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Roof/Floor Deflection Limit Calculator

Calculate the allowable absolute deflection for a given span and code deflection ratio limit.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax
19.69 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Allowable deflection

0.66 in

High confidence

At the values currently entered, the allowable deflection works out to 16.7 mm. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Add the equipment this sizes

This result is a specification — 0.656 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Standard code deflection limits are expressed as a span fraction (e.g. L/180, L/240, L/360); allowable deflection = span ÷ ratio

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Frequently asked questions

What do L/180, L/240, and L/360 mean?
These are span-fraction deflection limits — L/360 (span divided by 360) is a stricter limit commonly used for floors with plaster or similarly rigid finishes, while L/180 (roof, non-plaster) allows more deflection since it rarely sits below a brittle finish.
Which ratio should I use?
Use L/360 for floors with plaster or rigid finishes, L/240 for ceilings, and L/180 for roof members without a plaster ceiling below — always confirm against your governing building code, which may specify tighter limits for certain conditions.
Does a higher ratio number mean a stricter or looser limit?
Stricter — a higher ratio (e.g. 360 vs 180) divides the span into more parts, producing a smaller allowable deflection in mm, so L/360 is a tighter limit than L/180.