Joist span is where most framing plans go wrong first. This calculator sizes wood floor joists by species, grade, size, spacing, and deflection limit using the same NDS method the span tables are built from, so you can see what governs the span instead of just reading a number off a chart.
Open the CalculatorEvery joist span is governed by one of three limits, and knowing which one controls tells you how to fix a span that comes up short.
Bending. The joist has to resist the moment from floor load without exceeding the adjusted bending stress for its species and grade. Bending usually governs shorter, heavily loaded spans and lower grades.
Deflection. The joist has to stay stiff enough that the floor feels solid. Code minimum is L/360 under live load. A floor that meets L/360 is legal. It is not necessarily a floor you would want under a tile kitchen.
Shear. Rarely governs sawn floor joists at normal spans, but the calculator checks it anyway, because "rarely" is not "never."
2x10 Douglas Fir-Larch #2, 16 inches on center, 40 psf live load and 10 psf dead load, dry service.
Bending governs, and the allowable span is 15 ft 7 in, which is exactly what the code span table gives for this case. That agreement is the point: the calculator is running the method behind the table, not approximating it.
Now run the same joist at L/480 live load, the standard we recommend for most floors:
That nine-inch difference is the gap between a code-minimum floor and a quiet one.
Code minimum deflection is L/360 on live load. Bardowl Design specifies L/480 on total load, or a half-inch absolute cap, whichever is tighter, for floors that carry finished spaces. Stiffer floors cost a size increment now and save a callback later. The calculator lets you run both so the difference is a number, not an opinion.
Code minimum is L/360 under live load. For finished living space, especially under tile, stone, or long open rooms, use L/480 or tighter. The stiffer limit typically costs one size or one grade increment and eliminates most bounce complaints.
They trade against each other. Moving from 16 to 12 inches on center adds roughly a third more capacity per foot of floor, which can buy a longer span from the same member. Whether that beats upsizing depends on material cost and what the subfloor spans. The calculator lets you compare directly.
If the inputs match, it should not. Differences almost always come from inputs: wrong grade, wet service where dry was assumed, a missing repetitive member factor, or comparing a live-load deflection limit against a total-load one.
Deck joists add wet service and, for treated lumber, incising adjustments, and the code handles them in a separate section. Use the deck framing calculator for exterior work.
No. It sizes single members under uniform load with stated assumptions. Point loads, cantilevers, stacked walls, and anything with a load path question need design work.
Results from this calculator are preliminary sizing guidance for comparison and planning. They are not engineering, not sealed drawings, and not a substitute for design by a licensed professional under your local code. Final member sizing, connections, and load paths must be confirmed for your specific project.
Bardowl Design produces structural layouts and material specifications that builders and lumberyards can actually work from. Clear scope, defined assumptions, documented output.
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