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Lightning Rod (Air Terminal) Cone of Protection Calculator

Calculate the protected radius at ground level from a lightning air terminal, using the classical cone-of-protection method.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax
32.81 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Protected radius

32.81 ft

Check your inputs

The classical cone method's protection angle depends on the lightning protection class and terminal height per NFPA 780 tables, and this simplified single-cone method does not replace NFPA 780's more comprehensive rolling sphere method, which is required for a complete and code-compliant lightning protection system design by a certified LPI installer/designer.

Running these inputs gives 10 m as the protected radius. Expect some drift against the real job; the calculation is solid but conditions on site are not. Currently reading for United States under IRC 2024 — pick a different market above and the figures re-cast accordingly.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Classical cone-of-protection method: protected radius = rod height × tan(protection angle), where the protection angle depends on the terminal's height and the desired protection class per NFPA 780 (this simplified cone method is a traditional approach; NFPA 780 also provides the more comprehensive rolling sphere method for modern lightning protection system design)

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Frequently asked questions

What is the classical cone-of-protection method?
It models the zone protected by a lightning air terminal as a cone extending outward and downward from the terminal's tip, with protected radius = rod height × tan(protection angle) — a traditional approach per NFPA 780.
Where does the protection angle come from?
The protection angle depends on the terminal's height and the desired protection class, set by NFPA 780's class/height tables — it is not a fixed universal value, so confirm the correct angle for your specific installation before relying on this calculator.
Is the cone method sufficient for a complete lightning protection system design?
No. NFPA 780 also provides the more comprehensive rolling sphere method for modern lightning protection system design, which better accounts for complex rooflines and multiple terminals. A complete, code-compliant design should be performed by a certified LPI installer/designer.