Floor Joist Calculator

1. Enter your floor or deck length (the side the joists are spaced along) and width (the distance each joist has to span) in feet.
2. Pick your on-center spacing. 16 inches is the most common choice for floors and decks; use 12 inches for a stiffer floor or heavy loads, and 24 inches for light-duty framing.
3. Read the joist count and total lumber instantly.
4. Open More options to switch to metric, add a waste allowance, turn rim/end joists on or off, or estimate cost per board.

A joist calculator answers one core question: how many parallel boards do you need to frame a floor or deck? Joists are the horizontal boards that carry the subfloor or decking above them. They run across the width of the area and repeat at a fixed on-center (OC) spacing along the length.

The count is not simply length divided by spacing. Because you need a joist at *both* ends of the run, you add one extra board: a 20-foot run at 16-inch spacing has 15 gaps but 16 joists. Most builders then add two rim (band) joists that cap the ends of the run and tie the frame together, so the total board count is usually the field joists plus two.

Tighter spacing means more joists, a stiffer floor, and less bounce underfoot.

```
Field joists = ceil( length_in / spacing_in ) + 1
Total joists = field joists + 2 rim joists
Total lumber = (field joists x width) + (2 x length)
```
Length and width are converted to inches for the spacing math. The +1 adds the closing joist at the far end of the run, and the ceil (round up) covers a run that isn't an exact multiple of the spacing. Add a waste percentage to cover offcuts and blocking.

How many joists for a 12x20 deck? Using the 20-ft side as the run at 16-inch OC: 20 ft = 240 in, 240 / 16 = 15, +1 = 16 field joists spanning 12 ft, plus 2 rim joists = 18 joists.

Same deck at 24-inch OC: 240 / 24 = 10, +1 = 11 field joists + 2 rim = 13 joists — fewer boards, but a springier deck.

A 10x10 floor at 16-inch OC: 120 / 16 = 7.5, round up to 8, +1 = 9 field joists + 2 rim = 11 joists.

- 16-inch OC is the default for residential floors and most decks; it balances cost and stiffness.
- Go to 12-inch OC under tile, heavy loads, or long spans to cut deflection and bounce.
- Check your span tables — spacing plus joist depth (2x8, 2x10, 2x12) together set the maximum allowable span for your lumber species.
- Add about 10% waste for blocking, offcuts, and the occasional crooked board.
- Buy each joist in the next standard length (8, 10, 12, 16 ft) at or above your width so every joist is a single uncut board.