Roof Geometry and Truss Analysis, Done Right
Truss Calculator is a free two-tab engineering tool: lay out a roof with exact geometric math in the Simple tab, then verify the structure with a real direct-stiffness solver in the Analysis tab. Enter your building, get the lengths, counts, forces, and reactions — all computed instantly in your browser.
From span to lumber list, in secondsPhoto: Unsplash2
Calculators, one workflow
7
Preset truss families
20+
In-depth guides
100%
Computed in-browser
$0
Free forever, no account
What Is Truss Calculator?
Truss Calculator (truss-calculator.com) exists to answer two deceptively simple questions accurately: what lengths and counts does this roof need? and can this truss carry the load? The questions are simple to ask and surprisingly easy to get wrong. Misjudge a pitch factor and the sheathing order comes up short. Confuse span with run and every rafter is the wrong length. Assume a truss is fine because it looks right, and the bottom chord may be working harder than any chart suggested. Between those failure modes sits a body of engineering — roof geometry, slope multipliers, layout arithmetic, and structural analysis — that professionals navigate daily and DIYers meet for the first time, usually at the lumberyard counter, holding a tape measure and a phone.
We built Truss Calculator to put that body of engineering in everyone’s pocket. The tool is deliberately focused. It does not try to be a full CAD suite, a building-design package, or a permit filing system. It does two things — turn building dimensions into precise material geometry, and turn truss geometry and loads into member forces and reactions — and it does them with the same formulas a framer and a structural engineer would apply by hand: the Pythagorean theorem, the slope multiplier, the fencepost count, and the direct stiffness method.
Everything runs client-side. When you type a 24-foot span and a 6/12 pitch, those numbers are evaluated by JavaScript in your browser and answered before your finger leaves the key. There is no server round-trip, no account, no history. That architectural choice is also a privacy promise: we literally cannot lose data we never receive.
Engineering-Grade Math
Tab 1 runs exact roof geometry — Pythagorean lengths, pitch factors, and on-center counts. Tab 2 solves real structural analysis with the direct stiffness method, not ad-hoc shortcuts.
Privacy by Design
Spans, pitches, loads, and member data are computed entirely in your browser. Nothing you type is transmitted, logged, or stored on our servers.
Instant Two-Tab Workflow
Lay out the roof with the Simple Geometric calculator, then verify a truss's member forces with the Structural Analysis calculator — both update as you type.
Free, No Sign-Up
No accounts, no paywalls, no export limits, no watered-down trial version. Open the page, enter your numbers, get your answer, and get back to work.
Two Tabs, One Consistent Model
Real projects run in both directions. Sometimes you know the building and need the lumber list; sometimes you have a truss shape and need to know whether it carries the load. Truss Calculator handles both with the same underlying geometry, so answers stay consistent no matter which tab you enter through.
Tab 1: Layout & Materials
Simple Geometric (DIY / Framing)
- Enter building span, pitch (X/12 or degrees), and overhang
- Get rafter length, rake length, and rise to the sixteenth of an inch
- Choose on-center spacing to count rafter pairs and purlins
- Estimate sheathing area with the automatic pitch factor
- Add your local lumber prices for a full material cost estimate
- Switch between imperial and metric units at any time
Tab 2: Forces & Reactions
Structural Analysis (Engineering)
- Pick a preset truss — Pratt, Howe, Fink, King Post, Warren, and more — or draw a custom one
- Apply point and distributed loads anywhere on the structure
- Solve by the direct stiffness method, including indeterminate trusses
- See member forces color-coded tension versus compression on an interactive diagram
- Check reactions, deflections, and stress ratios per member
- The determinacy check (m + r = 2j) runs automatically on every model
From building width to member forces — both directions coveredPhoto: UnsplashHow Our Calculations Work
Trust in a calculator comes from being able to check its work. That is why every result on Truss Calculator shows the formula behind it. Here is the exact logic the engine applies, in order, for every calculation.
Roof Geometry
The geometric tab starts with the Pythagorean theorem: half-span and rise form a right triangle, and the rafter is its hypotenuse. Overhangs are computed as their own triangles past the wall, and every length is reported to the sixteenth of an inch — the precision lumber is actually sold and cut at.
Pitch Factor
Sloped roof area equals plan area times √(rise² + 12²) ÷ 12 — the slope multiplier behind every sheathing and coverage estimate. The tool shows which factor it applied so you can verify it against any published chart, from 1.054 at 4/12 to 1.414 at 12/12.
Layout Counts
Member counts follow the fencepost rule: ceil(span ÷ spacing) + 1 positions per run. Purlin counts run the same logic up the rafter length. These counts multiply by your prices into the cost estimate, so the arithmetic behind every dollar is visible.
Direct Stiffness Solution
The structural tab assembles each member's 4×4 axial stiffness matrix (EA/L), builds the global system K·d = F, enforces the supports, and solves for joint displacements. Member forces follow from displacement differences across each member — the same procedure commercial FEA packages run.
Verification & Reporting
Every structural result passes equilibrium checks, and the determinacy equation m + r = 2j is reported for every model. Force diagrams color tension and compression separately, and stress ratios show exactly how much capacity each member is using.
A Worked Example
Suppose a 24-foot gable shed at a 6/12 pitch with a 12-inch overhang. The run is 12 feet and the rise is 6 feet, so the half-rafter is √(12² + 6²) ≈ 13.42 feet; with the tail the full rafter lands near 14.51 feet. The pitch factor is √(6² + 12²) ÷ 12 ≈ 1.118, so the sheathing order is about 12% bigger than the footprint. At 24-inch on-center spacing, 13 rafter positions per side means 13 pairs — 26 rafters plus the ridge. Switch to the Analysis tab, load a Fink preset at the same span with your region’s snow load, and the solver reports the top chords in compression, the bottom chord in tension, and the reactions your shed walls must actually resist. Every one of those numbers is reproducible by hand, and the tool shows its work at each step.
Illustrative example; connection design, lumber grading, and code compliance still require evaluation by a qualified professional.
Under the Hood: How the Tool Is Built
Most “calculators” are a form that emails a server. Truss Calculator is built the opposite way — as a static site with the entire math engine compiled into the page itself. That single architectural decision produces the privacy, speed, and reliability everything else depends on.
Zero-Backend Architecture
Both calculators are compiled into static pages with the entire math engine bundled as client-side JavaScript. There is no calculation API to call, no database of your projects, and no server that can go down mid-estimate — the tool works the same on a jobsite phone with weak signal as it does on a studio desktop.
Shareable Calculation Links
The Structural Analysis tab can encode a complete model — truss type, span and height, web bays, node coordinates, loads, supports, and units — into a single URL. Send the link to a colleague, a supplier, or an instructor, and the identical model rebuilds on their screen, ready to check or modify.
Real-Time Recalculation
Results update on every keystroke with no submit button, because the arithmetic is fast enough to run between letters. That immediacy is deliberate pedagogy: change one variable and watch exactly which outputs move, which is how the intuition behind pitch factors and load paths is actually built.
Dual Units, One Engine
Imperial (feet, inches, kips, ksi) and metric (meters, millimeters, kilonewtons, GPa) are first-class throughout. Switching units converts every input, output, diagram label, and report value together — the underlying formulas never change, only the dress they wear.
The whole engine ships with the pagePhoto: UnsplashWhy static beats server-side for math
A server-side calculator adds three failure modes: latency on every keystroke, downtime at the worst moment, and a log file full of your project data. A client-side engine removes all three. The page loads once, the math runs on your silicon, and the only network activity left is loading the page itself and the ads that fund it.
It also makes the tool auditable in a way server tools are not: the same inputs always produce the same outputs, on every device, with no silent model changes between visits. When a correction ships, it ships to everyone at once.
Who Uses Truss Calculator
The same question — “what does this roof need?” — is asked by very different people in very different situations. We designed the interface to be fast enough for daily professional use and clear enough for a first-time DIYer working through their first shed roof.
Framers & Carpenters
Field-speed rafter lengths, purlin counts, and material totals between the truck and the roof. Confirm a pitch conversion or explain to a customer exactly why the job needs 13 rafter pairs, not 12.
General Contractors & Builders
Pre-size roof packages, sanity-check supplier quotes, and model how a pitch change ripples through rafter lengths, sheathing area, and cost before it reaches the change order.
DIY Homeowners
Planning a shed roof, a carport, or a porch? Understand what length boards the job actually needs — and whether the structure carries the load — before you buy a single stick of lumber.
Engineers & Designers
Fast pre-design exploration of truss geometry and load paths. The stiffness-method solver handles determinate and indeterminate models alike, with member forces you can verify by hand.
Students & Educators
A transparent teaching tool. Draw a truss, load it, and see the joint-by-joint results the method of joints predicts — building intuition for structural analysis courses, not just answers.
Building Officials & Inspectors
A neutral, manufacturer-independent reference for reviewing owner-built framing plans and answering homeowner questions during permit conversations.
Measure, calculate, then cut — in that orderPhoto: UnsplashReal projects, real constraints
A shed roof at the bottom of the garden. An addition whose pitch must match the existing house. A carport that spans wider than anything you have framed before. A cabin with a loft that loads the bottom chord. Each of these jobs stresses a different part of the math — geometry, layout, or structure — and each has a different correct answer.
That is precisely why a single “rafter chart” is never enough, and why the calculators model your actual inputs instead of handing you a generic lookup row. Change any parameter and watch the answer move — the tools recalculate in real time, so building intuition is as easy as editing a number.
How Truss Calculator Compares
No single tool is right for every job. Here is where this site sits against the two alternatives people usually weigh it against — the free chart apps and the commercial analysis suites — stated as plainly as we can manage.
| Truss Calculator | Rafter chart apps | Commercial FEA suites | |
|---|---|---|---|
| Price | Free, no account | Free with heavy ads | $2,000+/year per seat |
| Rafter & roof geometry | Exact per-input math with shown formulas | Fixed chart rows, interpolate by eye | Complete but buried in setup |
| Structural solve | Direct stiffness method, determinate & indeterminate | None — geometry only | Full FEA, far beyond trusses |
| Learning curve | Minutes | Minutes | Weeks of training |
| Privacy | Inputs never leave your browser | Varies; most log everything | License servers, cloud projects |
| Verification support | Determinacy check, matrices printed, equilibrium visible | A bare number | Full audit trail, for experts |
| Best for | Planning, pre-sizing, checking, learning | One-off rafter lookups | Sealed production design |
The honest conclusion: for sealed, permitted, production work, the commercial suite and the licensed engineer behind it win — and should. For everything between “what does this roof need?” and “is this quote sane?”, a tool that shows its math and respects your data fills a gap the other two leave open.
Why We Built Truss Calculator
The idea came from a familiar frustration. Planning a workshop roof, one of our founders found a dozen “rafter calculators” online — and they disagreed with each other. Some ignored the overhang entirely. Some counted purlins without the fencepost rule, so every estimate came up one short. None explained what to do when the geometry was fine but the structure needed checking. The authoritative sources — engineering texts and truss design standards — are wonderful for professionals and impenetrable at the lumberyard counter.
Meanwhile, the consequence of guessing wrong is not a wasted afternoon. A short sheathing order stalls the job; a rafter cut to the wrong pitch means a re-order; a truss assumed adequate without checking risks the whole structure. We believed a calculator could be both radically simple and engineering-honest at the same time, if it showed its work: named formulas, visible pitch factors, and stated assumptions instead of a bare number.
So Truss Calculator was built around three commitments. First, traceability: every answer names the formula or method that produced it. Second, completeness: geometry and structural analysis share one site, so the lumber list and the force table can never quietly disagree. Third, privacy: a calculation about your building should not require sending your building’s details to a server, so the math runs entirely in your browser.
Today the site serves framers double-checking a layout between jobs, contractors sanity-checking quotes, engineering students verifying the method of joints by hand, and homeowners planning their first shed roof with confidence instead of crossed fingers. If the tool has saved you one return trip to the lumberyard or one wrong stick of lumber, it has done its job.
Show the work, earn the trustPhoto: UnsplashData Sources & Editorial Standards
A calculator is only as trustworthy as its sources and its maintenance. These are the standards every page and every tool on Truss Calculator is held to.
Named Formulas, Always
We cite the exact relationships every result comes from — the Pythagorean theorem, the slope multiplier, the fencepost count, the direct stiffness method, and Euler buckling concepts. Where a value comes from a specific formula, the page says so.
Conservative by Default
When two defensible interpretations exist, we ship the more conservative one and label the alternative in the notes. Lumber is expensive; a mid-job supply run or an overloaded member is more expensive.
Assumptions Stated Up Front
Every tool states its baseline: pin-jointed members, loads applied at joints in the structural tab, and no automatic load combinations or wet-service reductions. If your job exceeds those conditions, the notes tell you which professional evaluation applies next.
Maintained, Not Abandoned
When a formula reference changes, a preset proves confusing, or a correction report checks out, we update the calculators and date-stamp the change instead of quietly hoping nobody notices.
Found an error? We treat every correction report seriously — send the page URL, your inputs, and the result you expected via our contact page, and we will verify against the underlying formulas and update the tool. Corrections that change results are documented on the page they affect.
The Principles Behind the Tool
Four commitments shape every feature decision on this site — what we build, what we refuse to build, and how we explain the results.
Free Forever
Structural and framing knowledge should never sit behind a subscription. The calculators, references, and guides are free to use, unlimited, without an account.
Local-First Privacy
Your inputs never leave your browser. There is no upload step, no history, and nothing to leak — because there is nothing stored.
Speed as a Feature
A calculator you use at the lumberyard has to load fast and answer in real time. Results update as you type, with no submit button and no page reloads.
Honest About Limits
We do not pretend a web tool replaces an engineer. Connection design, member grading, bracing, and code compliance are professional work — and we say so plainly on every page.
How Truss Calculator Stays Free
Free tools fail quietly when the money has to come from somewhere invisible. Ours comes from somewhere visible — the ad slots on each page — and we would rather explain the model than hide it.
Advertising Pays the Bills
Hosting, development time, and correction reviews are funded by clearly-labeled ads. That is the entire business model — no premium tier hiding better math, no paywall between you and a result.
No Data Business
We cannot sell or profile your calculations because your calculations never reach us. The ad-supported model works precisely because it does not need your project data to function.
Corrections Drive Updates
When a report verifies a discrepancy, the fix ships and the page documents it. A free tool stays trustworthy the same way an expensive one does: by showing its work and owning its mistakes.
Frequently Asked Questions About Truss Calculator
Is Truss Calculator free to use?▼
Yes — completely. There is no account, no trial, no usage cap, and no paywalled features. The site is supported by advertising, which is why you can run unlimited calculations without paying or signing up.
What is the difference between the two tabs?▼
Tab 1 (Simple Geometric) answers layout questions: how long are the rafters, how many purlins, what does the sheathing area and the lumber package cost. Tab 2 (Structural Analysis) answers engineering questions: which members are in tension or compression, how large are the forces, and what are the reactions at the supports. Most real projects use both — geometry to buy materials, analysis to check the structure carries the load.
Are my inputs stored anywhere?▼
No. Spans, pitches, spacings, prices, truss geometry, loads, and section properties are processed locally in your browser using client-side JavaScript. Nothing is transmitted to or persisted on our servers.
How accurate are the structural results?▼
The solver implements the direct stiffness method exactly — the same matrix procedure taught in structural engineering courses and used in commercial software. The results are exact for the model you build: pin-jointed members, loads entered where you place them, supports as you define them. Accuracy in the real world therefore depends on how well the model matches the building — load magnitudes, bracing conditions, and joint behavior are where engineering judgment enters.
Can I use this instead of hiring an engineer?▼
No. The calculators are planning and educational aids. They do not design connections, select lumber grades, check bracing and buckling of real assemblies, apply load combinations, or account for your jurisdiction's code amendments. Real trusses must be designed and sealed by a licensed engineer or a certified truss manufacturer for your actual loads, spans, and codes.
Does it work with metric units?▼
Yes. Both tabs support imperial and metric throughout — switch units and every input, output, and diagram label converts together, so the tool works the way you measure regardless of where you build.
What We Are Building Next
The tool is deliberately unfinished — the backlog is ordered by what users ask for most, and every item below traces back to real requests. Corrections always outrank features.
More Truss Families & Load Controls
Additional preset geometries for attic, gambrel, and mono-pitch profiles, plus guided load panels that help you distribute code-typical roof pressures to joints correctly.
Printable Calculation Reports
A formatted, print-friendly report of geometry inputs, member forces, and reactions — the artifact most requested by users who need a paper trail for quotes and site meetings.
A Growing Guide Library
New guides are commissioned from the questions that arrive through the contact page — the fastest way to see a topic covered is to ask for it.
More Application Walkthroughs
Step-by-step project guides for porches, dormers, garage headers, and gambrel sheds, each pairing the geometric and structural tabs the way real builds sequence them.
Ready to Size Your Roof?
Open the calculators, enter your span and pitch, and get geometry with its reasoning attached — free, private, and instant. Then check the structure in the Analysis tab, and browse the guides to understand the “why” behind every number.