Floor Joist Calculator

A floor joist calculator is a valuable tool used in construction to determine the correct size and spacing of floor joists.

While constructing a 20-foot wide room, a floor joist span calculator would take into account several factors:

• Span of the joists (20 feet in this case)
• Expected load on the floor (including furniture, people, and appliances)
• Type of wood or material used for the joists
• Desired spacing between joists

By inputting these parameters, the calculator can recommend an appropriate joist size and spacing, such as 2x12 joists spaced 16 inches on center.

Floor Joist Calculation Chart

Floor Joist Span Formula

The formula for calculating the maximum span of a floor joist is:

L = (d^2 / 240) x √(E / f)

Where:

• L = Maximum span in inches
• d = Depth of the joist in inches
• E = Modulus of elasticity of the wood
• f = Allowable bending stress of the wood

To calculate the maximum span for a 2×10 joist (actual depth 9.25 inches) made of Douglas Fir-Larch:

• d = 9.25 inches
• E = 1,900,000 psi
• f = 1,000 psi

The calculation yields:

L = (9.25^2 / 240) × √(1,900,000 / 1,000) ≈ 176 inches or 14.7 feet

This calculation indicates a 15-foot maximum span for 2×10 joists at 16 inches on center.

Calculating the Number of Floor Joists Needed

To determine the number of floor joists required, follow these steps:

• Measure the length of the area to be spanned.
• Decide on the required joist spacing (typically 16 or 24 inches on center).
• Divide the length by the spacing and round up to the nearest whole number.
• Add one extra joist for the end of the span.

For example, for a 20-foot room with joists spaced 16 inches on center:

Length = 240 inches (20 feet x 12 inches/foot)

Spacing = 16 inches

Calculation: 240 / 16 = 15

Total joists needed: 15 + 1 = 16 joists.