Floor Joist Span Chart: Max Span by Size & Spacing 2026

How far can a floor joist span? Read it off the table

A floor joist span is not a guess — it is a table lookup. The International Residential Code publishes the maximum span for each joist size at each spacing, for a stated species, grade and load. This chart is that table for Southern Pine No. 2 at 40 psf live load plus 10 psf dead load, L/360 deflection — the common case for a house floor. Find your joist size down the side, your spacing across the top, and read the span. On the defaults — a 2×10 at 16 inches on center — the maximum span is 14'0" (14.00 ft), the room needs 11 joists, each joist is 23.33 board feet, the total is 256.67 board feet, and the lumber costs $308.00 at $1.20 a board foot.

At the defaults — joist size 2×10, spacing 16 inches on center, room width 14 ft, lumber price $1.20 per board foot — the maximum span is 14'0" (14.00 ft), the joist count is 11, the board feet per joist are 23.33, the total board feet are 256.67, and the lumber cost is $308.00. Check the identities: maximum span = the IRC table value for the size and spacing; joist count = floor(room width × 12 / spacing) + 1; board feet per joist = (thickness × width × length) / 12, using nominal inches — a 2×10 is 2 in × 10 in; total board feet = board feet per joist × joist count; lumber cost = total board feet × price per board foot. The span does not change when the room gets wider — only the joist count and the board feet do. A 2×12 spans 1.77× a 2×6 at 16 inches on center (16.50 / 9.33 ft); 12-inch spacing adds 15.5% over 16-inch spacing for a 2×10 (16.17 / 14.00 ft); 24-inch spacing is 81.5% of the 16-inch span for a 2×10 (11.42 / 14.00 ft). Rounding convention: totals are computed from unrounded values, so rounded parts may differ by a cent from their rounded sum.

The worked example, line by line

Setup: joist size 2×10, spacing 16 inches on center, room width 14 ft, lumber price $1.20 per board foot.

Line 1 — the maximum span. Look up 2×10 at 16 inches on center in the IRC table: 14'0" (14.00 ft).

Line 2 — the joist count. floor(14 × 12 / 16) + 1 = floor(10.5) + 1 = 10 + 1 = 11 joists. The count is the number of spaces plus one — the joists at both ends.

Line 3 — the board feet per joist. 2 × 10 × 14 / 12 = 280 / 12 = 23.33 board feet. The thickness and width are the nominal inches — a 2×10 is 2 in × 10 in — and the length is the room width, 14 ft.

Line 4 — the total board feet. The unrounded 23.3333 board feet per joist × 11 joists = 256.67 board feet.

Line 5 — the lumber cost. The unrounded 256.6667 board feet × $1.20 = $308.00.

Reverse: Reverse: the smallest joist for a 12 ft span at 16 in o.c. is 2×10 (14'0"; the next smaller, a 2×8, spans 11'10" — 2 in short of 12 ft). For a 10 ft span at 16 in o.c. it is 2×8 (11'10"; the next smaller, a 2×6, spans 9'4"). For a 15 ft span at 16 in o.c. it is 2×12 (16'6"; the next smaller, a 2×10, spans 14'0"). For a 13 ft span at 12 in o.c. it is 2×8 (13'6"; the next smaller, a 2×6, spans 10'3"). For a 9 ft span at 24 in o.c. it is 2×8 (9'8"; the next smaller, a 2×6, spans 7'7"). For a 17 ft span at 12 in o.c. it is 2×12 (19'1"; the next smaller, a 2×10, spans 16'2"). A 20 ft span has no joist in the table — the longest is a 2×12 at 12 in o.c., 19'1" — so use an engineered I-joist / LVL or add a beam. 11 joists at 16 in o.c. cover 13.33 ft (10 spaces × 16 in = 160 in).

Every branch, checked

2×6 at 16 in o.c.: Max span 9'4" (9.33 ft), 11 joists, 14.00 bf/joist, 154.00 bf total, $184.80. A shallower joist spans less: 9 feet 4 inches against the 2×10's 14 feet — and it still needs 11 joists for the same room, because the count depends on the room and the spacing, not the size.

2×8 at 16 in o.c.: Max span 11'10" (11.83 ft), 11 joists, 18.67 bf/joist, 205.33 bf total, $246.40. A 2×8 reaches 11 feet 10 inches — 2 inches short of a 12-foot span, which is why the reverse below steps up to a 2×10.

2×12 at 16 in o.c.: Max span 16'6" (16.50 ft), 11 joists, 28.00 bf/joist, 308.00 bf total, $369.60. The deepest joist in the table spans 16 feet 6 inches — 1.77× the 2×6's span at the same spacing — and costs $61.60 more than the 2×10 for the room.

2×10 at 12 in o.c.: Max span 16'2" (16.17 ft), 15 joists, 23.33 bf/joist, 350.00 bf total, $420.00. Closer spacing buys span: 16 feet 2 inches instead of 14 feet — 15.5% more — but it also buys joists: 15 instead of 11, and the lumber costs $112.00 more.

2×10 at 24 in o.c.: Max span 11'5" (11.42 ft), 8 joists, 23.33 bf/joist, 186.67 bf total, $224.00. Wider spacing gives span back: 11 feet 5 inches, 81.5% of the 16-inch span — but only 8 joists, and the lumber costs $84.00 less.

Room 20 ft: Max span 14'0" (14.00 ft), 16 joists, 33.33 bf/joist, 533.33 bf total, $640.00. A wider room needs more joists and longer ones: 16 joists at 33.33 board feet each — the span stays 14'0" because the size and spacing did not change.

Room 10 ft: Max span 14'0" (14.00 ft), 8 joists, 16.67 bf/joist, 133.33 bf total, $160.00. A narrower room needs fewer, shorter joists: 8 joists at 16.67 board feet each — the span is still 14'0".

Price $1.50/bf: Max span 14'0" (14.00 ft), 11 joists, 23.33 bf/joist, 256.67 bf total, $385.00. The price changes nothing but the cost: the same 256.67 board feet at $1.50 a board foot cost $385.00, $77.00 more than at $1.20.

Reverse — the other way round: the smallest joist for a 12 ft span at 16 in o.c. is 2×10; for 10 ft it is 2×8; for 15 ft it is 2×12; for 13 ft at 12 in o.c. it is 2×8; for 9 ft at 24 in o.c. it is 2×8; for 17 ft at 12 in o.c. it is 2×12; no joist spans 20 ft; 11 joists at 16 in o.c. cover 13.33 ft.

What the chart assumes

The chart assumes Southern Pine No. 2, 40 psf live load plus 10 psf dead load, and L/360 deflection — the IRC Table R502.3.1(2) case. Change any of those and the spans change: a different species or grade, a heavier load, or a stricter deflection limit all move the numbers. The spans are code-table maximums, not a design: a span at the maximum is allowed by the table, but the floor still has to be built right — blocking, bearing and connections are outside this chart. This is not a deck joist chart — outdoor decks use a different table with different loads and exposure, so see the deck joist span chart for those. It is not an engineered-I-joist or LVL chart — those products have their own span tables from the manufacturer. It is not a rafter chart — rafters carry roof loads at a slope, and their spans are different. And it is not a structural design: confirm the spans with your local code and an engineer for anything outside the table, especially a span near the maximum or a load above 40 psf. With a 1 ft room the count is 1 joist (floor(12/16) + 1 = 1). The shortest span in the table is a 2×6 at 24 inches on center: 7'7" (7.58 ft); the longest is a 2×12 at 12 inches on center: 19'1" (19.08 ft). Rounding convention: totals are computed from unrounded values, so rounded parts may differ by a cent from their rounded sum; the reverses are nearest-value solutions, and the exact unrounded figure (160 in ÷ 12 = 13.3333 ft, shown as 13.33 ft) is stated wherever the rounded figure appears.

Output · formula · meaning

Who uses this

• The homeowner planning a new floor: they learn a 2×10 at 16 inches on center spans 14 feet — so they know the size they need for a 14-foot room, and they check the 2×8 branch (11'10") before ordering a size that falls 2 inches short of 12 feet.
• The DIYer counting joists: they learn a 14-foot room needs 11 joists at 16 inches on center — so they know how many to buy, and they check the 12-inch branch (15 joists) before budgeting for closer spacing.
• The builder pricing lumber: they learn 256.67 board feet costs $308.00 at $1.20 a board foot — so they know what the floor costs, and they check the $1.50 branch ($385.00) before quoting at today’s price.
• The renovator with a 20-foot span: they learn no solid-sawn joist in the table spans 20 feet — the longest is a 2×12 at 12 inches on center, 19'1" — so they know to price an engineered I-joist or LVL, or add a beam, and they check the 17-foot reverse (2×12, 19'1") before assuming a shorter span is fine.

Accuracy & limitations

The math is exact for the numbers you enter: joist count = floor(room width × 12 / spacing) + 1; board feet per joist = (thickness × width × length) / 12 in nominal inches; total board feet = board feet per joist × joist count; lumber cost = total board feet × price per board foot. The spans come from IRC Table R502.3.1(2) for Southern Pine No. 2 at 40 psf live load plus 10 psf dead load, L/360 — the table values are exact to the inch (14'0", 11'10", 19'1") and the decimal feet are the same spans divided by 12 (14.00, 11.83, 19.08 ft). What the chart cannot do is design your floor: check the span against your local code, because the table is a maximum and your floor still needs proper bearing, blocking and connections — and an engineer for anything outside the table. It assumes the load is 40 psf live plus 10 psf dead and leaves out heavier loads, cantilevers and engineered lumber. It is a span reference, not a structural design: for outdoor decks, see the deck joist span chart; for board feet on any board, see the board foot calculator; for concrete work on the same project, see the concrete driveway cost calculator.