# Roof Truss Calculator — How Many Trusses & Cost > Free roof truss calculator: work out how many trusses you need from roof length and on-center spacing — both gable-end trusses included — plus lineal footage and cost. Metric and imperial, all client-side. Turn a roof length and on-center spacing into a truss count — both gable-end trusses included — plus an ordered quantity with allowance, optional lineal footage and cost, in metric or imperial. - **HTML page:** https://siteometry.com/en/tools/roof-truss-calculator - **Category:** [Roofing & Siding](https://siteometry.com/en/tools/category/roofing-siding) - **Figures live on the page:** tables, diagrams and worked figures the text refers to ("the table below") are drawn by the page and are not reproduced here. Each section below links to its anchor on the page. - **Runs in the browser:** Siteometry calculators need no account and send no input anywhere — every number below is computed on the visitor's own device. ## How to count roof trusses 1. **Choose units & currency** — Pick metric or imperial and your currency; every field follows. 2. **Set the on-center spacing** — Pick a preset (24 in / 600 mm is typical) or enter a custom spacing. 3. **Add roof sections** — Enter each roof's length along the eaves; add a section per separate roof. 4. **Add width & overhang (optional)** — Enter the building width for lineal footage and the overhang for reach — neither adds trusses. 5. **Set the allowance (optional)** — Add spare trusses or a waste percentage to pad the ordered quantity. 6. **Read the take-off** — Read the truss count, the quantity to order, lineal footage and cost. _On the page: https://siteometry.com/en/tools/roof-truss-calculator#howto_ ## Roof truss take-offs, explained A roof-truss take-off is one count dressed up as a roof: how many prefabricated trusses stand side by side along the length of the building. Divide the roof length by the on-center spacing, round up to get the bays, and add one — because a truss sits at both ends as well as at every spacing mark in between. That final +1 is where both gable-end trusses come from, so they are already in the number this calculator gives you. Everything runs in your browser and nothing you type is uploaded. The count is float-tolerant (a 12 ft roof at 16 in on-center returns 10 trusses, not 11), each roof section is counted on its own and summed, and the ordered quantity rounds a waste percentage up to a whole truss before adding any flat spares — because a truss is indivisible. _On the page: https://siteometry.com/en/tools/roof-truss-calculator#rooftruss-guide-intro_ ## Parts of a roof truss A common Fink truss is a rigid triangle: a sloped top chord on each side, a flat bottom chord tying the walls together, and internal web members in a "W" that brace the two. The peak where the top chords meet is the apex; the corners where they land on the walls are the heels. The distance between the heels is the span — the width of the building, not the up-slope length of a chord. The take-off does not care what happens inside the triangle. Whatever the webbing, pitch or heel detail, one truss is still one truss, and the number you need depends only on the roof length and the spacing between trusses. _On the page: https://siteometry.com/en/tools/roof-truss-calculator#truss-anatomy_ ## How many trusses do you need? Start from the roof length measured along the eaves — the run of wall the trusses sit on, side by side — and the on-center spacing. Trusses are most often set 24 in (600 mm) apart; 16 in (400 mm) is used under heavier loads. The bays are the roof length divided by the spacing, rounded up, and the trusses are the bays plus one. The plan below shows a 32 ft roof at 24 in on-center: 16 bays give 17 trusses, with the first and last standing as the gable-end trusses. Add a section for each separately-framed roof — a main roof and an attached garage, for instance — and the tool sums them. _On the page: https://siteometry.com/en/tools/roof-truss-calculator#how-many-trusses_ ## How this calculator works The tool keeps the math in a pure engine and shows it in your chosen units. You set the spacing, enter each roof section's length, and it returns the truss count — both gable ends included — the quantity to order after allowance, optional lineal footage of bottom chord, and an optional cost, live as you type. Building width and eave overhang are optional. Width unlocks the lineal footage of bottom chord and a to-scale plan; overhang extends how far each truss reaches past the wall. Neither one changes the truss count. _On the page: https://siteometry.com/en/tools/roof-truss-calculator#how-calculator-works_ ## Count & allowance formulas The count comes straight from the roof length and spacing; the ordered quantity layers an optional allowance on top. Trusses are whole units, so the waste percentage is rounded up to a whole truss before any flat spares are added. _On the page: https://siteometry.com/en/tools/roof-truss-calculator#count-and-allowance_ ## Spacing and which length to enter Two inputs decide the count, and both are easy to get wrong. Spacing is the on-center distance between trusses: tighter spacing means more trusses over the same roof. The comparison below runs the same 32 ft roof at 16 in and at 24 in on-center — 25 trusses versus 17 — so the same building can need very different quantities. The length you enter is the building length along the eaves, the wall the trusses are spaced along. It is not the up-slope rafter or top-chord length, which runs the other way, across the width. Entering the slope length instead of the eave length is the single most common mistake in a truss take-off. _On the page: https://siteometry.com/en/tools/roof-truss-calculator#spacing-and-length_ ## Lineal footage & cost Enter a building width to turn the count into lineal footage of bottom chord, and a price per truss to turn it into money. Overhang only extends the per-truss span; it never adds a truss. _On the page: https://siteometry.com/en/tools/roof-truss-calculator#span-and-cost_ ## Worked examples Two take-offs that show how the count falls out of your inputs. Your dimensions will differ — these illustrate the method. _On the page: https://siteometry.com/en/tools/roof-truss-calculator#examples_ ## Common mistakes The three big ones are all double-counts or wrong lengths: adding one or two trusses "for the gable ends" when they are already in the count, adding trusses because the roof has an overhang, and entering the up-slope rafter length instead of the eave length. Each one throws the order off. The safe habit is to trust the +1: the count already lands a truss at both ends. Order the number shown, add spares only for genuine breakage or field changes, and measure the length along the wall the trusses sit on. _On the page: https://siteometry.com/en/tools/roof-truss-calculator#mistakes_ ## Frequently asked questions Common questions about counting roof trusses, gable ends, overhangs and spacing. ### Are both gable-end trusses included in the count? Yes. The formula is trusses = ⌈roofLength / spacing⌉ + 1: the +1 lands a truss at the near end and the round-up lands one at the far end, so both gable-end trusses are in the number by construction. You never have to add them yourself. ### Should I add one or two extra trusses for the gable ends? No. This is the most common counting mistake and some guides get it wrong. The count already places a truss at both gable ends, so adding "1 or 2 for the gables" double-counts them and over-orders. Order the number shown; add trusses only as deliberate spares for breakage or field changes. ### Does adding an eave overhang mean more trusses? No. An overhang changes how far each truss reaches past the wall, not how many trusses there are. The overhang is framed either by extending each truss's own top and bottom chords past the wall or by hand-framing the ends with lookouts — neither is an extra truss. In this tool the overhang only feeds the per-truss span and lineal footage; it can never change the count. ### Which length do I enter — the roof length or the rafter length? Enter the building length along the eaves: the run of wall the trusses are spaced along. Do not enter the up-slope rafter or top-chord length, which runs the other way, across the width of the building. The trusses are spaced along the eave length, so that is the length the count divides by the spacing. ### What spacing should roof trusses be? Roof trusses are most commonly set 24 in (600 mm) on-center. Heavier loads, tile roofs or specific engineering may call for 16 in (400 mm), and 19.2 in is an occasional imperial preset. Pick a preset or enter a custom spacing; the count divides your roof length by whatever spacing you set. Always follow your truss supplier's engineered layout. ### How many spare trusses or how much waste should I add? Trusses are made to order and are not consumed like sheet goods, so most take-offs need little or no waste allowance. Add a flat spare or two for breakage or a field change if you want a cushion. The tool rounds any waste percentage up to a whole truss and then adds your flat spares, since a truss is indivisible. ### Does it work in metric and imperial? Both. Switch units at the top and every length and spacing converts — roof length in m or ft, spacing in mm or in, lineal footage in whichever system you chose. Metric and imperial spacings are kept as their real values, not aliased to each other. ### Is the calculator private? Yes. Every calculation runs in your browser. Nothing you enter is uploaded, and your project is saved only in your own browser's local storage. _On the page: https://siteometry.com/en/tools/roof-truss-calculator#faq_ ## Related calculators - [Roofing Calculator — Pitch, Area & Materials](https://siteometry.com/en/tools/roofing) - [Rafter Length Calculator](https://siteometry.com/en/tools/rafter-length-calculator) - [Gutter & Downspout Calculator](https://siteometry.com/en/tools/gutter-downspout-calculator) - [Siding Calculator — Squares, Panels, Boxes & Trim](https://siteometry.com/en/tools/siding-calculator)