I-Joist Calculator

How the I-joist calculator counts joists

The count depends on two numbers: the width of the floor measured across the joists, and the spacing between joist centres. Divide the width by the spacing, round up, and add one for the joist at the far edge. A floor 16 ft wide at 16 in centres is 192 ÷ 16 = 12 spaces, so 13 joists; at $22 each that's $286. In metric, 4 m at 400 mm centres is 10 spaces and 11 joists, £198 at £18.

The width here is not the span. The span is the distance each joist runs between its supports, and it decides which I-joist you need; the width across the joists decides how many. The calculator only does the second job.

If the width doesn't divide exactly, the last space is simply narrower. A 13 ft floor at 16 in centres is 156 ÷ 16 = 9.75, which rounds up to 10 spaces and 11 joists, with the final gap 12 in instead of 16.

The spacings I-joists actually use

Manufacturers design around a short list of centres. Weyerhaeuser's TJI specifier's guide publishes its floor span tables at 12, 16, 19.2 and 24 in on centre, and James Jones's JJI-Joist technical manual uses 300, 400, 480 and 600 mm. The odd-looking ones exist because of the sheet sizes: 19.2 in is five spaces in an 8 ft board and 480 mm is five in a 2.4 m board, so the sheet edges still land on a joist. Both are accepted by the calculator.

On the same 16 ft wide floor, 12 in centres need 17 joists, 16 in need 13, 19.2 in need 11 and 24 in need 9. In metric, 4 m at 300, 400, 480 and 600 mm needs 15, 11, 10 and 8.

The imperial and metric centres are close but not the same: 16 in is 406 mm and 19.2 in is 488 mm. If you are working in metric with a product whose tables are in inches, use the product's own spacing rather than rounding it to 400 or 480.

A whole floor

Take a floor 16 ft by 24 ft with the joists spanning the 16 ft direction. The width across the joists is 24 ft, so at 16 in centres it's 288 ÷ 16 = 18 spaces and 19 joists, and at 19.2 in it's 15 spaces and 16 joists. Rim board runs along both 24 ft ends where the joists bear, so allow about 48 ft of it on top. The metric version, 4.8 m by 7.2 m spanning 4.8 m, is 19 joists at 400 mm and 16 at 480 mm, with about 14.4 m of rim.

Why fewer joists isn't automatically cheaper

Wider spacing saves joists but shortens how far each one can span. The TJI guide's table for an 11⅞ in TJI 210, at 40 psf live load and 10 psf dead load with L/480 deflection, gives 21'-1" at 12 in centres, 19'-3" at 16 in, 18'-2" at 19.2 in and 16'-11" at 24 in. The same guide notes that its floor spans assume glued and nailed span-rated subfloor panels, and that spans drop by 6 in with nails alone. So spacing and joist choice have to be settled together, from the manufacturer's tables for your loads, before this count means anything.

What the count leaves out

The result is the number of full-length joists across one floor bay. A real I-joist floor needs more pieces than that. Rim board or rim joists close the ends. Blocking, web stiffeners at some bearings and hangers, and extra or doubled joists under load-bearing walls and around openings, all come from the manufacturer's details for your layout. Where a floor has more than one bay, or joists change direction, run the calculator once per bay.

Bearing matters too. The TJI tables assume at least 1¾ in of bearing at the ends and 3½ in at intermediate supports without web stiffeners. The JJI-Joist site guide asks for at least 45 mm at the ends and 89 mm where a joist runs continuously over a wall.

On site

Both manufacturers warn that I-joists are unstable until they're fully braced: TJI lists blocking, hangers, rim board and sheathing among the bracing, and JJI lists longitudinal binders, diagonal bracing, stability blocking and rim boards. The TJI guide says not to walk on joists lying flat and not to stack materials on joists that aren't sheathed; the JJI guide asks for all binders, bracing, blocking and hangers to be completely installed and fully nailed as detailed. Plan deliveries so the deck goes down promptly behind the joists.

Services are the other common site problem. Both guides say never to cut or notch the flanges, and holes through the web have to follow the manufacturer's hole charts for size and distance from supports. Agree pipe and duct routes before the joists go in, because a joist cut in the wrong place may need replacing or an engineer's repair detail.

Treat the price per joist as a placeholder until you have a quote for the actual series and length. I-joists are sold by depth, series and length, and the price of an 11⅞ in joist at 16 ft is not the price of a 14 in joist at 20 ft. This count feeds that quote; the design behind it comes from the manufacturer's tables or a structural engineer.