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Floor joist span calculator

Find how far a 2×6, 2×8, 2×10 or 2×12 floor joist can span for your lumber species and grade, joist spacing and room use — on the same basis as the IRC floor joist span tables — and how many joists your floor needs.

1The joist

Stamped on every board, e.g. DF-L No. 2, SPF No. 2 or SYP No. 2. No. 2 is what most lumber yards sell.

Joist size
Spacing (on center)

From the center of one joist to the next. 16 in is standard; 19.2 in divides an 8 ft sheet into five equal spaces.

2The floor

10 psf: a wood or vinyl floor with no ceiling or a light one below. 20 psf: a drywall or plaster ceiling hung below, or a tile floor.

3Your floor

The clear distance between the supports, inside face to inside face.

Gives the number of joists at your spacing.

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  • The span you need

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How far can a floor joist span? Using the calculator

  1. 1Pick the species and grade stamped on the lumber, the joist size and the spacing you plan.
  2. 2Choose the room use: 40 psf for living areas, 30 psf for sleeping rooms, as in the IRC tables.
  3. 3Choose the dead load: 10 psf for a light floor with no ceiling below, 20 psf with a drywall ceiling hung below or a tile floor.
  4. 4Enter the span you need between the supports, and the floor width if you want the number of joists. Read the maximum span, whether yours fits, and the smallest size that does.

How joist spans are worked out (bending and L/360)

  1. 01Load per inch of joist w = load (psf) × spacing (in) ÷ 144
  2. 02Bending: span = √(8 × Fb × CF × 1.15 × S ÷ w total)
  3. 03Stiffness: span = ∛(384 × E × I ÷ (5 × w live × 360))
  4. 04Maximum span = the shorter of the two, to the nearest inch, never over 26 ft
  5. 05Joists across a floor = width ÷ spacing, rounded up, + 1

A floor joist must do two things: not break and not bounce. Strength is the bending check — the moment from the floor load, w × span² ÷ 8, must stay within the lumber's bending value Fb times its section modulus S (1.5 × depth² ÷ 6 for a 2× joist). Fb comes from the grading rules, multiplied by a size factor CF and by 1.15 because joists share the load with their neighbors through the subfloor. Stiffness is the deflection check — under the live load the joist may sag at most span ÷ 360, using the lumber's modulus of elasticity E and the moment of inertia I (1.5 × depth³ ÷ 12).

This is how the American Wood Council works out the span tables the IRC prints. The calculator's results were checked against 344 published spans for Douglas fir-larch, hem-fir, spruce-pine-fir and Southern pine, and every one matches to the inch. Where editions of the design values differ, it uses the value that reproduces the IRC table, which is the more cautious one.

Floor joist span example

A No. 2 Douglas fir-larch 2×10 at 16 in on center in a living area (40 psf live, 10 psf dead load) spans up to 15' 7". Strength sets the limit; stiffness alone would allow 16' 4 9/16".

A 14 ft span therefore fits, and a floor 12 ft wide needs 10 joists at 16 in.

Floor joist span table at 16 in on center

Maximum floor joist spans at 16 in on center (40 psf live, 10 psf dead load)
Species and grade2×62×82×102×12
Douglas fir-larch, Select Structural10′ 4″ (3.15 m)13′ 7″ (4.14 m)17′ 4″ (5.28 m)21′ 1″ (6.43 m)
Douglas fir-larch No. 19′ 11″ (3.02 m)13′ 1″ (3.99 m)16′ 5″ (5 m)19′ 1″ (5.82 m)
Douglas fir-larch No. 29′ 9″ (2.97 m)12′ 9″ (3.89 m)15′ 7″ (4.75 m)18′ 1″ (5.51 m)
Hem-fir, Select Structural9′ 9″ (2.97 m)12′ 10″ (3.91 m)16′ 5″ (5 m)19′ 11″ (6.07 m)
Hem-fir No. 19′ 6″ (2.9 m)12′ 7″ (3.84 m)16′ 0″ (4.88 m)18′ 7″ (5.66 m)
Hem-fir No. 29′ 1″ (2.77 m)12′ 0″ (3.66 m)15′ 2″ (4.62 m)17′ 7″ (5.36 m)
Southern pine, Dense Select Structural10′ 4″ (3.15 m)13′ 7″ (4.14 m)17′ 4″ (5.28 m)21′ 1″ (6.43 m)
Southern pine No. 19′ 9″ (2.97 m)12′ 10″ (3.91 m)16′ 1″ (4.9 m)19′ 1″ (5.82 m)
Southern pine No. 29′ 4″ (2.84 m)11′ 10″ (3.61 m)14′ 0″ (4.27 m)16′ 6″ (5.03 m)
Spruce-pine-fir, Select Structural9′ 6″ (2.9 m)12′ 7″ (3.84 m)16′ 0″ (4.88 m)19′ 6″ (5.94 m)
Spruce-pine-fir No. 1 / No. 29′ 4″ (2.84 m)12′ 3″ (3.73 m)15′ 5″ (4.7 m)17′ 10″ (5.44 m)

From the calculator's own method, checked against the IRC and Southern Pine span tables. With a ceiling hung below or a tile floor (20 psf dead load), or at a wider spacing, the spans are shorter — use the calculator above.

Tips before you buy

  • Read the grade stamp: "DF-L No. 2" or "SPF No. 1/No. 2" tells you the species group and grade. Unstamped lumber should not be used for floor joists.
  • If a span is close to the limit, go one size up or move the joists closer. A stiffer floor feels better and costs little more.
  • Span is the clear distance between supports. The joist itself is longer: add the bearing at each end.
  • Walls, bathtubs, water heaters and kitchen islands are point loads. Double the joists under them or have them checked.
  • Notches and holes weaken joists. The IRC limits them (R502.8): no notches in the middle third, holes no closer than 2 in to the edges.

Frequently asked questions

How far can a 2×10 floor joist span?

At 16 in on center in a living area (40 psf live, 10 psf dead load), a No. 2 Douglas fir-larch 2×10 spans 15 ft 7 in, Southern pine No. 2 14 ft 0 in, spruce-pine-fir No. 1/No. 2 15 ft 5 in and hem-fir No. 2 15 ft 2 in. At 12 in on center it spans about 2 ft more; at 24 in about 3 ft less.

How far can a 2×8 floor joist span?

About 12 ft 9 in for Douglas fir-larch No. 2 at 16 in on center in a living area, 14 ft 2 in at 12 in on center and 10 ft 5 in at 24 in. With a ceiling hung below (20 psf dead load) the 16 in figure drops to 11 ft 8 in.

What spacing should floor joists have?

16 in on center is the usual choice: it suits 4 × 8 ft subfloor sheets and most flooring. 12 in gives a longer span and a stiffer floor; 19.2 in (five spaces to 8 ft) and 24 in save lumber but need a thicker subfloor and shorter spans.

What do live load and dead load mean?

Live load is what moves: people and furniture. The IRC uses 40 psf for living areas and 30 psf for sleeping rooms. Dead load is the floor's own weight: joists, subfloor, finish and anything hung below. 10 psf covers a light floor; 20 psf covers a drywall ceiling below or a tile floor.

How many floor joists do I need?

Divide the floor width by the spacing, round up and add one for the end joist. A 12 ft floor at 16 in on center needs 144 ÷ 16 = 9 spaces, so 10 joists — plus rim joists around the edge and doubled joists at openings.

Can I use this for a deck?

Deck joists are outdoors and wet, so they use lower, wet-service values and the deck tables in AWC DCA 6 or IRC R507. Use this calculator for indoor floors only, and the deck calculator for the boards and joists of a deck.

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Sources

Formulas last reviewed: September 30, 2026 ·How we check our formulas

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