How Many Trusses Do I Need?
The number of trusses you need equals your building length divided by your on-center spacing, plus one extra truss to close out the run. Enter your building length and spacing below for the exact count and an estimated material total.
Calculate Your Exact Truss Count
Enter your building specifications below. Results and the roof diagram update live as you type.
Live Roof Diagram
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Estimate Your Project Budget
Automatically calculated from your inputs above in the calculator.
Estimates only. Actual costs vary by region, supplier, and site conditions.
The Truss Count Formula, Explained
The formula is straightforward: truss count = ceiling(building length ÷ spacing) + 1. The "+1" accounts for the extra truss needed to close out both ends of the run — without it, you would be one truss short at the far gable end.
For example, a 40 ft long building at 24" (2 ft) on-center spacing needs 40 ÷ 2 = 20, plus 1 = 21 trusses. The same building at 16" (1.33 ft) spacing needs 40 ÷ 1.33 ≈ 30, plus 1 = 31 trusses — roughly 48% more trusses for the closer spacing.
Always round up (not to the nearest whole number) when dividing length by spacing, since a partial truss position still needs a full truss — and confirm your building length is measured the same way your supplier measures it (typically outside wall to outside wall) before finalizing an order.
Frequently Asked Questions
How many trusses do I need for a 40 ft roof?
At standard 24" on-center spacing, a 40 ft long building needs 21 trusses (40 ÷ 2 = 20, plus 1 to close the run). At 16" spacing, the same building needs 31 trusses.
Do you round up or down when calculating truss count?
Always round up. Dividing building length by spacing gives the number of spacing intervals; any partial interval still requires a full truss, and one additional truss is added to close out both ends of the run.