Floor Joist Span Calculator
Maximum clear spans straight out of IRC 2021 Table R502.3.1(1) and Table R502.3.1(2), for SPF, Douglas Fir-Larch, Hem-Fir and Southern Pine in SS, No.1, No.2 and No.3, from 2x6 to 2x12 at 12, 16, 19.2 and 24 inches on center. Work either direction: size a joist for the span you have, or read the maximum span for a joist you already own.
Built for wall-removal checks, additions and bid-stage framing questions, where the answer decides whether a job is a table lookup or an engineer's scope.
The table span is the clear span, not the joist you order
- 2x10 SPF No.2 at 16 in o.c., living area, 40 psf live load and 10 psf dead load
- Table answer: 15 ft 5 in clear span, face of support to face of support
- IRC 2021 R502.6 bearing: add 1 1/2 in at each end on wood or metal, 3 in on masonry or concrete
- So the member itself runs 15 ft 8 in minimum on wood bearing, before you cut
Ordering off the clear span alone is the mistake that sends a lumber package back.
Four things that decide the number
30 psf vs 40 psf floor joist span
Sleeping rooms and attics served by a fixed stair use Table R502.3.1(1) at 30 psf live load. Every other living area, including kitchens, baths, halls and living rooms, uses Table R502.3.1(2) at 40 psf. It is not a small difference: a 2x12 SPF No.2 at 16 in o.c. reads 19 ft 11 in on the sleeping table and 17 ft 10 in on the living table. Pick the wrong table and you are 2 ft out on the same piece of lumber.
Clear span vs joist length
Every figure in the IRC floor joist tables is a clear span measured between the faces of the supports, not the overall length of the member and not a centre-to-centre dimension. Bearing sits on top of it. The calculator asks for the clear span for that reason, and the reference figure above shows what the bearing allowance does to the piece you actually order and cut. That figure already carries the floor joist deflection limit of L/360.
Species group and grade
Species and grade move the answer more than most framers expect. At 2x10, No.2, 16 in o.c. in a living area, Douglas Fir-Larch reads 15 ft 7 in, SPF 15 ft 5 in, Hem-Fir 15 ft 2 in and Southern Pine 14 ft 0 in. Southern Pine sitting last is not a typo, it follows the 2013 revision of southern pine design values. Grade matters just as hard: the same SPF 2x10 drops from 16 ft 0 in at Select Structural to 11 ft 8 in at No.3.
On center spacing
The tables tabulate 12, 16, 19.2 and 24 in o.c. and nothing else. The 2x10 floor joist span moves with it: SPF No.2 in a living area runs 17 ft 3 in at 12 in o.c., 15 ft 5 in at 16 in, 14 ft 1 in at 19.2 in and 12 ft 7 in at 24 in. Tightening the spacing is often cheaper than going up a depth when you are 6 in short on a span. Species rides along with it: the 2x8 floor joist span in Southern Pine No.2 at 16 in o.c. reads 11 ft 10 in, against 12 ft 3 in in SPF.
When this stops being a table question
The IRC floor joist tables assume a uniformly loaded, repetitive member with no point load and no bearing partition above. Step outside that and the table no longer applies, whatever number it returns. Take it to a licensed structural engineer when a bearing wall over floor joist span is involved, when a girder end, post or ridge beam reaction lands on the member, when you are doubling or sistering joists, when the member is engineered lumber rather than sawn dimension lumber, when a cantilever exceeds IRC 2021 R502.3.3, or when the building official asks for stamped calculations. Removing a load-bearing wall is an engineered condition in every jurisdiction we could find.
Every cell in this tool was checked against the printed table
Most floor joist span calculators on the web will not tell you which edition they are running, which table they pulled, or whether anyone verified the transcription. Here is ours in full.
1,024 cells, zero mismatches
The dataset behind this tool carries 1,024 span values across four species groups, four grades, four joist sizes, four spacings and four load conditions. Every one of them was diffed programmatically against the published table text, and the diff returned zero mismatches across the whole set.
Checked by hand as well
A programmatic diff can agree with a bad source. Two flagged cells plus an eighteen-cell random sample were re-read by a person against PDF page images of the printed table on 4 September 2026, and all of them matched. Where an outside handout disagreed with the code text on one cell, the code text won.
What the tables assume
Repetitive member use, three or more members at 24 in o.c. or closer. Visually graded dimension lumber at the grade shown. Dry service, moisture content at or below 19 percent. Live-load deflection limited to L/360 per IRC 2021 Table R301.7. Anything outside those four conditions is outside the table, whatever number it returns.
9’-4″
The shortest member in the table, and the one that forces a mid-span beam on almost any modern room.
12’-3″
Common in older houses, which is why so many additions have to change depth at the tie-in.
15’-5″
The workhorse depth for remodels, and the one most wall-removal checks come down to.
17’-10″
The deepest sawn joist the table carries. Past this it is engineered lumber or a beam.
All four are SPF No.2 at 16 in o.c. in a living area at 40/10 psf. Raise the dead load to 20 psf, for a heavy floor assembly or a plaster ceiling below, and that 2x10 falls from 15 ft 5 in to 14 ft 1 in. The dead-load column is the one people skip.
Code edition, and who actually decides
This tool runs the 2021 IRC tables. The edition your building department has adopted may be older, and states and municipalities amend the model text, so a locally amended table beats anything on this page. IRC R104 and R106 give the building official the authority, and the AHJ is the final word on your permit. A prescriptive span table is a worst-case lookup, not an engineering calculation, and nothing here is a stamped design.
Sources
- ICC Digital Codes — 2021 International Residential Code, Chapter 5, Floors, for Tables R502.3.1(1) and R502.3.1(2), R502.3.3 and R502.6.
- 2021 Seattle Residential Code, Chapter 5 (PDF), which reproduces the IRC 2021 floor joist span tables verbatim and was the text the dataset was verified against. Seattle's local amendments in that chapter affect deck tables, not R502.3.1.
- American Wood Council, Span Tables for Joists and Rafters, PS 20-20 sizes (PDF), the parent standard the IRC tables are derived from.
Data retrieved 27 August 2026, verified 4 September 2026. Deliberately absent: no doubled or sistered joist spans, because the IRC does not tabulate them; no engineered lumber spans, because those come from the manufacturer; no soil, snow or seismic adjustment, because floor joist tables do not carry one.
Floor joist span FAQs
Do I use the 30 psf table or the 40 psf table?
Sleeping rooms and attics with fixed stairs use IRC 2021 Table R502.3.1(1) at 30 psf live load. All other living areas, including kitchens, baths, halls and living rooms, use Table R502.3.1(2) at 40 psf. Both assume 10 psf dead load in the standard column and a live-load deflection limit of L/360 per IRC Table R301.7. Match the room, not the habit.
What is the maximum span for a 2x12 floor joist in No. 2 grade?
It depends on species, spacing and which load table applies, so there is no single number. SPF No.2 at 16 in o.c. reads 17 ft 10 in in a living area and 19 ft 11 in in a sleeping room. Run your actual species group and on center spacing rather than carrying one remembered figure across jobs.
How far can I span a doubled 2x10 at 16 inches on center?
Doubling a joist does not double its span, and the IRC tables do not tabulate doubled members, so that is an engineering question rather than a lookup. Single-member spans come from Table R502.3.1(1) for sleeping rooms and Table R502.3.1(2) for living areas. Species, grade and spacing set the number; a doubled or sistered member needs calculation.
Is the table span the clear opening or the full joist length?
Clear span between bearing faces, not overall length. Add the required bearing on top: IRC 2021 R502.6 calls for 1 1/2 inches of bearing on wood or metal and 3 inches on masonry or concrete. Ordering lumber off the clear span alone leaves you short at the cut list, which is where the error usually shows up.
Does the span change when a bearing wall sits above the joist?
Yes, and the table no longer applies at all. The IRC floor joist tables assume uniform load with no bearing partition or point load over the joist. A wall above puts a line load on the member and calls for a girder, a doubled or engineered member sized by calculation, or an engineer's stamp. Do not read a number off the table and carry on.
The span passes code but the floor still bounces. Why?
Code minimum is not stiffness. IRC 2021 Table R301.7 permits L/360 live-load deflection for floors, which a homeowner still reads as bounce. Cut the span, tighten spacing to 12 inches on center, go up a depth, or add a mid-span beam. Blocking and strongbacks help distribute load but do not change the tabulated floor joist span.
A span check is free. The job it belongs to is not
Which is why it matters that the homeowners asking you to price a wall removal or an addition are the ones who can actually fund it. Filtering for that before the site visit is a marketing problem, not a framing one.
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