Reference

Geodesic dome strut length calculator

Every geodesic dome calculator is doing the same core arithmetic: multiply a chord factor by your radius. What separates a number on a screen from something you can cut is everything that comes after — which strut class each length belongs to, how much shorter the steel gets once the hub takes its bite, and whether the stock you bought can carry the load.

Chord factors, and why they are all you need

A geodesic dome is a sphere approximated by triangles. Every strut in it is a chord across that sphere, and the ratio of a chord's length to the sphere's radius is fixed by the geometry alone — it does not care how big your dome is. That ratio is the chord factor.

So the calculation is simply strut length = chord factor × radius. A 3V icosahedral dome has three distinct chord factors, conventionally labelled A, B, and C. Multiply each by a 3 m radius and you have your three cut lengths. Multiply by 4 m instead and every strut scales with it, in the same proportion.

DomeFab's chord factors are checked against the published tables builders have used for decades — Kenner, Desert Domes, and Domerama — for 1V through 4V, along with the canonical strut counts. A 3V 5/8 sphere should come to 165 struts, split 30 A, 55 B, and 80 C; if a calculator tells you something else, one of you is wrong.

Why the number of strut classes grows with frequency

Frequency is how many times each edge of the base icosahedron is subdivided. Higher frequency means more, smaller triangles, a rounder dome — and more distinct strut lengths to keep track of.

1V — one strut length. Barely a dome; it is the icosahedron itself.
2V — two classes. The usual starting point for a small greenhouse or play dome.
3V — three classes. The workhorse frequency for most homebuilt domes.
4V and up — six classes and climbing. Rounder and stronger per unit of material, but the parts list and the labour both grow with it.

DomeFab covers 1V through 6V. Past about 4V, the practical constraint stops being geometry and starts being how many uniquely-labelled sticks you are willing to cut and sort correctly on a windy site.

The part chord factors do not tell you

Hub take-off

A chord factor gives you the distance between two vertices — the theoretical points on the sphere. Your strut does not run between two points; it runs between two connectors that occupy real space. Cut to the theoretical length and every strut in the dome will be too long, by an amount that depends on the hub design and the angle it meets at.

DomeFab computes the cut length after hub offsets for the build system you actually chose, and gives you the end and bevel angles with it. That is the difference between a strut list and a cut list.

Whether the strut survives the winter

Chord factors are indifferent to whether you build in 3/4" EMT conduit or 2×6 timber. The dome does not care what the numbers say if the members buckle. DomeFab screens every strut class against your location's factored snow and wind combination, comparing member demand to Euler buckling and material capacity, and flags each class pass, caution, or fail before you buy stock.

Getting from lengths to a build

Once the lengths are right, the remaining work is logistics: how to get those cuts out of standard stock lengths with the least waste. DomeFab bin-packs the cut list against real catalog stock — 10 ft EMT, 10.5 ft fence top rail, 21 ft Schedule 40 pipe, 8 and 12 ft SPF dimensional lumber — and prices the result as a costed bill of materials.