Roof Truss Quantity Calculator
This roof truss quantity calculator works out how many trusses your roof needs, based on building length and your chosen on-center spacing, along with an estimated board footage and material take-off.
Calculate Your Truss Quantity
Enter your building specifications below. Results and the roof diagram update live as you type.
Live Roof Diagram
Results
Estimate Your Project Budget
Automatically calculated from your inputs above in the calculator.
Estimates only. Actual costs vary by region, supplier, and site conditions.
How Many Roof Trusses Do You Need?
Truss quantity is calculated by dividing the building length by your on-center spacing, then adding one extra truss to close out the run. A 40 ft long building at 24" on-center spacing needs 21 trusses; the same building at 16" spacing needs 31.
Closer spacing increases truss count and total material, but each individual truss can sometimes use lighter framing; wider spacing reduces truss count but often requires larger chord sizes or engineered members to handle the load between supports.
The calculator above also estimates total board footage from your truss count and member lengths, giving you a rough material take-off to check against a supplier's quote.
Frequently Asked Questions
Calculate how many roof trusses would be needed
Divide the building length by your on-center spacing (commonly 24"), then add one truss to close out the run. Enter your length and spacing above to see the exact count instantly.
Roof truss calculation examples
The Guides section includes worked calculation examples and formulas.