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Roof Truss Spacing & Count Calculator

Calculate total number of roof trusses needed for a given building length and truss spacing.

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
39.37 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Trusses needed

21 trusses

High confidence

At the values currently entered, the trusses needed works out to 21. Note that truss spacing (in, o.c.) sits at the high end of the range this calculator was checked against, so treat the output as indicative rather than settled. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

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.

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

  • Truss count = ceil(building length ÷ spacing) + 1, the standard method for spacing repetitive framing members along a run

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

Why add 1 to the truss count?
A truss sits at both ends of the run as well as at every spacing interval in between — dividing the length by the spacing gives the number of gaps, so one additional truss is needed to cap both ends of the run.
Why is truss spacing rounded up to the next whole gap?
Rounding up (ceiling) ensures no gap between trusses exceeds your specified maximum on-center spacing — the last gap may end up slightly tighter than the target spacing, which is the standard, conservative approach to laying out repetitive framing.
Does this account for gable-end trusses or hip sets?
No — this is a straight run count only. Gable-end trusses, hip corner sets, and any girder trusses for openings are additional to this count and should be added separately per your truss layout drawing.