Framing & structure
Floor joist span calculator
Find the maximum floor joist span for a species, grade, size and spacing, with the bending, deflection and shear limits shown side by side.
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What a floor joist span calculator does
A floor joist span calculator finds the longest clear span a joist can cover for a given species, grade, size, spacing and floor load. Enter those and it returns the maximum span, the span each check allows on its own (bending, deflection and shear) and which one governs, plus a span table for every joist depth at your spacing.
It uses the same allowable-stress method as the American Wood Council span tables that the IRC reproduces. For Douglas Fir-Larch No.2 joists under 40 psf live and 10 psf dead load at L/360, it returns the published spans to the inch: 9′-9″ for a 2x6, 12′-9″ for a 2x8, 15′-7″ for a 2x10 and 18′-1″ for a 2x12 at 16 in centres. Metric grades (EN 338 C16 and C24, AS/NZS MGP10 and MGP12) are converted to the same basis so you can compare them on one table.
It is a preliminary estimate for a simple span with a uniform load. It does not check point loads, cantilevers, bearing, notches and holes, vibration or fire, and it does not replace the span table your jurisdiction adopts or an engineer's design.
How to use the floor joist span calculator
- Choose Imperial or Metric. Imperial is the default, and your choice carries across the free tools.
- Pick the Species and grade stamped on the lumber (Douglas Fir-Larch No.2 is the default) and the Joist size, from 2x6 to 2x12 or 45 × 140 mm to 45 × 290 mm.
- Set the Joist spacing (12, 16, 19.2 or 24 in; 300, 400, 450 or 600 mm), the Live load (40 psf for living areas, 30 psf for sleeping rooms) and the Dead load (10 psf, or 20 psf with a heavy finish or a ceiling below).
- Choose the Live-load deflection limit: L/360 is the usual floor limit, L/480 gives a stiffer floor for tile or stone, and L/240 suits some decks and storage floors.
- Read the Maximum clear span and which check governs. To test a particular layout, enter a Span to check (optional) and read the pass or fail, utilisation and deflection. Download the span table as PDF or CSV.
Worked example
Imperial: Douglas Fir-Larch No.2 2x10 joists at 16 in centres under 40 psf live and 10 psf dead span 15′-7″, matching the IRC span table; bending governs, and deflection alone would allow 16.38 ft.
Metric: C24 45 × 240 mm joists at 400 mm centres under 1.5 kPa live and 0.5 kPa dead span 5.87 m at L/360; deflection governs, and bending alone would allow 7.24 m.
Floor joist span table: Douglas Fir-Larch No.2
These are the calculator's spans for Douglas Fir-Larch No.2 joists, 40 psf live load and L/360 deflection, the case in IRC Table R502.3.1(2) for living areas. Every figure matches the published table. Spans are clear spans between the faces of supports, rounded to the nearest inch as the tables do.
Adding a heavier finish or a drywall ceiling below takes the dead load to 20 psf, which shortens bending-governed spans by roughly 9%. The deflection span does not change, because deflection is checked under live load only, so a deflection-governed span shortens only where bending takes over.
| Size and spacing | 10 psf dead | 20 psf dead |
|---|---|---|
| 2x6 at 12 in | 10′-9″ | 10′-8″ |
| 2x6 at 16 in | 9′-9″ | 9′-3″ |
| 2x6 at 24 in | 8′-3″ | 7′-6″ |
| 2x8 at 12 in | 14′-2″ | 13′-6″ |
| 2x8 at 16 in | 12′-9″ | 11′-8″ |
| 2x8 at 24 in | 10′-5″ | 9′-6″ |
| 2x10 at 12 in | 18′-0″ | 16′-5″ |
| 2x10 at 16 in | 15′-7″ | 14′-3″ |
| 2x10 at 24 in | 12′-9″ | 11′-8″ |
| 2x12 at 12 in | 20′-11″ | 19′-1″ |
| 2x12 at 16 in | 18′-1″ | 16′-6″ |
| 2x12 at 24 in | 14′-9″ | 13′-6″ |
How species and grade change the span
Species and grade set two numbers: the allowable bending stress Fb and the modulus of elasticity E. Bending controls most 2x8 to 2x12 floors at 40 psf; deflection controls shallow joists and stiffer limits. A higher grade raises both values, so the gain shows up on every size.
Southern Pine is graded with size-specific values, and its No.2 bending stress drops from 1,100 psi for a 2x4 to 800 psi for a 2x10 and 750 psi for a 2x12. That is why Southern Pine No.2 falls further behind Douglas Fir-Larch No.2 as the joist gets deeper: 5 in shorter as a 2x6, 19 in shorter as a 2x12.
| Species and grade | 2x6 | 2x8 | 2x10 | 2x12 |
|---|---|---|---|---|
| Douglas Fir-Larch No.1 | 9′-11″ | 13′-1″ | 16′-5″ | 19′-1″ |
| Douglas Fir-Larch No.2 | 9′-9″ | 12′-9″ | 15′-7″ | 18′-1″ |
| Hem-Fir No.2 | 9′-1″ | 12′-0″ | 15′-2″ | 17′-7″ |
| Spruce-Pine-Fir No.1/No.2 | 9′-4″ | 12′-3″ | 15′-5″ | 17′-10″ |
| Southern Pine No.2 | 9′-4″ | 11′-10″ | 14′-0″ | 16′-6″ |
Joist spacing and bedroom floors (30 psf)
The IRC uses 40 psf live load for living areas and 30 psf for sleeping rooms, which is why there are two floor joist tables. Closer spacing shares the floor between more joists: going from 24 in to 16 in centres puts one third less load on each joist.
19.2 in spacing divides an 8 ft sheet into five bays. It is common with engineered I-joists and less so with sawn lumber, but the calculator handles any spacing you enter.
| Spacing | 2x6 | 2x8 | 2x10 | 2x12 |
|---|---|---|---|---|
| 12 in | 11′-10″ | 15′-7″ | 19′-10″ | 23′-4″ |
| 16 in | 10′-9″ | 14′-2″ | 17′-5″ | 20′-3″ |
| 19.2 in | 10′-1″ | 13′-0″ | 15′-11″ | 18′-6″ |
| 24 in | 9′-3″ | 11′-8″ | 14′-3″ | 16′-6″ |
Metric joist spans: C16, C24, MGP10 and MGP12
Metric grades publish characteristic (5th-percentile) strengths for limit-state design. The calculator converts them to an allowable-stress equivalent (× 0.8 for medium-term load ÷ 1.3 material factor ÷ an average 1.4 load factor) so the same unfactored floor load can be used. At 1.5 kPa live and 0.5 kPa dead, deflection governs almost every metric case.
UK and EU practice often adds an absolute deflection cap and a vibration check for domestic floors, and Australian span tables (AS 1684) include their own serviceability rules, so national span tables can be shorter than these figures.
| Grade and centres | 45 × 140 | 45 × 190 | 45 × 240 | 45 × 290 |
|---|---|---|---|---|
| C16 at 400 mm | 3,080 | 4,180 | 5,280 | 6,380 |
| C24 at 400 mm | 3,420 | 4,650 | 5,870 | 7,090 |
| C24 at 600 mm | 2,990 | 4,060 | 5,130 | 6,200 |
| MGP10 at 400 mm | 3,310 | 4,500 | 5,690 | 6,870 |
| MGP12 at 400 mm | 3,590 | 4,870 | 6,160 | 7,440 |
Worked example: 2x10 joists at 16 in
Take Douglas Fir-Larch No.2 2x10 joists (1½ × 9¼ in actual) at 16 in centres under 40 psf live and 10 psf dead. Each joist carries 50 psf × 16/12 ft = 66.7 lb per foot. The adjusted bending stress is Fb′ = 900 psi × 1.1 size factor × 1.15 repetitive-member factor = 1,138.5 psi, and the section modulus is 1.5 × 9.25² ÷ 6 = 21.39 in³.
The bending span is √(8 × 1,138.5 × 21.39 ÷ 5.556 lb/in) = 187.3 in, or 15.61 ft. The deflection span at L/360 under the 53.3 plf live load, with E = 1,600,000 psi and I = 98.93 in⁴, is 16.38 ft, and shear allows 49.95 ft. Bending governs, and the maximum clear span is 15′-7″.
Who it is for and when to use it
Laying out a floor
Pick a joist depth and spacing for a room before drawing the framing plan, then see how much a closer spacing or a deeper joist buys.
Checking an existing floor
Enter the joist size, spacing and the span you measured to see whether an old floor is near its limit before adding a heavy finish, a bath or a partition.
Comparing species and grades
See what a No.1 grade or a different species changes before ordering, or compare metric and US grades on one table.
Talking to an engineer or plan reviewer
Arrive with the span, load and deflection assumptions written down, so the conversation is about the exceptions rather than the basics.
Method, formulas & assumptions
Allowable-stress method (NDS): Fb′ = Fb × CF (size factor) × Cr 1.15 (repetitive member) × CD 1.0; total load w = (live + dead) × spacing; live load wL = live × spacing. Bending span = √(8 Fb′ S ÷ w). Deflection span = (384 E I ÷ (5 wL × limit))^⅓ for the L/360, L/480 or L/240 limit on live load only, as AWC span tables do. Shear span = 4 Fv A ÷ (3 w). The maximum clear span is the smallest; the table repeats the check for every joist depth at the chosen spacing. US species use NDS 2018 Supplement reference values; EN 338 and AS/NZS MGP grades convert characteristic values to the same basis (× 0.8 ÷ (1.3 × 1.4)).
Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.
Common mistakes to avoid
- Using the centre-to-centre distance between supports. Span tables and this calculator use the clear span between the faces of the supports.
- Choosing the 30 psf bedroom load for a floor that also serves living areas. Use 40 psf unless the whole span is sleeping rooms.
- Forgetting the dead load of a stone or tile floor, or a plaster ceiling. Both can push the dead load well past 10 psf.
- Assuming a joist that passes L/360 feels stiff. Long, shallow joists can pass the deflection check and still bounce; L/480 or a deeper joist helps.
- Applying a uniform-load span to a joist that carries a wall, a tub or a post. Concentrated loads need a separate check.
- Notching or drilling the joist after sizing it on the full section. Holes and notches reduce capacity; see the notch and hole rules before cutting.
Scope & limits
A preliminary estimate for a simple, uniformly loaded span in dry service with the top edge held by the subfloor. It does not check point loads, cantilevers, bearing length, notches and holes, vibration, fire, wet service or connections, and it is not a code span table: confirm with the span table your jurisdiction adopts or an engineer.
Frequently asked questions
How far can a 2x10 floor joist span?
A Douglas Fir-Larch No.2 2x10 at 16 in centres spans 15′-7″ under 40 psf live and 10 psf dead load at L/360, and 18′-0″ at 12 in centres. Other species, grades and loads change the figure.
How far can a 2x8 floor joist span?
A Douglas Fir-Larch No.2 2x8 at 16 in centres spans 12′-9″ under 40 psf live and 10 psf dead at L/360, or 14′-2″ for a 30 psf bedroom floor.
Does this calculator match the IRC span tables?
For Douglas Fir-Larch No.2 at 40 psf live and L/360 it reproduces IRC Table R502.3.1(2) to the inch at every size and spacing. It uses the same AWC method, so other cases should be close, but the adopted code table is the reference.
What does L/360 mean?
It limits live-load deflection to the span divided by 360. On a 15 ft span that is 0.5 in. L/480 is stiffer and is often used under tile or stone.
Why does bending govern some sizes and deflection others?
Deflection grows with the cube of the span while bending capacity grows with depth squared, so shallow joists and strict limits tend to be deflection-governed, and deeper joists at heavier loads tend to be bending-governed.
Is the span measured from the centre of the supports?
No. The calculator returns the clear span between the faces of the supports, the same convention as AWC and IRC span tables.
Can I use it for decks or garage floors?
Only as a first check with the right loads. Decks need wet-service adjustments and treated-lumber values, and garage or storage floors carry heavier or concentrated loads that this calculator does not model.
Does it work for I-joists or open-web trusses?
No. Engineered joists have manufacturer span tables. Use this calculator for sawn lumber only.
Do I need an account to download the result?
No. All calculations, local file processing and exports on this page are free, without an account.
Where are my inputs stored?
The tool keeps inputs in this browser tab. It does not upload them or create a cloud copy. Save or download anything you want to keep before leaving the page. Standard site analytics may operate independently of the tool.
Can I work in metric instead of imperial?
Yes. Use the Imperial / Metric switch above the inputs. Imperial (feet, inches, pounds, gallons) is the default; switching converts every dimensional field and result, and your choice is remembered across the free tools in this browser.
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