Calculate Distance Between Floor Joists for Subfloor

Joist spacing is measured on centre — from the centre of one joist to the centre of the next — not between their faces. The three standard layouts are 16 in, 19.2 in and 24 in on centre, chosen because each divides evenly into the 96 in length of a sheet of sheathing (six, five and four spaces respectively), so panel edges always land on a joist.

Count joists with the grid formula: (floor dimension ÷ spacing) + 1. A 16 ft wide floor at 16 in on centre is (192 ÷ 16) + 1 = 13 joists, running the direction of the shorter span. Add doubled joists under parallel partition walls and around any stair or chimney opening, plus blocking or bridging at mid-span for joists deeper than 2×10.

Match sheathing thickness to spacing. At 16 in on centre, 19/32 in (5/8) plywood or OSB is the code minimum and 23/32 in (3/4) is what most builders use. At 19.2 in you want 23/32 in. At 24 in on centre you need 7/8 in or a rated 24 in OC subfloor panel; using 3/4 in over 24 in joists is the classic cause of a bouncy, squeaky floor.

Check the span, not just the spacing. A 2×10 southern yellow pine joist at 16 in on centre spans roughly 16 ft under a normal residential live load; the same joist at 24 in on centre loses about two feet of allowable span. Deflection, not strength, is usually the limit — the L/360 rule means a 15 ft span may only sag half an inch under load and still feel springy, which is why many builders design one step stiffer than the minimum.

Finally, lay the panels with the long edge perpendicular to the joists, stagger the end joints by at least one joist bay, leave a 1/8 in gap at panel edges for expansion, and glue plus screw rather than nail. Glue is what stops squeaks; screws are what keep it that way.