists are proprietary: their spans come from the manufacturer's literature or code evaluation report, not from sawn-lumber design values[7]. APA's PRI-400 standard gives a span system for APA-rated residential I-joists[8]. Use the maker's table for I-joists, LVL and trusses, and this calculator for 2x8 to 2x12 sawn lumber. For a quick count of the joists themselves, the deck calculator and the lumber calculator total the pieces.
Planning numbersThe AWC span calculator and the code tables your building department adopted are the reference. Notched joists, point loads and cantilevers need more than this check.
Frequently asked questions
How far can a 2x10 floor joist span?
A No. 2 Douglas fir-larch 2x10 at 16 in spans 15 ft 7 in at 40/10 psf; at 24 in it spans 12 ft 9 in.
How far can a 2x8 joist span?
A No. 2 spruce-pine-fir 2x8 at 16 in spans 12 ft 3 in under 40 psf live load.
What is the span of a 2x12 floor joist?
A No. 2 southern pine 2x12 at 16 in spans 16 ft 6 in under 40/10 psf.
Is joist span measured centre to centre?
No. The span tables use the clear span between the faces of the supports.
Sources
- 2018 NDS Supplement: Design Values for Wood Construction, Chapter 4 (Tables 4A and 4B)
- 2018 National Design Specification for Wood Construction, Chapter 4: Sawn Lumber (adjustment factors)
- 2021 International Residential Code, Chapter 5: Floors (R502 joists, R507 decks)
- 2024 International Residential Code, Chapter 5: Floors (R502.1.2, R502.3, R507.6)
- Span Tables for Joists and Rafters, 2024 Edition
- Span Options Calculator for Wood Joists and Rafters
- 2018 National Design Specification for Wood Construction, Chapter 7: Prefabricated Wood I-Joists
- APA PRI-400 Performance Standard for Residential I-Joists
- DCA 6: Prescriptive Residential Wood Deck Construction Guide (2015 IRC basis)