Deck Joist Calculator

About this calculator

This calculator estimates how many joists a deck frame needs by dividing the deck's length by the chosen on-center spacing (commonly 12in, 16in or 24in) and adding one for the starting joist, then figures the required lumber length for each joist plus the side beams (rim joists) that run along the deck's perimeter to tie the frame together.

DIY deck builders and contractors use this before ordering lumber, since spacing directly affects how much the decking material can span without sagging underfoot — closer 12in spacing is typical under composite decking, while solid wood decking can often use wider 16in or 24in spacing, and getting the joist count and beam lengths right up front avoids under- or over-ordering material.