Floor Joist Calculator

Find the smallest floor joist size that spans your floor from the IRC span tables by species, spacing and live load, plus the number of joists, lumber and rim joist.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To size floor joists, look up the allowable span for each joist size in the IRC table for your species, spacing and live load, and pick the smallest size whose span meets or exceeds yours; the number of joists is the floor length in inches divided by the spacing, plus one. A 12 ft span at 16 in on center in Douglas fir-larch #2 with a 40 psf living-area load needs 2x8 joists (12 ft 9 in allowed), and a 20 ft long floor takes 16 joists, 192 ft of joist lumber and 40 ft of rim joist.

Results

Joist size: 2x8; Douglas fir-larch #2 at 16 in on center spans up to 12 ft 9 in (2018/2021 IRC Table R502.3.1(2), 40 psf live, 10 psf dead)
Number of joists: 16 joists; 20 ft of floor / 16 in spacing + 1
Joist lumber: 192 ft; 16 pieces at 14 ft; joists that rest on a plate or beam need at least 1.5 in of bearing at each end
Rim joist: 40 ft; Same depth as the joists, one along each end of the joist run (2 x 20 ft)
Joist hangers: 16 hangers; Only if the joists hang from a ledger or the face of a beam; use hangers sized for 2x8
Important: Planning estimate; Spans assume #2 grade, uniform loads and no point loads, bearing walls or tubs above. A structural engineer, the local code and the inspector govern.

Floor Joist formula

Joist size = smallest of 2x6, 2x8, 2x10, 2x12 whose IRC allowable span >= joist span
Joists = Floor length (in) / spacing (in), rounded up, + 1
Joist lumber = Joists x span;  Rim joist = 2 x floor length
Hangers = Joists  (only when joists hang from a ledger or beam face)

Worked example

A 14 ft span at 16 in on center in Southern pine #2 for a living area (40 psf): a 2x8 spans only 11 ft 10 in, but a 2x10 spans 14 ft 0 in, so 2x10 is the smallest size that works. For a 24 ft long floor: 24 x 12 / 16 = 18, plus 1 = 19 joists, or 19 x 14 = 266 ft of 2x10, plus 2 x 24 = 48 ft of rim joist.

The same 14 ft span framed as a bedroom floor (30 psf) at 24 in on center: a 2x10 in Southern pine #2 spans 12 ft 10 in, not enough, and a 2x12 spans 15 ft 1 in, so 2x12 joists at 24 in. Joists = 24 x 12 / 24 + 1 = 13.

Quick reference

Maximum floor joist spans, #2 grade, 40 psf live load, 10 psf dead load (2018/2021 IRC Table R502.3.1(2))
Size	Spacing	Douglas fir-larch	Hem-fir	Southern pine	Spruce-pine-fir
2x6	16 in	9 ft 9 in	9 ft 1 in	9 ft 4 in	9 ft 4 in
2x8	12 in	14 ft 2 in	13 ft 2 in	13 ft 6 in	13 ft 6 in
2x8	16 in	12 ft 9 in	12 ft 0 in	11 ft 10 in	12 ft 3 in
2x10	12 in	18 ft 0 in	16 ft 10 in	16 ft 2 in	17 ft 3 in
2x10	16 in	15 ft 7 in	15 ft 2 in	14 ft 0 in	15 ft 5 in
2x10	24 in	12 ft 9 in	12 ft 5 in	11 ft 5 in	12 ft 7 in
2x12	12 in	20 ft 11 in	20 ft 4 in	19 ft 1 in	20 ft 7 in
2x12	16 in	18 ft 1 in	17 ft 7 in	16 ft 6 in	17 ft 10 in

Reading the IRC joist span tables

The IRC publishes two floor joist tables: R502.3.1(1) for sleeping areas and attics at 30 psf live load, and R502.3.1(2) for all other rooms at 40 psf. Each lists the maximum clear span for every common species, grade, size and spacing, with columns for 10 and 20 psf dead loads. This calculator uses the 10 psf dead load columns for #2 grade, which covers ordinary subfloor, finish flooring and drywall ceilings; a tile floor, a thick lightweight concrete topping or a plaster ceiling pushes you toward the 20 psf columns.

Measure the clear span between the inside faces of the supports, not the length of the board. If the number sits right at the table limit, go up one size or tighten the spacing: the tables allow a deflection of span / 360, which feels springy under a large room, and many framers design to L/480 for hardwood or tile floors.

Layout and lumber

Joists are laid out from one end of the supporting wall at 12, 16 or 24 inches on center so 4 x 8 subfloor sheets break on a joist; the count is the floor length divided by the spacing, rounded up, plus one for the end. Buy joists in the next standard length above the span, and remember that joists resting on a sill plate or beam need at least 1.5 inches of bearing at each end while joists in hangers can be cut to the exact clear span.

A rim joist of the same depth closes both ends of the joist run and carries the wall above; blocking or bridging at mid-span is required when joists are deeper than 2x12 and is good practice on long 2x10 and 2x12 spans. Double the joists under parallel partitions and around stair openings, and treat any bearing wall, tub or point load above as a job for an engineer rather than a span table.

Key facts

The IRC sets residential floor live loads at 40 psf for living areas and 30 psf for sleeping areas, with a 10 psf dead load in the standard span tables.
Floor joist spans are limited to L/360 deflection, so a 12 ft joist may sag no more than 0.4 in under live load.
A 2x8 Douglas fir-larch #2 joist at 16 in on center spans 12 ft 9 in; a 2x10 spans 15 ft 7 in; a 2x12 spans 18 ft 1 in.
The 2018 and 2021 IRC span tables R502.3.1(1) and (2) contain the same values; they changed in 2015 when Southern pine design values were reduced.
Joists need at least 1.5 in of bearing on wood or metal and 3 in on masonry, and the ends must be laterally supported by a rim joist or blocking.

Frequently asked questions

What size floor joist do I need for a 12 foot span?

At 16 in on center with a 40 psf living-area load, a 2x8 works in Douglas fir-larch #2 (12 ft 9 in allowed) and in Spruce-pine-fir #2 (12 ft 3 in), but Hem-fir #2 (12 ft 0 in) and Southern pine #2 (11 ft 10 in) need a 2x10.

How far can a 2x10 floor joist span?

At 16 in on center under a 40 psf living-area load: 15 ft 7 in for Douglas fir-larch #2, 15 ft 5 in for Spruce-pine-fir #2, 15 ft 2 in for Hem-fir #2 and 14 ft 0 in for Southern pine #2. Bedroom floors at 30 psf allow 17 ft 5 in in Douglas fir-larch #2.

How many floor joists do I need?

Divide the floor length in inches by the spacing, round up, and add one. A 20 ft floor at 16 in on center needs 20 x 12 / 16 + 1 = 16 joists.

Which IRC edition are these spans from?

The 2018 and 2021 IRC Tables R502.3.1(1) and R502.3.1(2), 10 psf dead load columns, #2 grade. They have not changed between those editions, but your local code may amend them, so confirm with the building department.

Sources and references

Chapter 5 Floors, 2021 IRC (Tables R502.3.1(1) and R502.3.1(2) floor joist spans) (ICC)
Span Options Calculator for Wood Joists and Rafters (American Wood Council)
Engineered Wood Construction Guide (Form E30) (APA The Engineered Wood Association)

Next steps for this project

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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.