Deck Joist Calculator
One direction count: floor(span ÷ spacing) + 1.
Step 1: Units
Step 2
Enter project measurements
Use decimals if needed (e.g. 3.5).
How to use this calculator
Enter deck width perpendicular to joists and joist spacing on center.
What the math is doing
Joists = floor(width ÷ spacing) + 1.
Frequently asked questions
Beam locations?
May require doubled joists locally.
Cantilever?
Follow span tables.
Hangers?
Count separately.
Blocking?
Linear feet accessory.