How Many Floor Joists Do I Need? Count, Spacing & Worked Examples
Planning count, not a structural design. The joist count is simple division plus one end joist — but the size and span still need a span table. This guide gives you the count method, then run your floor through our floor joist calculator.
The count formula
Joists = ceiling(floor width in inches ÷ spacing in inches) + 1. The +1 is the end joist at the far edge. A 20 ft wide floor at 16 inches on center: 240 ÷ 16 = 15, plus 1 = 16 joists. A 15 ft wide floor at the same spacing: 180 ÷ 16 = 11.25, rounded up to 12, plus 1 = 13 joists. If you forget the +1, the last bay of your floor has no joist — the most common counting error.
Width vs span — do not swap them
Width is the distance across the joists (the direction you step from joist to joist). Span is the length of each joist, support to support. The count uses the width; the span sets the board length you buy. A floor 20 ft across with 12 ft joists needs 16 joists of 12 ft each — not 12 joists of 20 ft. Measure the width across the joists, in the same direction as the subfloor seams run.
Spacing: 12, 16, 19.2, or 24 inches?
16 inches on center is the residential standard: it matches 4×8 ft subfloor sheets (each sheet edge lands on a joist) and suits most loads. 12 inches is for heavy loads, tile floors (which need a stiffer floor so grout does not crack), and long spans. 19.2 inches appears in engineered layouts because five bays of 19.2 inches exactly span an 8 ft sheet. 24 inches suits light loads, short spans, and some engineered systems only — check the table before using it.
Rim joists, blocking, and doubled joists
Rim joists are the two end boards perpendicular to the field joists, each as long as the span — count them separately. Blocking (short pieces between joists) stiffens the floor and stops joists twisting; plan roughly one row per 8 ft of span if your layout calls for it. Doubled joists (two nailed together) go under partition walls running parallel to the joists, around stair and chimney openings, and under heavy point loads like bathtubs — add them on top of the field count.
Worked examples
- 20 ft wide, 16 in spacing, 12 ft span: ceiling(240 ÷ 16) + 1 = 16 joists, 2 rim joists, 16 × 12 + 2 × 12 = 216 linear ft.
- 15 ft wide, 16 in spacing, 10 ft span: ceiling(180 ÷ 16) + 1 = 13 joists, 2 rim joists, 13 × 10 + 2 × 10 = 150 linear ft.
- 12 ft wide, 12 in spacing, 8 ft span: ceiling(144 ÷ 12) + 1 = 13 joists, 2 rim joists, 13 × 8 + 2 × 8 = 120 linear ft.
The span-table warning
This guide counts joists; it does not size them. A joist that is too small for its span gives a bouncy floor, cracked drywall, and cracked tile — and a joist that is too long for its size can fail. Always check the size and spacing against your local span tables (IRC tables for dimensional lumber, manufacturer tables for I-joists and LVL) for your species, grade, spacing, and load, and get an engineered design for anything unusual.
Frequently asked questions
How many floor joists do I need for a 20-foot floor?
A 20 ft wide floor at 16 inches on center needs 16 joists, plus 2 rim joists. Order 18 joists with 10% waste.
What is the standard floor joist spacing?
16 inches on center for most residential floors. 12 inches for heavy loads and tile; 24 inches only for light loads and short spans.
Do I count the rim joists separately?
Yes — two rim joists, each as long as the span, running perpendicular to the field joists. They are not part of the field-joist count.
How do I measure floor width for joists?
Measure across the joists — the direction you step from joist to joist — not along them. The span is the joist length, support to support.
Can I use 24-inch spacing for a bedroom floor?
Only if your span table allows it for your joist size, species, and load. For most dimensional lumber at bedroom spans, 16 inches is the safe default.
The rest of the floor frame
Joists are one part of the frame — the wall stud calculator counts the walls they sit on, and the stair calculator covers the stair opening in the floor.