Floor Joist Calculator
Size your floor joists. Enter the clear span, joist spacing, live and dead loads, and species — we find the shallowest 2× joist that passes bending and live-load deflection (L/360), with the max span, load per joist, capacity used and which check governs, plus by-size and by-spacing comparison tables and a downloadable PDF.
| Joist | Max span | Capacity used | Status |
|---|---|---|---|
| Enter a span to compare | |||
| Spacing | Recommended | Capacity used | Status |
|---|---|---|---|
| Enter a span to compare | |||
How to use the floor joist calculator
Four quick steps turn a span and a load into a recommended joist — the shallowest 2× size that passes both bending and live-load deflection, with the max span, load per joist, capacity used and comparison tables.
Enter the span
The clear span the joist must cross, bearing to bearing, in feet.
Set spacing & loads
Pick the on-center spacing and the live and dead loads.
Choose the species
Select SPF, Douglas Fir-Larch, Southern Pine or Hem-Fir #2.
Read results & save a PDF
Get the recommended size, max span and tables — then download a report.
The formulas behind the joist size
The load per joist comes from the spacing and the pressures; each joist is then checked as a simple beam on bending and live-load deflection.
▶ Worked example
A 14 ft clear span at 16" o.c. carrying 40 psf live + 10 psf dead, in Douglas Fir-Larch #2.
Load per joist w = (16 ÷ 12) × (40 + 10) = 66.7 plf.
Checking each 2× size, the shallowest that passes both bending and L/360 deflection is a 2×10.
Recommended: 2×10 · 80.5% used (bending governs) · max span 15.6 ft
A 2×8 would be overstressed at this span, and a 2×12 would reach about 18.1 ft.
Floor joist quick reference
The sizing formulas, the species strength values used, and the maximum span each 2× size reaches for a typical floor.
| Quantity | Formula |
|---|---|
| Load per joist | (spacing ÷ 12) × (live + dead) |
| Max moment | w × L² ÷ 8 |
| Section modulus | 1.5 × d² ÷ 6 |
| Effective Fb | Fb × 1.15 × size factor |
| Bending use | M ÷ (effective Fb × S) |
| Deflection use | (5 wₗ L⁴ ÷ 384EI) ÷ (L ÷ 360) |
| Species / grade | Bending Fb | Stiffness E | Relative span |
|---|---|---|---|
| SPF #2 | 875 psi | 1.4M psi | Good |
| Douglas Fir-Larch #2 | 900 psi | 1.6M psi | Strong |
| Southern Pine #2 | 1,000 psi | 1.6M psi | Strongest |
| Hem-Fir #2 | 850 psi | 1.3M psi | Moderate |
Effective bending strength also includes the 1.15 repetitive-member factor and a depth size factor (1.3 for 2×6 down to 0.9 for 2×14).
| Joist | Max span | At 14 ft: capacity used | Governs |
|---|---|---|---|
| 2×6 | 9.7 ft | Over (297%) | Deflection |
| 2×8 | 12.8 ft | Over (130%) | Deflection |
| 2×10 | 15.6 ft | 80.5% | Bending |
| 2×12 | 18.1 ft | 59.9% | Bending |
| 2×14 | 20.2 ft | 47.9% | Bending |
By spacing for a 14 ft span (DFL #2): 12″ o.c. → 2×8, 16″ o.c. → 2×10, 19.2″ o.c. → 2×10, 24″ o.c. → 2×12. Wider spacing and heavier loads call for a deeper joist.
Everything the calculator returns
One calculation gives the full sizing picture — the recommended joist, the numbers behind it, and side-by-side comparisons.
Joist sizing at a glance
The limits and factors built into the check, and the default result.
Built for the way you work
Whether you are framing a floor, planning a renovation, or checking a drawing, the joist sizing starts from the same inputs.
Need a fast, defensible joist size and spacing for a bid or a crew, with a by-size table to see the margin each option leaves.
- Compare species in seconds
- Check which limit governs
- Hand the PDF to the inspector
Building a deck, addition or floor and want to know the joist size and spacing before buying lumber or pulling a permit.
- Start at 16″ o.c., 40 psf
- Mind the floor feeling solid
- Permit structural work
Reviewing a floor framing plan and want a quick sanity check on span, spacing and size before the code tables or an engineer.
- Test several spacings fast
- Flag spans near the limit
- Cross-check against R502
Frame a solid floor
Seven things that keep a joist estimate honest and the floor feeling solid.
Tighter spacing spans farther
Going from 24″ to 16″ o.c. cuts the load on each joist, so a shallower size works or the same size reaches farther.
Deflection decides the feel
A floor that passes bending can still feel bouncy. L/360 limits the live-load sag — long spans often hit it first.
Species and grade count
Douglas Fir-Larch and Southern Pine span farther than SPF or Hem-Fir in the same size. Confirm what your yard stocks.
Add blocking or bridging
Mid-span blocking ties the joists together, shares load and stops twisting — a stiffer, quieter floor for little cost.
Don’t over-notch
Notches and holes for pipes and wires weaken a joist. Keep them small, near mid-depth, and away from the span’s middle third.
Use the right live load
40 psf for living areas, 30 for bedrooms, more for other uses. Picking the wrong one skews the whole result.
Confirm the span tables
The IRC joist span tables (R502) give prescriptive sizes. Use them and your building official to confirm the estimate.
Frequently asked questions
Common questions about sizing floor joists, spans, loads, spacing and when to call an engineer.
A floor joist is sized from its clear span, the joist spacing, and the loads it carries. First work out the load on one joist: multiply the spacing in feet by the total load, which is the live load plus the dead load. Then check candidate sizes against that load.
Each joist is treated as a simple beam, with a maximum bending moment of the load times the span squared, divided by eight. The shallowest 2x size that keeps both the bending stress and the live-load deflection within the limits is the one to use.
For example, a 14 ft span at 16 inch spacing carrying 40 psf live plus 10 psf dead works out to about 67 plf per joist, and a 2×10 in Douglas Fir-Larch passes at about 80% of allowable with bending governing. This calculator does the whole check and picks the size.
It depends on the spacing, the loads and the species. For a Douglas Fir-Larch #2 2×10 at 16 inch spacing carrying a 40 psf live load plus 10 psf dead, the maximum span is about 15.6 feet before either bending or deflection reaches its limit.
Closer spacing lets it span farther; wider spacing or heavier loads shorten it. A weaker species or a lower grade also reduces the span.
So a 2×10 commonly spans around 15 to 16 feet in a living area, but the exact figure depends on your inputs. This calculator gives the maximum span for each size at your spacing, load and species.
Dead load is the permanent weight of the floor itself — the joists, subfloor, finish flooring and anything fixed in place. For a typical residential floor it is taken at about 10 to 15 psf. It does not change over time.
Live load is the weight of the things that come and go — people, furniture and belongings. Residential codes use 40 psf for living areas and 30 psf for bedrooms, with higher values for other uses. Both add up to the total load the joist carries.
For example, a 40 psf live plus 10 psf dead floor has a 50 psf total load. Deflection is checked against the live load alone, while bending is checked against the total. This calculator takes both and applies each to the right check.
Both are standard; the choice is a trade-off between joist size and the number of joists. At 16 inch spacing each joist carries less load, so it can be shallower or span farther; at 24 inch spacing you use fewer joists but each needs to be deeper or the span shorter.
16 inch on center is the most common for floors because it gives a stiffer floor and suits standard sheet goods. 24 inch is used with deeper or engineered joists to save material and labor, and 12 or 19.2 inch spacings cover the cases in between.
For example, a 14 ft span in Douglas Fir-Larch needs a 2×10 at 16 inch spacing but a 2×12 at 24 inch. This calculator shows the recommended joist at each common spacing.
L/360 is the standard deflection limit for floor joists under live load: the joist may sag no more than its span divided by 360. For a 14 ft, or 168 inch, span that is about 0.47 inch.
The limit is about stiffness and comfort, not safety. A joist strong enough not to break can still feel bouncy or crack a plaster or tile ceiling below if it flexes too much, so the code caps the live-load deflection to keep floors feeling solid.
Longer spans often hit the L/360 deflection limit before they run out of bending strength, which is why a floor can need a deeper joist than strength alone would suggest. This calculator checks both and flags which one governs.
Yes. The species and grade set the two properties that drive the result: the bending strength (Fb) and the stiffness (E). Douglas Fir-Larch and Southern Pine are stronger and stiffer than SPF or Hem-Fir, so they span farther in the same size.
Grade matters too — a #2 joist is weaker than a #1 or select structural piece of the same species. The repetitive-member and size factors then adjust the strength for joists used in a row.
For example, at 14 ft and 16 inch spacing a Douglas Fir-Larch 2×10 passes comfortably, while a weaker species might be closer to its limit or need the next size up. This calculator includes four common species so you can compare.
The repetitive-member factor is a strength bonus allowed when three or more joists are spaced closely and tied together by the subfloor, so they share load. Because a single weak joist is helped by its neighbors, the code lets you increase the bending strength by 15%, a factor of 1.15.
It applies to joists, rafters and studs at 24 inch spacing or less with a load-distributing element like sheathing — which covers almost all floor framing. It is one reason a joist in a floor spans farther than the same board tested alone.
This calculator applies the 1.15 repetitive-member factor, along with the depth size factor, to the bending check, matching how floor joists are sized in practice.
No. This calculator is a planning aid that estimates a joist size from a simplified uniform load and a simple-beam check. It assumes a uniform load, sawn 2x lumber, adequate bearing and bridging, and no point loads, openings, cantilevers or notching.
Real projects are governed by the prescriptive span tables in the building code (IRC R502) or by an engineered design, which account for load combinations, bearing, connections and special conditions a quick estimate does not.
Use this to get a sense of the size and compare options, then confirm the final joist, spacing, bearing and any openings against the current code and your building official or a licensed engineer.
Disclaimer: US units (feet, psf, inches). Load per joist = (spacing ÷ 12) × (live + dead). Each joist is checked as a simple beam: moment M = wL² ÷ 8 against bending capacity (Fb × 1.15 repetitive × size factor × S), and live-load deflection 5wₗL⁴ ÷ 384EI against L/360. It assumes a uniform load, sawn 2× lumber, adequate bearing and bridging, and no point loads, openings, cantilevers or notching. It is NOT a substitute for the IRC joist span tables (R502) or an engineered design. A planning aid only — confirm species values, bearing, connections and any openings with the current code and your building official or a licensed engineer.