Floor Joist Calculator

What Is a Floor Joist Calculator?

A floor joist calculator estimates how many floor joists and end joists you need for a decking or subfloor framing project. Enter room dimensions, joist size, on-center spacing, and lumber prices to get joist count, recommended lengths, and a material cost estimate.

How Floor Joist Count Is Calculated

Floor joists run perpendicular to the subfloor span direction. The number of joists along the floor length is:

$$ n = 1 + \frac{L - w}{s} $$

Where $L$ is the floor length in inches, $w$ is the actual joist thickness (1.5 inches for nominal 2-inch lumber), and $s$ is the on-center spacing in inches. The result is rounded up to the nearest whole joist.

Standard on-center spacings are 12, 16, 19.2, and 24 inches. Sixteen inches is the most common choice for residential floors.

End Joists and Cost

Two end joists run perpendicular to the main floor joists along the edges of the floor frame. Each end joist spans the full floor length. Add a waste allowance of 10% to cover cuts, defects, and miscalculation.

For wall framing stud counts, use the Framing Calculator. For shed projects, see the DIY Shed Cost Calculator.