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Joist Calculator
Use this free joist calculator to find the floor joist size you need for a given span. Enter the span, the joist spacing, the live and dead loads, and the lumber species and grade, and the calculator picks the smallest joist that works — 2×6, 2×8, 2×10, or 2×12 — and shows the maximum span of each size. The sizes are based on published lumber design values and the standard bending and deflection checks, and every result is a planning estimate: confirm the final size with your local building code and a professional before you build.
| Joist size | Max span |
|---|---|
| 2×6 | — |
| 2×8 | — |
| 2×10 | — |
| 2×12 | — |
| Joists needed (20 ft floor) | — |
Planning estimate only. Spans are calculated from published design values (Fb and E for No. 2 grade lumber) using the bending and deflection checks in the residential code style. They do not replace your local building code, a span table adopted by your jurisdiction, or a structural engineer. Confirm before building.
How it works
A floor joist must pass two checks, and the calculator runs both for every size. The first is bending: the load on the joist creates a bending moment at midspan, and the joist's bending strength must be greater than that moment. The bending strength is the design bending stress Fb for the species and grade times the section modulus S of the joist. For Southern Pine No. 2, Fb is 1,000 psi. A 2×8 has a section modulus of 13.14 cubic inches, so its bending capacity is 1,000 × 13.14 = 13,140 inch-pounds, while a 2×10, with a section modulus of 21.39 cubic inches, can take 21,390 inch-pounds. The load is the total of live and dead load times the spacing: at 16 inch spacing with 50 pounds per square foot total, each joist carries 66.7 pounds per foot of span.
The second check is deflection, which is usually what limits a floor joist in practice. A joist that is strong enough can still feel bouncy if it sags too much, so the code limits the sag under live load to the span divided by 360 for normal floors. For a 12 foot span, that is 144 ÷ 360 = 0.4 inches of sag at midspan. Deflection depends on the joist's stiffness, which is the modulus of elasticity E times the moment of inertia I, and it grows with the fourth power of the span, so a small increase in span makes a big difference to sag. The calculator computes the span at which deflection reaches the limit and the span at which bending reaches its limit, and takes the smaller of the two as the maximum span for that size.
Spacing changes everything. Each joist carries a strip of floor as wide as the spacing, so at 12 inch spacing a joist carries 25 percent less load than at 16 inches, and at 24 inch spacing it carries 50 percent more. That is why the same 2×10 that spans 14 feet 8 inches at 16 inch spacing spans further at 12 inches and less at 24 inches. Wider spacing also means fewer joists to buy and install, but it needs thicker subfloor to bridge the gap, so 16 inches is the standard compromise for most homes.
Live and dead load are the two kinds of weight a floor carries. Dead load is the permanent weight of the floor itself — joists, subfloor, flooring, and the ceiling below — usually 10 pounds per square foot, or 15 with tile and a heavy finish. Live load is the weight that comes and goes: people, furniture, and appliances. Residential code uses 40 pounds per square foot for living areas and 30 for sleeping areas. A room with a large bathtub, a stone floor, a waterbed, or a home gym needs a higher allowance, and that is when you should ask an engineer rather than stretching a table.
Species and grade matter because lumber is not all the same. Southern Pine is the strongest common joist lumber, with an Fb of 1,000 psi and an E of 1,600,000 psi in No. 2 grade. Douglas Fir-Larch No. 2 is close behind at 900 psi, Spruce-Pine-Fir No. 2 at 875 psi, and Hem-Fir No. 2 at 850 psi. A lower grade, such as No. 3 or stud grade, has much lower design values and is not suitable for floor joists at these spans. Always buy lumber stamped with the species and grade you designed for, and reject pieces with large knots, splits, or excessive bow.
Bending span = √(8 × Fb × S ÷ w) · Deflection span = ∛(384 × E × I ÷ (5 × w × limit))
Max span = smaller of the two
Worked example
You are framing a living room floor with a 12 foot span, joists at 16 inches on center, 40 pounds per square foot live load and 10 pounds per square foot dead load, in Southern Pine No. 2, with the standard L/360 deflection limit.
Step 1: Load per joist — (40 + 10) × 16 ÷ 12 = 66.7 pounds per foot, or 5.56 pounds per inch.
Step 2: Try a 2×8 — bending gives √(8 × 1,000 × 13.14 ÷ 5.56) = 137.6 inches, or 11 feet 6 inches, and deflection under live load gives about 12 feet 10 inches, so the maximum span is 11 feet 6 inches. That is less than 12 feet, so the 2×8 fails.
Step 3: Try a 2×10 — bending gives √(8 × 1,000 × 21.39 ÷ 5.56) = 175.5 inches, or 14 feet 8 inches, and deflection gives about 16 feet 5 inches, so the maximum span is 14 feet 8 inches. That is more than 12 feet, so the 2×10 works.
Step 4: Joist count — for a 20 foot long floor at 16 inch spacing, you need 20 × 12 ÷ 16 + 1 = 16 joists.
The answer is 2×10 joists, 16 of them for a 20 foot floor. If you switch to the stricter L/480 limit for a tile floor, the 2×10 deflection limit drops to about 14 feet 11 inches, which is still above its bending limit of 14 feet 8 inches, so the maximum span stays 14 feet 8 inches — but on a longer span the stricter limit would force a larger joist.
FAQ
What size joist do I need for a 12 foot span?
For a 12 foot span at 16 inch spacing with 40 pounds per square foot live load and 10 pounds per square foot dead load, Southern Pine No. 2 needs a 2 by 10 joist. A 2 by 8 in the same species spans only about 11 feet 6 inches under those loads, so it falls short. At 12 inch spacing a 2 by 8 can reach further, and at 24 inch spacing you would need a larger joist still.
What is the difference between live load and dead load?
Dead load is the permanent weight of the floor itself, the joists, subfloor, flooring, and ceiling below, usually 10 to 15 pounds per square foot. Live load is the weight that comes and goes: people, furniture, and appliances, usually 40 pounds per square foot for living areas and 30 for sleeping areas under residential codes. Both loads act on the joist, so both are added together when sizing it.
What does L/360 deflection mean?
L/360 is the deflection limit for floors: the joist may sag by no more than the span divided by 360 under live load. For a 12 foot span that is 144 inches divided by 360, or 0.4 inches of sag at midspan. Floors with tile or stone need the stricter L/480 limit, which allows only 0.3 inches of sag on the same span, and that usually means a larger joist.
Does joist spacing change the size I need?
Yes. Closer spacing means each joist carries a narrower strip of floor, so it can span further. A joist at 12 inch spacing carries 25 percent less load than the same joist at 16 inch spacing, and at 24 inch spacing it carries 50 percent more. That is why the same 2 by 10 that spans 14 feet 8 inches at 16 inch spacing spans further at 12 inches and less at 24 inches.
Can I use this joist calculator instead of a building permit?
No. This calculator gives planning estimates based on published design values for standard lumber grades, and it does not replace the building code or a structural engineer. Always confirm the joist size with your local building code and a licensed professional before you build, especially for long spans, heavy loads such as tile, stone, or a bathtub, or any notched or drilled joists.