How to Use the Roof Truss Calculator
This roof truss calculator has two modes. Pick Rafter Length to find the dimensions of your roof trusses, or Truss Count to estimate how many trusses you need and what they will cost.
How to Calculate Rafter Length
The rafter length is the diagonal distance from the wall plate to the ridge. It equals the hypotenuse of a right triangle whose legs are the run (horizontal half-span) and the rise (vertical height of the roof):
Rafter Length = √(Rise² + Run²)
You can also work from the roof pitch instead of the rise. Enter the pitch as a percentage, an angle in degrees, or the traditional X:12 ratio. The calculator converts pitch to rise automatically:
Rise = Run × Pitch (%)/100 Rafter Length = Run × √(Pitch²/10000 + 1)
The calculator works both ways — supply any two of the three values (rise, run, rafter length) and it returns the third.
How to Calculate Truss Count and Roof Truss Cost
Switch to Truss Count mode and enter:
- Roof Length — the total length of the ridge (along the building).
- On-Center Spacing — the distance between the centerlines of adjacent trusses (default 2 ft / 0.6 m).
The number of trusses is:
Truss Count = ⌈Roof Length ÷ Spacing⌉ + 1
Once you have the count, enter the price per truss to get the material cost. Check Include Installation Cost to also add the contractor's hourly rate and the expected installation duration.
What Is On-Center (OC) Spacing?
On-center spacing (OC or o.c.) is the distance measured from the center of one framing member to the center of the next. Common values are 24 in (2 ft) in North America and 600 mm (0.6 m) in metric countries. Using OC spacing ensures that the edges of plywood or OSB sheathing always land on the center of a truss, giving a solid nailing surface.
Types of Roof Trusses
- King Post — the simplest design; one vertical post from the apex to the tie beam. Best for short spans up to ~26 ft (8 m).
- Queen Post — two vertical posts, suits medium spans of 26–40 ft (8–12 m).
- Fink (W-Truss) — the most common residential truss; W-shaped internal webbing handles spans up to 50 ft (15 m).
- Howe Truss — diagonal members slope toward the center; efficient in wood and steel construction.
- Scissor Truss — gives a vaulted ceiling effect; the bottom chord slopes upward from both walls.
- Mono Truss — a single-slope (shed) truss used for lean-to additions.
- Attic Truss — creates usable attic space inside the truss; heavier but doubles as floor framing.
Measurement Systems
The calculator fully supports both US Customary / Imperial (feet, inches) and Metric (meters) inputs. Switch between them with the toggle at the top of the form — the default spacing value adjusts automatically (2 ft → 0.6 m).
Currencies
Cost estimates are available in 16 world currencies: USD ($), RUB (₽), EUR (€), GBP (£), CAD, AUD, JPY, CNY, BRL, MXN, INR, KRW, CHF, SEK, PLN, and TRY. Select your preferred currency from the dropdown before calculating.
Frequently Asked Questions
- What is a roof truss?
- A roof truss is a pre-engineered structural framework of rafters, a tie beam, and internal webbing that supports the roof load and transfers it to the load-bearing walls.
- How many trusses do I need for a 40-foot roof?
- With standard 24-inch OC spacing: ⌈40 ÷ 2⌉ + 1 = 21 trusses. Enter your exact dimensions in the calculator for a precise result.
- What is a typical roof pitch?
- Residential roofs commonly range from 4:12 (18.4°) to 9:12 (36.9°). Low-slope roofs start at about 2:12; steep roofs exceed 12:12.
- How much does a roof truss cost?
- Prefabricated wood trusses typically cost $100–$400 USD each depending on span, design, and lumber prices. Installation labor adds roughly $30–$80 per truss. Use the calculator's cost section for a project-specific estimate.
- Can I use this calculator for metric measurements?
- Yes. Toggle to Metric (m) and enter values in meters. The standard on-center spacing defaults to 0.6 m (600 mm).