Roof Truss Calculator

Convert the building length to inches. Divide by the truss spacing and round up to get the bays. Add one truss to close the last bay. A 40 foot building is 480 inches. At 24 inches on center that is 20 bays and 21 trusses. At 16 inches it is 30 bays and 31 trusses.

1. Measure the building length along the ridge. Trusses repeat in this direction.
2. Multiply the length in feet by 12 to get inches.
3. Divide by the spacing in inches on center. Round up.
4. Add one for the truss at the far end.
5. Check the last bay. A short bay is fine, but tell your supplier.

To check the roof surface above the trusses, use our roofing calculator. For wall studs and plates below them, use the framing calculator.

The building is 40 feet long and 24 feet wide, with a 6 in 12 roof, a 12 inch overhang and trusses at 24 inches on center. The count is 21 trusses. Each truss has a 26.6 degree roof angle, a 6 foot rise and a 14.53 foot top chord. The Fink layout uses 74.70 linear feet of lumber per truss.

A 31 foot building at 24 inches is 372 inches. That is 15.5 bays, which rounds to 16 bays and 17 trusses. The last bay is 12 inches.

Most residential roofs use 24 inches on center, measured from the middle of one truss to the middle of the next. Heavier roofing can call for closer spacing. Steeper roofs, heavy rainfall and attic storage can too. Local code and your truss designer set the final number.

Source: Today's Homeowner, read October 2026. Pole barns have no fixed standard. The same source gives 4 to 12 or 16 feet. Closer spacing raises the count: a 40 foot building needs 21 trusses at 24 inches and 31 at 16 inches.

The truss type changes the web members, not the chords. For a 24 foot span at 6 in 12, every layout has a 24 foot bottom chord and two 14.53 foot top chords. The Fink uses 74.70 linear feet, the King post 72.49 and the Howe 82.04.

• King post: one center post, with two struts to the top chords. Angi describes it as limited in span.
• Queen post: two posts joined by a horizontal strut. Here the posts sit one third of the span from each heel.
• Fink: four webs in a W shape. Angi calls it cost-effective for wide spans.
• Howe and Pratt: vertical posts with diagonals. Calcs.com says Pratt diagonals slope toward the center and Howe diagonals slope away. The Howe diagonal here is 8.49 ft and the Pratt diagonal is 6.71 ft.

Source spans disagree with each other, so this page gives no span limits. Ask a truss supplier what each layout can carry for your loads.

Pitch is inches of rise per 12 inches of run. The roof angle is atan(pitch / 12). The top chord is the run plus the horizontal overhang, times the square root of one plus the pitch squared over 144. A 6 in 12 roof is 26.57 degrees, matching Inch Calculator.

Rise is measured from the heel to the peak over a 12 foot run. For other conversions between pitch, angle and percent, use the roof pitch calculator.

A board foot is nominal thickness times nominal width times length in feet, divided by 12. A 2x4 is 0.667 board feet per linear foot, a 2x6 is 1.0 and a 2x8 is 1.333. The Fink truss above uses 74.70 linear feet, so 49.8 board feet in 2x4 stock.

These figures use one lumber size for every member and add no waste. Real trusses mix sizes and grades. For site-built framing lumber, see the lumber calculator.

Every manufactured truss needs a truss designer. The design drawings show joint locations, bearing widths, design loads, reactions, and the lumber size, species and grade of each member. IRC R802.10.1 says the drawings go to the building official at inspection and arrive with the shipment.

• The building designer supplies the loads, including snow drift. The truss designer designs each truss for those loads (Brent Maxfield, S.E., STRUCTURE magazine).
• ANSI/TPI 1 and BCSI-B3 put continuous lateral bracing on the building designer (same source).
• Use this calculator to count trusses and check a quote. Do not cut your own trusses from these lengths.

The IRC text is read through the South Carolina regulation that reproduces it. Your jurisdiction may use a different edition, so ask your building department.

This page counts trusses and finds geometry for a symmetric gable roof. It does not size members, plates or bearing, check loads or bracing, or give span limits. It does not cover hip, scissor, attic, mono or gambrel roofs, and it has no metric input yet. Prices appear only if you type a price per truss. Consult your truss supplier or a structural engineer before you order or build.

For roof sheathing, use the plywood calculator. For a shed roof budget, use the shed cost calculator.

Published October 2026. Last updated October 2026. Next review October 2027.

Alex Rivera, PEPE

Reviews: volume calculators · 38 calculators reviewed

Alex Rivera is a Licensed Professional Engineer (PE) with 18 years of structural and civil engineering experience. He holds PE licenses in California (#C-89412) and Texas (#P.E.-98765). He previously served as Engineer of Record on 250+ residential foundation designs at Thornton Tomasetti. At CalcSummit, he writes and personally reviews every structural, concrete, rebar, deck, and framing calculator against current IRC and ACI 318 standards.

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