Calculate the number of joists.

To calculate the number of joists needed, you will first need to determine the spacing between each joist. Typically, joists are spaced 16 inches or 24 inches apart, but this can vary depending on the specific requirements of your project.

Once you have determined the spacing, you can calculate the number of joists by dividing the total length of the area to be covered by the spacing between each joist.

For example, if you have a total length of 20 feet and are spacing your joists 16 inches apart, you would calculate it as follows:

Total length in inches = 20 feet x 12 inches/foot = 240 inches
Spacing between joists = 16 inches

Number of joists = Total length in inches / Spacing between joists
Number of joists = 240 inches / 16 inches = 15 joists

Therefore, you would need 15 joists for a total length of 20 feet with joists spaced 16 inches apart.

To calculate the number of joists, you need the following information:

- Span: The distance between the supporting beams.
- Spacing: The distance between the joists.
- Joist size: The width and height of the joists.
- Length: The length of the joists.

Once you have this information, you can use the following formula:

```
Number of joists = (Span / Spacing) + 1
```

For example, if you have a span of 12 feet, a spacing of 16 inches, and joists that are 2x8, you would need:

```
(12 - 12 / 16) + 1 = 9 joists
```