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 10, 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.

Your measurements

Deck from above: boards run the length, joists run the width beneath them at a fixed spacing.

Joist spanftClear distance between the inside faces of the supports, not the board length. A 12 ft clear span on plates needs a 12 ft 3 in joist, so you buy 14 footers.Floor length (along the supporting wall)ftThe direction the joists are spaced along. Layout starts from one end so 4x8 subfloor sheets break on a joist.Joist spacing16 in is standard. 12 in stiffens a floor for tile or stone. 24 in needs 23/32 in tongue and groove subfloor and most tile warranties want 16 in.Species and gradeRead the grade stamp at the yard. Most northern yards stock SPF, the Southeast stocks Southern pine, the West Coast Douglas fir. #2 is the usual framing grade; the stamp says.Live load30 psf is bedrooms and attics only. A bathroom with a tub, a kitchen or a room that might change use gets 40 psf.

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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 on a plate or beam need 1.5 in of bearing at each end (R502.6). Sight each one and set the crown up
Rim joist: 40 ft; Same depth as the joists, one along each end of the joist run (2 x 20 ft). R502.7 requires a rim or full-depth blocking at every bearing point
Joist hangers: 16 hangers; Only if the joists hang from a ledger or the face of a beam; use hangers sized for 2x8, with a 10d common or 1.5 in N10 hanger nail in every hole, never deck or drywall screws
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. This calculator uses the 10 psf columns for #2 grade, which covers 3/4 inch subfloor, wood or carpet and a drywall ceiling below. Tile on cement board, a gypcrete topping or a plaster ceiling belongs in the 20 psf columns, which cut about a foot off a 2x10 span.

Measure the clear span between the inside faces of the supports, not the board. A 12 foot clear span on plates needs 12 feet 3 inches of joist, so you buy 14 footers. Within a few inches of the table limit, go up a size or tighten the spacing. The tables allow span/360 of deflection, about 3/8 inch on 12 feet, which bounces under a big room. Framers set hardwood at L/480, and TCNA wants L/720 under stone tile.

Layout and lumber

Lay out from one end of the sill at 12, 16 or 24 inches on center so 4x8 subfloor breaks on a joist; the count is floor length over spacing, rounded up, plus one. Joists on a plate or beam need 1-1/2 inches of bearing at each end, 3 inches on masonry (R502.6). Joists in hangers are cut 1/8 inch shy of the clear span so the seat carries them. Every hanger hole takes a 10d common or 1-1/2 inch N10 nail, never a deck screw. Sight each board and set the crown up.

A rim joist of the same depth closes both ends and carries the wall above; R502.7 requires it, or full-depth blocking, at every bearing point. R502.7.1 requires bridging every 8 feet only for joists deeper than 2x12, but a row of mid-span blocking on 2x10 and 2x12 spans over 12 feet quiets bounce. Double the joists under parallel partitions and around stair openings. A cracked or over-notched joist gets a full-length sister, glued and nailed with three rows of 10d commons at 16 inches. A bearing wall, tub or point load above is a job for an engineer, not a span table.

What this calculator leaves out

Not counted: 23/32 inch tongue and groove subfloor, glued and screwed; the beam and posts that split a wide floor into two spans; hangers, hanger nails and blocking; sill seal and a treated sill on concrete or block. Yards stock 2x10s and 2x12s to 20 feet. Longer floors go to I-joists or LVL sized by the supplier and delivered by boom truck.

R502.8.1 limits plumber's notches to 1/6 the depth outside the middle third and holes to 1/3 the depth, 2 inches from any edge; past that is a sister or an engineer. Every floor gets a permit and a framing inspection before drywall.

Code limits that apply

IRC Table R502.3.1(2)checked
Max span at 40 psf live, 10 psf dead, L/360; 2x8 Douglas fir #2 at 16 in OC spans 12 ft 9 in
If you miss it: A joist a few inches past the table bounces under hardwood and gets flagged at framing; the fix is sistering or a mid-span beam
IRC Table R502.3.1(1)checked
Bedrooms and attics at 30 psf live; a 2x10 Douglas fir #2 at 16 in OC reaches 17 ft 5 in
If you miss it: A bedroom floor sized at 30 psf becomes a bath with a tub or a kitchen; the floor is under-sized for the new use
IRC Table R503.2.1.1(1)checked
Subfloor span rating matches joist spacing; 24 in OC takes 23/32 in tongue and groove panels
If you miss it: 5/8 in OSB on 24 in centers flexes between joists and tile cracks along every joist line
IRC R502.6
Joist ends bear 1-1/2 in min on wood or metal, 3 in on masonry or concrete
If you miss it: A joist cut 1/4 in short sits on 1-1/4 in of plate and gets a red tag; the fix is a full-length sister, not a shim
IRC R502.7
Joist ends held by a rim joist, full-depth blocking or a hanger at every bearing point
If you miss it: 2x10s with no rim or blocking roll at the plate, squeak and crack the drywall; blocking goes in before the inspection
IRC R502.7.1
Bridging or blocking every 8 ft along the span for joists deeper than a 2x12
If you miss it: Deep joists with no mid-span bridging twist and drum underfoot; a row of blocking goes in before drywall covers them
IRC R502.8.1
Notches 1/6 the depth, none in the middle third; holes 1/3 the depth, 2 in from any edge
If you miss it: A plumber's 3 in drain through a 2x8 leaves nothing; the inspector calls for a full-length sister or an engineered repair

Provision	Limit	If you miss it
IRC Table R502.3.1(2)checked	Max span at 40 psf live, 10 psf dead, L/360; 2x8 Douglas fir #2 at 16 in OC spans 12 ft 9 in	A joist a few inches past the table bounces under hardwood and gets flagged at framing; the fix is sistering or a mid-span beam
IRC Table R502.3.1(1)checked	Bedrooms and attics at 30 psf live; a 2x10 Douglas fir #2 at 16 in OC reaches 17 ft 5 in	A bedroom floor sized at 30 psf becomes a bath with a tub or a kitchen; the floor is under-sized for the new use
IRC Table R503.2.1.1(1)checked	Subfloor span rating matches joist spacing; 24 in OC takes 23/32 in tongue and groove panels	5/8 in OSB on 24 in centers flexes between joists and tile cracks along every joist line
IRC R502.6	Joist ends bear 1-1/2 in min on wood or metal, 3 in on masonry or concrete	A joist cut 1/4 in short sits on 1-1/4 in of plate and gets a red tag; the fix is a full-length sister, not a shim
IRC R502.7	Joist ends held by a rim joist, full-depth blocking or a hanger at every bearing point	2x10s with no rim or blocking roll at the plate, squeak and crack the drywall; blocking goes in before the inspection
IRC R502.7.1	Bridging or blocking every 8 ft along the span for joists deeper than a 2x12	Deep joists with no mid-span bridging twist and drum underfoot; a row of blocking goes in before drywall covers them
IRC R502.8.1	Notches 1/6 the depth, none in the middle third; holes 1/3 the depth, 2 in from any edge	A plumber's 3 in drain through a 2x8 leaves nothing; the inspector calls for a full-length sister or an engineered repair

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

A joist with less than 1-1/2 in of bearing on the plate is a red tag (R502.6); cut short, it gets sistered, not shimmed.
Deck or drywall screws in joist hangers shear under load; fill every hole with 10d commons, 1-1/2 in N10 hanger nails or Simpson SD screws.
Joists set crown down sag at mid-span and the floor telegraphs it; sight every board and mark the crown up before layout.
No blocking or rim at the bearing ends lets 2x10s roll and squeak; R502.7 requires full-depth blocking or a rim joist there.
A plumber's notch deeper than 1/6 the joist or a hole within 2 in of an edge violates R502.8.1; the fix is a full-length sister.
2x10s over 16 ft do not ride in a pickup and most yards stock to 20 ft; longer spans mean I-joists and a delivery fee.

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.

Can a plumber notch or drill my floor joists?

Within IRC R502.8.1 limits. Notches in the top or bottom may be 1/6 the depth, never in the middle third of the span, and 1/4 the depth at the ends. Holes may be 1/3 the depth, at least 2 inches from any edge or notch. On a 2x8 that is a 2-3/8 inch hole, which just clears a 2 inch drain; 3 inch pipe does not fit any joist and runs below or between them. A joist cut past those limits gets a full-length sister or an engineered repair.

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)

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