How joist count and spacing work
Joists run parallel to each other across the width of the frame, so the count comes from the width, not the span. Divide the width in inches by the on-center spacing to get the number of bays, then add one — the extra joist is the one that closes off the far end. A 16 ft wide deck at 16″ on center is 12 bays, so 13 joists. Total lumber is simply the joist count multiplied by the span, which is what you hand to the yard. Each joist that lands on a ledger or beam face normally takes a hanger at both ends, so the hanger count is roughly twice the joist count; joists that bear on top of a beam instead of hanging from it don't need one, so trim the number to match how your frame is actually built.
Spacing and span are linked. Tightening from 24″ to 16″ on center, or from 16″ to 12″, lets the same size lumber cross a longer distance and stiffens the floor noticeably underfoot — which matters for tile, stone and long-run flooring, where deflection cracks grout. Going the other way saves lumber but almost always means stepping up a joist depth. This calculator counts and prices the lumber; it does not tell you whether the size you picked is strong enough for your span.
Typical maximum spans (for orientation only)
The figures below are rough, commonly quoted ranges for solid sawn joists at 16″ on center under ordinary residential loading. They are typical values, not code values. Real allowable span changes with wood species and grade, live and dead load, wet-service exposure, cantilever, notches and holes, and whether it's an interior floor or an exterior deck.
| Nominal size | Typical max span at 16″ OC |
|---|---|
| 2×6 | about 8–9 ft |
| 2×8 | about 10–12 ft |
| 2×10 | about 13–15 ft |
| 2×12 | about 16–18 ft |
Do not size structural framing from this page. Joist sizing is set by your local building code and by the published span tables for the exact species and grade you buy. Confirm your spans with your building department, a licensed engineer, or the lumber supplier's span tables before you order or build.