Distance joists must bridge unsupported
Perpendicular dimension for joist count and cost
Span Check
Max Allowable Span
16 ft 4 in
2x10 Douglas Fir-Larch, 16" OC
Your Span
14 ft
Within allowable span
2x10 joists can span 14 ft at 16" OC
Span Details
Material Estimate
12,800+ estimates calculated this month
Douglas Fir-Larch Span Table (#2 Grade, 40 psf Live Load)
| Size | 12" OC | 16" OC | 24" OC |
|---|---|---|---|
| 2x6 | 10 ft 8 in | 9 ft 8 in | 8 ft |
| 2x8 | 14 ft 1 in | 12 ft 9 in | 10 ft 7 in |
| 2x10 | 17 ft 11 in | 16 ft 4 in | 13 ft 5 in |
| 2x12 | 21 ft 10 in | 19 ft 10 in | 16 ft 4 in |
Planning tool only. These are rule-of-thumb spans based on IRC tables for #2 grade lumber with 40 psf live load and 10 psf dead load. Always verify with your local building department and a structural engineer for load-bearing applications. Actual allowable spans depend on lumber grade, moisture content, point loads, and local code amendments.
Last updated: 2026-02-23
Quick Answer: How Far Can Joists Span?
Joist span depends on three things: lumber size, wood species, and spacing. The most common residential setup is 2x10 Douglas Fir-Larch at 16" on center, which spans up to 16 ft 4 in. If you need more span, step up to 2x12 (19 ft 10 in max at 16" OC) or switch to 12" OC spacing. For spans over 20 feet, you will need engineered lumber or an intermediate beam.
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Inputs You'll Need
- Joist size (2x6, 2x8, 2x10, or 2x12)
- Wood species (Douglas Fir-Larch, SPF, Southern Pine, or Hem-Fir)
- Spacing (12", 16", or 24" on center)
- Span distance (feet) and room width for material quantities
Joist Span Table: Douglas Fir-Larch #2 Grade
These are maximum allowable spans for #2 grade Douglas Fir-Larch floor joists with 40 psf live load and 10 psf dead load, per IRC Table R502.3.1(1). This is the most widely used species for framing in the western United States.
| Joist Size | 12" OC | 16" OC | 24" OC |
|---|---|---|---|
| 2x6 | 10 ft 8 in | 9 ft 8 in | 8 ft 0 in |
| 2x8 | 14 ft 1 in | 12 ft 9 in | 10 ft 7 in |
| 2x10 | 17 ft 11 in | 16 ft 4 in | 13 ft 5 in |
| 2x12 | 21 ft 10 in | 19 ft 10 in | 16 ft 4 in |
Joist Span Table: Southern Pine #2 Grade
Southern Pine is the strongest common framing species and allows the longest spans. It is the standard in the southeastern United States.
| Joist Size | 12" OC | 16" OC | 24" OC |
|---|---|---|---|
| 2x6 | 10 ft 11 in | 9 ft 11 in | 8 ft 8 in |
| 2x8 | 14 ft 5 in | 13 ft 1 in | 11 ft 5 in |
| 2x10 | 18 ft 5 in | 16 ft 9 in | 14 ft 7 in |
| 2x12 | 22 ft 5 in | 20 ft 5 in | 17 ft 9 in |
Worked Examples
Example A (standard bedroom, 14 ft span):
- Joist: 2x10 Douglas Fir-Larch at 16" OC
- Max span: 16 ft 4 in - 14 ft is within limits
- Room width: 12 ft - joists needed: (144/16) + 1 = 10 joists
- Linear feet: 10 x 14 = 140 LF
- Lumber cost: 140 x $1.15-$1.65 = $161 - $231
Example B (deck, 10 ft span):
- Joist: 2x8 Southern Pine at 16" OC
- Max span: 13 ft 1 in - 10 ft is within limits
- Deck width: 16 ft - joists needed: (192/16) + 1 = 13 joists
- Linear feet: 13 x 10 = 130 LF
- Lumber cost: 130 x $0.80-$1.20 = $104 - $156
Example C (large room, 18 ft span):
- Joist: 2x12 Douglas Fir-Larch at 16" OC
- Max span: 19 ft 10 in - 18 ft is within limits
- Room width: 20 ft - joists needed: (240/16) + 1 = 16 joists
- Linear feet: 16 x 18 = 288 LF
- Lumber cost: 288 x $1.65-$2.35 = $475 - $677
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Pro Tips From the Field
- Crown every joist up. Dimensional lumber has a natural bow (crown). Install every joist with the crown facing up so the load straightens it over time instead of making the bow worse.
- Stay 80% of max span. Just because code says 2x10 can span 16 ft 4 in does not mean you should push it to the limit. Building at 80% of max span gives you a stiffer floor with less bounce. Your clients will notice the difference.
- Add blocking at mid-span. Solid blocking or cross-bridging between joists at the center of the span reduces lateral movement and floor squeaks. Required by code for spans over 8 feet in some jurisdictions.
- Double joists under parallel walls. Any partition wall running parallel to the joists needs doubled joists underneath, even if it is not load-bearing. This prevents drywall cracks and gives you a nailing surface for the bottom plate.
- Check for plumbing and HVAC runs. Before you finalize joist size, make sure you have enough depth for ductwork and drain pipes. A 2x10 gives you about 9.25 inches of actual depth, which may not be enough for a 6-inch duct plus subfloor.
Common Mistakes to Avoid
- Using the wrong lumber grade. Span tables assume #2 grade or better. If you buy economy or utility grade lumber, the allowable span drops significantly. Check the grade stamp on every piece.
- Ignoring point loads. Span tables assume uniform loads. A bathtub, island, or heavy appliance sitting between joists creates a point load that requires sistered or doubled joists in that area.
- Not accounting for notches and holes. Cutting notches or drilling holes in joists for plumbing reduces their strength. Code limits notch depth to 1/3 of the joist depth in the outer 1/3 of the span, and hole diameter to 1/3 of depth.
- Skipping the engineer on long spans. Rule-of-thumb tables work for standard residential rooms. Open floor plans, multi-story buildings, and spans over 16 feet should be reviewed by a structural engineer.
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Joist Span & Sizing Guide
Floor joist spans by species, sizing rules, spacing trade-offs, and quantity calculations.
How Far Can Floor Joists Span?
Floor joist spans depend on the lumber size, wood species, and spacing. For #2 grade Douglas Fir-Larch at 16" on center: 2x6 spans up to 9 ft 8 in, 2x8 spans 12 ft 9 in, 2x10 spans 16 ft 4 in, and 2x12 spans 19 ft 10 in.
- 2x6 joists: Max span 7 ft 6 in to 10 ft 11 in depending on species and spacing
- 2x8 joists: Max span 9 ft 11 in to 14 ft 5 in
- 2x10 joists: Max span 12 ft 7 in to 18 ft 5 in
- 2x12 joists: Max span 15 ft 3 in to 22 ft 5 in
These values are based on IRC Table R502.3.1(1) for residential floor joists with 40 psf live load and 10 psf dead load. Always check your local building code for specific requirements.
Key Takeaways
- 2x10 at 16" OC spans up to 16 ft 4 in (Douglas Fir)
- 2x12 at 16" OC spans up to 19 ft 10 in (Douglas Fir)
- Based on IRC R502.3.1 for 40 psf live / 10 psf dead load
What Size Joists Do I Need?
The joist size you need depends on the span distance, spacing, and load requirements. For most residential floors, 2x10 at 16" OC handles spans up to about 16 feet, which covers the majority of rooms.
- Small rooms (under 10 ft): 2x6 or 2x8 joists at 16" OC
- Standard rooms (10-16 ft): 2x10 joists at 16" OC (most common residential choice)
- Large rooms (16-20 ft): 2x12 joists at 12" or 16" OC
- Over 20 ft: Engineered lumber (LVL, TJI) or add a beam to reduce the span
Closer spacing (12" OC vs 16" OC) allows longer spans but uses more lumber. The trade-off is usually worth it only when you are close to the max span for a given size.
Key Takeaways
- 2x10 at 16" OC is the most common residential floor joist
- Spans over 20 ft typically need engineered lumber or a beam
- Closer spacing allows longer spans but uses more lumber
Joist Spacing: 12" vs 16" vs 24" On Center
16" on center is the standard joist spacing for most residential floors. Tighter spacing (12" OC) increases the allowable span by about 10-15%. Wider spacing (24" OC) reduces material but shortens the max span significantly.
- 12" OC: Maximum span, stiffest floor, most lumber. Used when you need every inch of span or want a rock-solid floor.
- 16" OC: Standard spacing for residential construction. Good balance of span, stiffness, and material cost. Required by most codes for standard floor sheathing.
- 24" OC: Saves lumber but reduces span by 15-20%. Floor may feel bouncy. Requires thicker subfloor (3/4" or 1" T&G). Common for non-habitable spaces like attics.
If you are using 24" OC spacing, check that your subfloor sheathing is rated for the wider span. Standard 3/4" plywood works, but 1/2" will sag between joists.
Key Takeaways
- 16" OC is the residential standard
- 12" OC adds 10-15% more span capacity
- 24" OC requires thicker subfloor sheathing
How to Calculate Number of Joists Needed
Divide the area width (in inches) by the joist spacing, then add 1 for the end joist. For a 12-foot wide room at 16" OC: (12 x 12) / 16 + 1 = 10 joists.
- Formula: Joists = (Width in inches / Spacing) + 1
- 12 ft wide at 12" OC: (144 / 12) + 1 = 13 joists
- 12 ft wide at 16" OC: (144 / 16) + 1 = 10 joists
- 12 ft wide at 24" OC: (144 / 24) + 1 = 7 joists
Each joist needs to be long enough to span the room plus 1.5 inches on each end for bearing on the sill plate or beam. Order lumber 1-2 feet longer than the span and cut to fit.
Key Takeaways
- Formula: (Width in inches / Spacing) + 1
- 16" OC uses about 25% less lumber than 12" OC
- Order joists 1-2 ft longer than the span for bearing
How to Use This Calculator
Select your joist size
Choose from 2x6, 2x8, 2x10, or 2x12 dimensional lumber. 2x10 at 16" on center is the most common choice for residential floor framing.
Pick the wood species
Select Douglas Fir-Larch, Spruce-Pine-Fir, Southern Pine, or Hem-Fir. Species affects the allowable span because each has different bending strength values.
Set the joist spacing
Choose 12", 16", or 24" on center. Standard residential construction uses 16" OC. Tighter spacing allows longer spans but uses more lumber.
Enter your span and area width
Type the distance your joists need to bridge (span) and the perpendicular room width. The calculator checks if your joist can handle the span and estimates materials and cost.
Joist Span Calculation Formulas
Max Span = lookup(Species, Size, Spacing) from IRC table
Joist Count = (Area Width × 12 / Spacing) + 1
Board Feet = Joist Count × (2 × Depth × Span) / 12Where:
- Species
- = Wood species group (Douglas Fir-Larch, SPF, Southern Pine, Hem-Fir)
- Size
- = Nominal joist dimensions (2x6, 2x8, 2x10, 2x12)
- Spacing
- = On-center spacing in inches (12, 16, or 24)
- Area Width
- = Room dimension perpendicular to joists (feet)
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Frequently Asked Questions
How far can a 2x10 joist span?
A 2x10 joist spans up to 16 ft 4 in at 16" on center using #2 grade Douglas Fir-Larch (the most common residential setup). At 12" OC spacing, the same joist reaches 17 ft 11 in. Southern Pine 2x10 joists span slightly farther at 16 ft 9 in at 16" OC. These values follow IRC Table R502.3.1 for 40 psf live load and 10 psf dead load.
How far can a 2x8 joist span?
A 2x8 joist spans up to 12 ft 9 in at 16" on center with Douglas Fir-Larch #2 grade lumber. At 12" spacing, the max increases to 14 ft 1 in. Southern Pine 2x8 spans up to 13 ft 1 in at 16" OC. For spans beyond these limits, upgrade to 2x10 or reduce the spacing to 12" OC.
How far can a 2x12 joist span?
A 2x12 joist spans up to 19 ft 10 in at 16" on center with Douglas Fir-Larch. At 12" OC, the max reaches 21 ft 10 in. Southern Pine 2x12 is the longest-spanning dimensional lumber at 20 ft 5 in at 16" OC. For spans beyond 20 feet, you will typically need engineered lumber like LVLs or I-joists, or add a bearing wall or beam.
What is the standard joist spacing for a house?
16 inches on center is the standard joist spacing for residential floor framing in the United States. This spacing works with standard 4x8 sheet goods (plywood and OSB subfloor) and provides a good balance between span capacity and material cost. 12" OC is used when you need longer spans, and 24" OC is sometimes used for non-habitable spaces but requires thicker subfloor sheathing.
What size floor joists for a 14 foot span?
For a 14-foot span, use 2x10 joists at 16" on center with Douglas Fir-Larch, SPF, or Southern Pine (#2 grade). All four common species can handle 14 feet with 2x10 at 16" OC. If you want a stiffer floor with less bounce, go with 2x10 at 12" OC or step up to 2x12. Never use 2x8 for a 14-foot span, as the max for 2x8 at 16" is about 12 ft 9 in.
How many joists do I need?
Divide the room width (in inches) by the joist spacing, then add 1. For a 12-foot wide room at 16" OC: (144 / 16) + 1 = 10 joists. At 12" OC the same room needs 13 joists, and at 24" OC it needs 7 joists. Each joist should be ordered 1-2 feet longer than the span to allow for bearing on the supporting walls or beams (minimum 1.5 inches of bearing at each end).
What is the difference between joist span and joist length?
Span is the unsupported distance between bearing points. Length is the total piece of lumber. A joist always needs to be longer than the span because each end must rest on a wall, beam, or sill plate. Code requires a minimum of 1.5 inches of bearing at each end. In practice, order joists 6-12 inches longer than the span and trim to fit on site.
Can I use 2x6 joists for a floor?
Yes, but only for short spans under 10 feet. A 2x6 at 16" OC spans a maximum of about 9 ft 8 in with Douglas Fir-Larch. This works for small closets, hallways, and bump-outs. For any standard room, 2x8 or 2x10 is the better choice. Using undersized joists creates a bouncy floor that feels cheap, even if it technically passes code.
When should I use engineered joists instead of dimensional lumber?
Use engineered joists (I-joists, LVLs) for spans over 20 feet, for open floor plans without intermediate support, or when you need consistent depth and straightness. Engineered I-joists can span 24-30+ feet and do not twist, shrink, or crown like dimensional lumber. The trade-off is higher material cost ($3-$6 per linear foot vs $1-$2 for dimensional) and the need for proper web stiffeners at bearing points.
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