Floor Joist Calculator: Joists, Rim Boards and Subfloor Sheets

Enter the length of the floor along the line of the joists spacing, the span of each joist and the spacing. You get the joists, the stock length, the rim joists, the blocking, the hangers and the 4 by 8 subfloor sheets.

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Example job shown. Change the numbers to match yours.

Joists

13 joists

One at each end and one every 16 in

Stock length of each joistThe next even length that covers the span of 12.0 ft: 12 ft
Rim joists (the two end boards)Each as long as the floor, 16.0 ft: join two pieces on a joist if it is longer than the stock: 2 boards
Blocking piecesOne between each pair of joists in each row: 12 pieces
Joist hangersOne at each hung end of each joist: 26 hangers
Subfloor sheets of 4 × 8 ftWith 10% extra: 7 sheets

The count is the length of the floor divided by the spacing, plus one for the last joist. The size of joist (2×8, 2×10 and so on) depends on the span, the spacing, the species and grade of the lumber and the load, and comes from the span tables in your local code or from your plans: this calculator does not choose it. Add doubled joists under walls that run the same way, and headers and trimmers around stairs and chimneys. The rim joist closes the ends of the floor and holds the joists upright.

Before you buy: round up to whole packs, check the size printed on the label, and add a little spare. Your supplier has the final say.

How this calculator works

Worked example: a floor 16 ft long with joists 16 in on center needs 16 × 12 ÷ 16 = 12 spaces, plus one: 13 joists. For a 12 ft span the stock length is 12 ft. One row of blocking is 12 pieces, and hangers at both ends are 26. The floor is 192 sq ft, which with 10% extra is 7 sheets of 4 × 8 ft. At 12 in it is 17 joists, at 19.2 in 11, and at 24 in 9.

Joists across a floor 16 ft long, for each spacing

Joists to buy for the length of floor at each spacing.

Length of floor	12 in	16 in	19.2 in	24 in
8 ft	9	7	6	5
10 ft	11	9	8	6
12 ft	13	10	9	7
16 ft	17	13	11	9
20 ft	21	16	14	11
24 ft	25	19	16	13

Tips before you buy

Choose the joist size from the span table in your code or from your plans, not from a guess.
Mark the layout from one end, and check that the edges of the subfloor sheets land in the middle of a joist.
Crown the joists, with the high edge up, and sight along each one before you fix it.
Notch and drill joists only where the code allows, away from the middle of the span and the ends.
Glue the subfloor to the joists as well as screwing it, to stop squeaks.

Tools & materials for this project

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Joist hangers 2×10 Structural screws Subfloor adhesive Subfloor sheet 3/4 in Chalk line

Frequently asked questions

How many joists do I need?

The length of the floor in inches divided by the spacing, rounded up, plus one: 13 joists for 16 ft at 16 in on center.

How far apart should floor joists be?

Usually 16 inches on center, and 12 or 19.2 or 24 inches in some floors. The spacing goes with the joist size, the span and the subfloor thickness.

What size joist do I need?

It depends on the span, the spacing, the species and grade of the lumber and the load. Use the span tables in your local code or ask the building department: this calculator counts but does not size.

Why is 19.2 inches a spacing?

Five spaces of 19.2 in make 8 ft, so the edges of a 4 × 8 ft sheet land on a joist at 19.2 in just as they do at 12, 16 and 24 in.

How many subfloor sheets do I need?

7 sheets of 4 × 8 ft for this floor, with 10% extra.

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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.

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