Calculate how many roof trusses you need from the roof length and on-center spacing (24 in by default), or find equal spacing, maximum run and quoted cost.
Roof Truss Count Formula
The calculator works in three directions. For a simple row with one truss position at each end, divide the distance between those centers by the maximum allowed bay spacing, round up to a whole number of gaps, and add one position. Equal spacing can be slightly smaller than the entered maximum, or the final bay can be shorter:
N = roundup(L / S) + 1
To find the spacing that a set number of trusses will sit at, it rearranges the same relationship:
S = L / (N - 1)
To find the longest roof a set number of trusses can cover, it multiplies the number of gaps by the spacing:
L = (N - 1) * S
Variables:
- N is the number of trusses
- L is the end-center layout run in the direction trusses repeat, not the outer roof dimension or an individual truss span
- S is the maximum permitted on-center (OC) spacing for count and maximum-run calculations, or the equal spacing produced by the spacing calculation
- roundup means round the division up to the next whole number, because any leftover distance still needs a truss over it
- the plus one adds the truss that sits at the far end, since a truss sits at both ends of the roof
Pick what you want to find at the top of the calculator. In “Number of trusses needed” mode you enter the roof length and the OC spacing and get the truss count, with optional fields for genuinely additional trusses and separately entered material and labor quotes. The base layout includes both ends; special trusses may replace existing positions and must not be double-counted. In “On-center spacing” mode you enter the roof length and a truss count and get the spacing those trusses land at, which is useful when the number of trusses is fixed by a delivery or budget. In “Max roof length covered” mode you enter a truss count and a spacing and get the longest roof they cover. The formulas require matching units; the calculator converts feet, meters, inches, and centimeters automatically.
Trusses Needed by Roof Length and Spacing
This table shows base layout counts at four example maximum spacings, using the formula above and including both end positions. Use spacing and quantities from the approved design and placement layout; changing the count alone does not establish structural capacity.
| Roof length | 24 in OC | 19.2 in OC | 16 in OC | 12 in OC |
|---|---|---|---|---|
| 10 ft | 6 | 8 | 9 | 11 |
| 20 ft | 11 | 14 | 16 | 21 |
| 24 ft | 13 | 16 | 19 | 25 |
| 30 ft | 16 | 20 | 24 | 31 |
| 40 ft | 21 | 26 | 31 | 41 |
| 50 ft | 26 | 33 | 39 | 51 |
| 60 ft | 31 | 39 | 46 | 61 |
For a budget, obtain project-specific material and installation quotes. Multiply total trusses by each entered per-truss quote. Blank cost fields omit that cost category; zero means no charge. Different truss types may need separate supplier pricing.
Use a written supplier quote for the specified truss types and a separate installer quote for the scope of labor. No unsupported current-price table is used.
The estimate excludes delivery, crane, taxes and other framing unless those costs are included in your entered quotes. It does not claim current regional market prices.
Example Problems
Example 1: Count trusses from roof length.
Your roof is 36 ft long and you are framing at 24 in on center. Divide the length by the spacing, round up, and add one:
N = roundup(36 * 12 / 24) + 1 = roundup(18) + 1 = 19 trusses. Using hypothetical entered quotes of $180 material and $50 labor per truss gives $3,420 material plus $950 labor, totaling $4,370 before additional trusses or excluded costs.
Example 2: Find the spacing for a fixed truss count.
You have 17 trusses to cover a 32 ft roof and want to know the spacing. There are 16 gaps between 17 trusses, so divide the length by the gaps:
S = (32 * 12) / (17 – 1) = 384 / 16 = 24 in on center. That is the equal geometric spacing; the approved design must permit it.
Frequently Asked Questions
How many trusses do I need for a 40-foot roof?
At an approved maximum spacing of 24 in, a 40 ft end-center layout run needs 21 positions, because 40 ft is 480 inches, 480 divided by 24 is 20, and adding the end truss gives 21. A maximum spacing of 16 in gives31 positions over that same run; an approved design must establish the appropriate spacing and truss types.
Why does the formula add one truss?
Dividing the roof length by the spacing counts the gaps between trusses, not the trusses themselves. A row of trusses always has one more member than the number of gaps, because a truss sits at both ends of the roof. That is why you round the division up and then add one.
Can roof trusses be spaced more than 24 inches apart?
Spacing can exceed24 in when the engineered design and roof support system permit it. There is no universal24 in maximum. Follow the approved truss design and placement layout, including loads, roof coverings or purlins, connections and bracing. Confirm quantities and spacing with the truss supplier and applicable local requirements.
