King Post Truss Calculator
A king post truss is the simplest possible triangular truss — a single vertical post from the ridge to the bottom chord, with two sloped top chords. It is best suited to shorter spans and is modeled here using our gable roof truss calculator, since the outer geometry is identical.
Size Your King Post Truss
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.
King Post, Howe, and Pratt Truss Basics
The king post truss uses one central vertical member (the "king post") running from the ridge down to the middle of the bottom chord, resisting sag in the tie beam. It is typically used for spans up to about 16–20 ft, where a single post provides enough support.
For longer spans, builders move to a queen post truss (two verticals) or web-braced designs like the Howe truss (diagonal webs in compression, verticals in tension) and Pratt truss (the reverse arrangement) — both add more triangulated bracing to carry heavier loads over greater distances.
Because the outer envelope of a king post truss is the same simple triangle as a gable truss, this calculator uses the gable roof-type option to approximate span, rafter length, and material needs — a truss designer will detail the internal web arrangement for your specific load.
Frequently Asked Questions
Simple truss calculator
Enter span, pitch and spacing for a fast, simple result — ideal for quick estimates before ordering.
How to calculate roof truss size?
Measure the building span (wall-to-wall width) and choose the roof pitch. Truss size is set by the top-chord length (from span & pitch), the overall rise, and the required load. Enter span, pitch and load into the calculator and it returns member lengths, height and quantity.