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The side of the floor the joists are laid out along, outside to outside of the framing.
Joists run across the floor and are spaced along this side. It is also the length of each rim joist, which closes the joists' ends along it.
Tools needed: Tape measure
The side each joist crosses, outside to outside of the rims.
Each joist is this width less the two rim joists it is fixed between. Where a beam or wall carries the joists part-way across, choose the lapped option below and the line is cut into two pieces.
The centres from your framing plan or span table.
Centre to centre, the figure the span table was read at. Metric floors are commonly 400, 450 or 600 mm; North American floors 12, 16, 19.2 or 24 in.
One joist's thickness, which is also one rim's.
Two rims come off the floor's width to give each joist's length, and one thickness comes off each block. A 2x joist is 1 1/2 in (38 mm) under PS 20.
A single length across the floor, or two lengths meeting over a beam or wall.
Where a beam or bearing wall carries the joists mid-floor, each line is two shorter pieces that lap over it. Section R502.6.1 of the IRC laps them at least 3 in (76 mm) and nails them together.
Joists your plan doubles under a partition, a bath or at an opening.
Each is one more joist line, cut like the others. Trimmers and headers round a stair opening are entered here too, at their full line length, which errs long.
The rows of solid blocking your plan shows; the IRC requires them on joists deeper than a 2x12.
Section R502.7.1 of the IRC braces joists deeper than a nominal 2x12 at the intervals it sets with blocking, bridging or a strip under the joists; many plans block smaller joists at mid-span too. Each row is one block in every bay.
Joist pieces to cut
26 joists
Joist lines at the spacing from one end of the floor to the other, each one piece between the rims; rims along both ends of the joists, and a block in every bay of each row.
- Joist lines at the spacing
- 26 lines
- Extra joist lines
- 0 lines
- Length of each joist piece
- 19.25 ft
- Joists, lineal
- 500.5 ft
- Rim joists, lineal (both ends of the joists)
- 66 ft
- Blocks
- 0 blocks
- Blocking, lineal
- 0 ft
- Joist-depth timber in all, lineal
- 566.5 ft
They open the calculator with your figures already in it
Floor Joist Quantity Calculator (Joists, Rim Joists and Blocking): 26 joists — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- California Residential Code (2025) § R502.7: joists supported laterally at the ends by full-depth solid blocking, or by attachment to a full-depth header, band or rim joist; § R502.7.1: joists deeper than a nominal 2 in by 12 in supported laterally by solid blocking, diagonal bridging or a 1 x 3 strip at intervals not exceeding 8 feet (2438 mm); § R502.6.1: joists from opposite sides of a bearing lap not less than 3 inches (76 mm)
- Southern Pine Inspection Bureau, "Nominal vs. Actual Lumber Sizes": dressed sizes per PS 20-20, the American Softwood Lumber Standard; a nominal 2-inch member dresses to 1 1/2 in
Inputs used
- Floor Length (joists spaced along it)
- 33 ft
- Floor Width (the way the joists run)
- 19.5 ft
- Joist Spacing (on centre)
- 16 in
- Joist Thickness
- 1.5 in
- Joist Lines
- One piece rim to rim
- Lap Over the Bearing
- 3 in
- Extra Joist Lines (doubled joists)
- 0
- Rows of Blocking Across the Joists
- 0
Intermediate steps
- Joist lines at the spacing
- 26 lines
- Extra joist lines
- 0 lines
- Length of each joist piece
- 19.25 ft
- Joists, lineal
- 500.5 ft
- Rim joists, lineal (both ends of the joists)
- 66 ft
- Blocks
- 0 blocks
- Blocking, lineal
- 0 ft
- Joist-depth timber in all, lineal
- 566.5 ft
Confidence note: Joist lines at the spacing from one end of the floor to the other, each one piece between the rims; rims along both ends of the joists, and a block in every bay of each row.
What this calculation does not cover
- Nothing here sizes a joist. The depth, grade and spacing that carry the floor come from your code's span tables or an engineer, and the page counts what you enter.
- A rectangle framed one way. Openings for stairs and chimneys, their trimmer and header joists, cantilevers and a floor framed in two directions are not counted; run each part separately and add any trimmers as doubled joists.
- The joist lines are laid from both ends of the floor, so the last bay is the short one. Lines that must land under walls above, or under the edges of the sheathing, can add a line or two that this does not show.
- Each block is a full bay, the spacing less one joist's thickness. The short bay at the end of a row takes a shorter block, so the blocking's lineal figure is slightly high.
- Hangers, nails, the sill plate and the subfloor are separate items. Rim board in engineered sizes has a page of its own.
- Nothing is added for off-cuts or for rounding to stock lengths; each piece is its cut length.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-10 · v1.0.0
Regulatory standards & verification citations2
- California Residential Code (2025) § R502.7: joists supported laterally at the ends by full-depth solid blocking, or by attachment to a full-depth header, band or rim joist; § R502.7.1: joists deeper than a nominal 2 in by 12 in supported laterally by solid blocking, diagonal bridging or a 1 x 3 strip at intervals not exceeding 8 feet (2438 mm); § R502.6.1: joists from opposite sides of a bearing lap not less than 3 inches (76 mm)
- Southern Pine Inspection Bureau, "Nominal vs. Actual Lumber Sizes": dressed sizes per PS 20-20, the American Softwood Lumber Standard; a nominal 2-inch member dresses to 1 1/2 in
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Tools and safety for this job
To cut framing lumber and frame joists and a deck. Generic types, no brands, no prices.
Protection this work requires
- Hardwood dust is a recognised carcinogen: extract at the tool and wear a P2/N95 respirator when cutting or sanding indoors.
- Saws, grinders and breakers run above 85 dB, where hearing damage accumulates and does not recover: defenders or plugs for every cut, not just the long ones.
- Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.
- Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.
Show the 10 tools this job needsHide tools
Essential
Circular saw
Claw hammer
Combination or framing square
Cordless drill/driver
Spirit level
Tape measure
Recommended
Impact driver
Mitre saw
String line and pins
Optional
Hand saw
Also needed as materials: circular saw blades, drill and driver bits, impact driver bits, mitre saw blades.
what each carpentry tool is for, and the spec that decides which to buy where one does.