A deck ledger is the board that fastens the deck directly to the house, and it carries roughly half of the entire deck load straight into the building. Under-fastening a ledger is one of the most common and most dangerous deck failures, so the building code prescribes exactly how many bolts you need and how far apart they go.
How it works
The required on-center bolt spacing depends on two things: how far the deck joists span, and what fastener you use. A longer joist span means each foot of ledger supports a wider strip of deck, so the bolts must come closer together. The tool looks up the maximum spacing from IRC Table R507.9.1.3(1) for your span band and fastener, then counts how many fasteners fit along the ledger:
spacing = lookup(joist span band, fastener type)
count = floor((ledger_length_in - 2 x 2) / spacing) + 1The two-inch offset keeps the end bolts away from the board ends. The result is split into a top and bottom row because the code requires the fasteners staggered in two rows.
Tips and notes
Through-bolts allow wider spacing than lag screws because they clamp the full thickness of the band joist. Always pair the ledger with proper flashing to keep water out of the connection, hold fasteners at least two inches from the top and bottom edges of the ledger, and never lag into brick veneer or a band joist hidden behind foam sheathing. This calculator is a planning aid that mirrors the published table; your local building official has the final say on any deck attachment.