NFPA 780 Risk Assessment Calculator: Free Annex L Screen

An NFPA 780 risk assessment calculator compares N_D, the strikes a structure can expect each year, with N_C, the frequency NFPA 780-2026 Annex L tolerates for it. If N_D is higher, a lightning protection system is recommended; if not, it is optional on the screen.

This is the simplified screen of Section L.5, run by the same engine as a full Lumex^™ assessment on the 2026 edition's values. Enter the structure's size, where it stands, the local strike density and four simple choices about the building, and the result updates as you type. The working underneath shows each figure with the section or table it comes from. For the method behind it, read the NFPA 780 Annex L guide.

The calculator

Annex L simplified risk screen

Fill in the structure and its setting. The page opens on the worked example below, so change any value to see your own result.

Your structure

Collection area shape

Rectangular structure

One prominent part covers the rest

equation (L.4.1.1), or the note to Figure L.4.1.2(a)

Length L

m

not used for a prominent part

Width W

m

not used for a prominent part

Height H

m

for a prominent part, its own height

Ground strike-point density N_SG

/km²/yr

Section L.2.1

Where the structure stands, C_D

Among taller structures or trees

Among structures no taller than it

Isolated, nothing within three times its height

Isolated on a hilltop

Table L.4.2

Structure material, for C2

Metal

Nonmetallic

Combustible

Table L.5.1.2(a)

Roof material, for C2

Metal

Nonmetallic

Combustible

Table L.5.1.2(a)

Contents, C3

Low value, does not burn

Ordinary value, does not burn

High value, burns moderately

Very valuable: flammable liquids, computers or electronics

Irreplaceable cultural items

Table L.5.1.2(b)

Occupancy, C4

Unoccupied

Normally occupied

Hard to evacuate, or panic is likely

Table L.5.1.2(c)

Consequence of a strike, C5

Services need not keep running, no harm to the environment

Services must keep running, no harm to the environment

Damage would harm the environment

Table L.5.1.2(d)

A critical facility listed in Section L.5.3.1

N_D against N_C

Protection recommended

on the L.5 screen of NFPA 780-2026 Annex L

N_D, strikes per year

0.03172

N_C, tolerable per year

0.0015

A_D, m²

7930

N_D is above N_C, so under Section L.5.2.1 a lightning protection system is recommended. The detailed assessment of Section L.6 shows which risk drives that and which measures bring it down.

Quantity	How it was found	Value	Source
L	structure length, m	60	NFPA 780-2026 L.4.1.1
W	structure width, m	30	NFPA 780-2026 L.4.1.1
H	structure height, m	8	NFPA 780-2026 L.4.1.1
N_SG	ground strike points per km² per year	4	NFPA 780-2026 L.2.1
A_D	LW + 6H(L + W) + 9 × π × H² = 60 × 30 + 6 × 8 × (60 + 30) + 9 × π × 8²	7930	NFPA 780-2026 equation (L.4.1.1)
C_D	Isolated, nothing within three times its height	1	NFPA 780-2026 Table L.4.2
N_D	N_SG × A_D × C_D × 10⁻⁶ = 4 × 7930 × 1 × 10⁻⁶	0.03172	NFPA 780-2026 equation (L.3)
C2	Nonmetallic structure, metal roof	1	NFPA 780-2026 Table L.5.1.2(a)
C3	Ordinary value, does not burn	1	NFPA 780-2026 Table L.5.1.2(b)
C4	Normally occupied	1	NFPA 780-2026 Table L.5.1.2(c)
C5	Services need not keep running, no harm to the environment	1	NFPA 780-2026 Table L.5.1.2(d)
C	C2 × C3 × C4 × C5 = 1 × 1 × 1 × 1	1	NFPA 780-2026 L.5.1.1, L.5.1.2
N_C	1.5 × 10⁻³ / C = 0.0015 / 1	0.0015	NFPA 780-2026 equation (L.5.1.1)

The method

How the Annex L screen decides

Section L.5 of NFPA 780-2026 asks one question: does lightning strike this structure more often than the structure can tolerate? It answers with two yearly frequencies and a comparison. Nothing else in the screen depends on wiring, services or protection measures; those belong to the detailed method of Section L.6.

The threat, N_D. Equation (L.3) multiplies three things: the ground strike-point density N_SG (Section L.2.1), the collection area A_D in square metres, and the location factor C_D from Table L.4.2, then scales by 10^-6 to turn square metres into square kilometres. The working beside the result shows the figures put into it.

The collection area, A_D. For a rectangular structure equation (L.4.1.1) gives A_D = LW + 6H(L + W) + 9πH²: the roof, a band around it three times the height wide, and rounded corners of the same reach. Where a single tall part, such as a tower or a stack, covers every part of the lower building, the note to Figure L.4.1.2(a) takes 9πH² from the height of that part alone.

The tolerance, N_C. Equation (L.5.1.1) divides a default tolerable frequency of 1.5 × 10⁻³ events per year by a coefficient C, where C = C2 × C3 × C4 × C5 (Section L.5.1.2). C2 comes from the structure and roof materials in Table L.5.1.2(a), C3 from the contents in Table L.5.1.2(b), C4 from the occupancy in Table L.5.1.2(c) and C5 from the consequence of a strike in Table L.5.1.2(d). A building that burns easily, holds valuable contents, is hard to evacuate or would stop an essential service gets a larger C, and so a smaller N_C: it tolerates fewer strikes.

The decision. Section L.5.2.1 compares the two. When N_D is at or below N_C, protection is optional; when it is above, protection is recommended. A critical facility listed in Section L.5.3.1 is different: Section L.5.3.2 advises protection or the detailed assessment whatever the screen shows.

Strike points, not flashes. The 2026 edition works in ground strike-point density N_SG, where the 2020 edition used flash density. If your source gives a flash density, convert it under Section L.2.2 before entering it, or a result will understate the threat. The NFPA 780-2026 changes guide covers the switch.

Worked example

A 60 m by 30 m building, 8 m high

Illustrative inputs, not a real site: a rectangular building with a nonmetallic structure and a metal roof, ordinary contents, normally occupied, standing on its own where N_SG is 4 strike points per km² per year. The figures below are computed live by the calculator's engine.

The collection area comes to 7930 m² by equation (L.4.1.1). With C_D = 1 for an isolated structure, equation (L.3) gives N_D = 0.03172 strikes per year. The four coefficients multiply to C = 1, so N_C = 0.0015 per year. N_D is above N_C, so the screen recommends a lightning protection system for this building.

Quantity	How it was found	Value	Source
L	structure length, m	60	NFPA 780-2026 L.4.1.1
W	structure width, m	30	NFPA 780-2026 L.4.1.1
H	structure height, m	8	NFPA 780-2026 L.4.1.1
N_SG	ground strike points per km² per year	4	NFPA 780-2026 L.2.1
A_D	LW + 6H(L + W) + 9 × π × H² = 60 × 30 + 6 × 8 × (60 + 30) + 9 × π × 8²	7930	NFPA 780-2026 equation (L.4.1.1)
C_D	Isolated, nothing within three times its height	1	NFPA 780-2026 Table L.4.2
N_D	N_SG × A_D × C_D × 10⁻⁶ = 4 × 7930 × 1 × 10⁻⁶	0.03172	NFPA 780-2026 equation (L.3)
C2	Nonmetallic structure, metal roof	1	NFPA 780-2026 Table L.5.1.2(a)
C3	Ordinary value, does not burn	1	NFPA 780-2026 Table L.5.1.2(b)
C4	Normally occupied	1	NFPA 780-2026 Table L.5.1.2(c)
C5	Services need not keep running, no harm to the environment	1	NFPA 780-2026 Table L.5.1.2(d)
C	C2 × C3 × C4 × C5 = 1 × 1 × 1 × 1	1	NFPA 780-2026 L.5.1.1, L.5.1.2
N_C	1.5 × 10⁻³ / C = 0.0015 / 1	0.0015	NFPA 780-2026 equation (L.5.1.1)

The limits

What the screen does not tell you

The screen is quick because it ignores most of what makes lightning costly. It does not look at the power and telecom lines entering the building, the equipment inside, surge protection, fire provisions or the people at risk of touch and step voltages. It cannot say which protection measure would help most, and it gives no figure for loss of life, loss of service, loss of historical value or economic loss.

The detailed assessment of Section L.6 does all of that. It builds the risks R1 to R4 from strikes to and near the structure and its incoming services, tests each against its own tolerable value from Table L.6.2, and shows which component drives the result. Use it when the screen recommends protection and you need to know what kind, when the building is a critical facility, or when a client or insurer asks for more than a yes or no.

Run the full assessment.

Lumex^™ runs both Annex L methods on the same engine as this calculator, keeps every factor traced to its NFPA 780-2026 table, proposes protection and produces a report for the engineer to sign.

Start a free trial or read

how Lumex^™ runs NFPA 780

. NFPA 780 is published by the NFPA, and Lumex^™ is independent of it: buy the standard for design and certification.

FAQs

Questions answered

What does this NFPA 780 calculator work out?

It runs the simplified risk assessment of NFPA 780-2026 Section L.5. From the structure's size and setting and the local strike density it finds N_D, the number of strikes the structure can expect each year, by equation (L.3). From four coefficients for the building, its contents, its occupancy and the consequence of a strike it finds N_C, the tolerable frequency, by equation (L.5.1.1). Then it compares the two under Section L.5.2.1.

What does it mean if N_D is greater than N_C?

Under Section L.5.2.1 of NFPA 780-2026 a lightning protection system is recommended when N_D is above N_C, and optional when N_D is at or below it. Optional is not the same as forbidden: an authority having jurisdiction, an insurer or a contract can still require protection, and Section L.1.1.1 puts those requirements ahead of any assessment.

Where do I get the strike density N_SG?

Section L.2 of the 2026 edition uses the ground strike-point density N_SG, in strike points per square kilometre per year, read from the map in Figure L.2.1 or from a lightning location service. If you only have a flash density, Section L.2.2 converts it to N_SG before it goes in. The calculator asks for N_SG directly, because that is what the 2026 equations take.

Is the simplified screen enough for a hospital or a fire station?

No. Section L.5.3.1 lists critical facilities such as emergency services, hospitals, utilities, telecom, schools and transport hubs, and Section L.5.3.2 advises either a lightning protection system or the detailed assessment of Section L.6 for them. Tick the critical facility box and the calculator, like the full product, will not treat the screen alone as enough.

How is the collection area A_D found?

For a rectangular structure, equation (L.4.1.1) adds the roof area to a band three times the height wide around it and the quarter circles at the corners: L times W, plus 6H times L plus W, plus 9 pi H squared. Where one tall part covers the whole of the lower structure, the note to Figure L.4.1.2(a) takes 9 pi H squared from that part's height alone. Pick the shape in the calculator to choose between them.

Is Annex L mandatory?

No. Annex L is informative, so it is guidance rather than a requirement of NFPA 780-2026 itself. It becomes binding only when something else adopts it, such as an authority having jurisdiction, an insurer, a specification or a contract. Some federal criteria do call for it, so check what applies to your project.

Are these the same values the Lumex^™ product uses?

Yes. The calculator runs the same NFPA 780-2026 engine as a full Lumex^™ assessment, on the same reference values, and its verdict comes from the same rule. Each value in the working names the section or table it comes from, so you can check it against your own copy of the standard.

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