Joist spacing calculator
Everything above is arithmetic on the run and the spacing you enter. Ends, headers and doubled joists at openings add members this layout does not know about, so check the count against your framing plan before ordering.
Enter the length the joists cross and the spacing, and the tool counts the members and any rows of blocking. The joist count is the run divided by the spacing, rounded up for the last bay, plus one for the joist that lands on the end.
Exemple détaillé
A 14-foot run at 16-inch centres is 168 ÷ 16 = 10.5, which rounds up to 11 bays, so 12 joists. One row of blocking sits in each of the 11 gaps between joists, so that is 11 blocking pieces. Two rows would be 22.
In metric: a 4.2 m run at 400 mm centres is 4,200 ÷ 400 = 10.5, rounded up to 11 bays and 12 joists, with the same 11 blocking pieces a row.
Sur le chantier
A floor over a centre beam. 24 × 32 ft, joists across the 24 with a beam down the middle. The count runs along the 32 ft: 384 ÷ 16 = 24 bays, 25 joists per side. They lap on the beam, so it is 50 pieces of 12-footers-and-a-bit — plus the two rim joists along the 32 ft and any doubles under a partition. The length each joist has to span is the engineer's business; this page only counts them.
A deck at 12 in centres. 12 × 16 ft, joists across the 12, count along the 16: at 16 in it is 12 bays and 13 joists; at 12 in it is 16 bays and 17. Four more sticks — and if the plans say 12, it is 17. The end joists usually double as rim; the count here is the joists, the rim is on top.
Blocking, cut to length. 25 joists is 24 gaps a row. At 16 in centres with 1½ in joists each block is 14½ in, so a row is 24 × 14.5 = 348 in = 29 lin ft, and two rows is 58 ft — four 16 ft sticks with the offcuts, or the crooked joists you culled. In metric, 38 mm joists at 400 mm centres give a 362 mm block: 24 × 362 = 8,688 mm, about 8.7 m a row.
What went wrong?
Quick referencefor joists
- Solives
- (run in inches ÷ spacing), rounded up, + 1
- At 16 in o.c.
- 8 ft 7 · 12 ft 10 · 16 ft 13 · 20 ft 16 · 24 ft 19 · 32 ft 25
- At 12 in o.c.
- 8 ft 9 · 12 ft 13 · 16 ft 17 · 20 ft 21 · 24 ft 25
- At 24 in o.c.
- 8 ft 5 · 12 ft 7 · 16 ft 9 · 20 ft 11 · 24 ft 13
- At 400 mm o.c.
- 2,4 m 7 · 3,6 m 10 · 4,8 m 13 · 6 m 16 · 7,2 m 19
- Blocking
- (joists − 1) × rows · each piece = spacing − 1½ in (14½ at 16, 22½ at 24) · 400 − 38 = 362 mm
- What this page does not do
- span — the size a joist must be for its length is the span table's and the engineer's
Waste & overagewhat to add, and why
- Framers buy one or two extra joists per floor for the crooked ones and the double under a partition; blocking cuts from the offcuts, so it rarely needs extra stock.
Mistakes to catchbefore you order
- Forgetting that a 14 ft run at 16 in centres needs a joist at each end — the tool counts both, so do not add one 'for the end' again.
- Typing 16 into a field set to metres; 16 in is 406 mm, and a 16 m spacing gives one joist.
- Counting doubled joists under bearing walls and around stair openings as a spacing problem — they are extra pieces from your plan, added on top of the count here.
What an inspector looks forthe building inspector, at the framing call
On this part of the job
- Spacing measured across the opening, and that the layout still holds at the far end.
- Span and joist size against the span table the code adopts or against the engineer's design. This tool spaces joists; it does not size them, and the inspector will be reading a table you should have read first.
- Bearing at each end — how much of the joist actually sits on the support, and whether the hanger is the right one and fully nailed with the fasteners it was tested with.
- Blocking, bridging or strapping where the code or the drawing calls for it, and doubled joists under parallel partitions.
- Notches and holes: where in the span they fall and how big they are. Drilled joists are the most common framing failure on a floor.
- Anything the drawing shows as engineered lumber left exactly as the supplier's layout shows it — those come with their own installation sheet.
Have this ready
- The floor plan with the joist direction, size and spacing marked.
- The joist supplier's layout and installation sheet for engineered product.
- A tape, and the hangers' own nailing instructions if you have them.
Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.
La formule
joists = ⌈length ÷ spacing⌉ + 1 · blocking = (joists − 1) × rows
The run divided by the spacing is the number of bays; rounding up covers the partial bay at the end, and the plus one is the joist on the far side of the last bay. There is always one fewer gap than there are joists, which is why blocking multiplies the joist count minus one.
Questions
- How many joists do I need at 16 inch centres?
- Divide the length you are filling by the spacing, round up for the last bay, and add one for the joist that lands on the far end. A 14-foot run at 16-inch centres is 168 ÷ 16 = 10.5, rounded to 11 bays, so 12 joists. In metric, a 4.2 m run at 400 mm centres is 4,200 ÷ 400 = 10.5, so 11 bays and 12 joists. This tool spaces and counts them; it does not size them.
- Does this tell me what size joist to use or how far it can span?
- No — and that is deliberate. Allowable span depends on the joist size, species, grade, load and deflection limit, all of which live in span tables and structural design. This tool only lays out spacing and counts members. Take the span and size question to your span book, your plans, or an engineer.
- What spacings are common for joists?
- Sixteen inches on centre is the usual default and where this tool starts (400 mm in metric, with 300 and 600 mm the other common layouts). Nineteen-point-two (five joists to the eight-foot module) and twenty-four inch layouts also appear. Enter whatever your drawings specify — the field is editable and the count follows the spacing you type.
- How do I count rows of blocking or bridging?
- Blocking runs perpendicular between the joists, usually in one or more rows across the run. Set the number of rows and the tool multiplies the gaps between joists by the rows to give the number of blocking pieces. Whether blocking is required, and where, is a code and design matter.
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