Construction

Floor Joist Calculator

Count floor joists from length and spacing.

Floor length

Joist spacing

Calculated Output

Joists needed 13

Formula: joists = floor length × 12 ÷ spacing + 1

Step-by-Step Breakdown

1. Values used:

2. Floor length = 1

3. Joist spacing = 1

5. Joists needed = 13

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The math behind it

joists = length (in) ÷ spacing + 1 (for the rim joist). 16 in on-center is standard.

20 ft at 16 in → 16 joists.

Double joists?

Add extra joists under load-bearing walls or large openings.