Flash Guide Number Calculator

Solve the flash exposure triangle for manual flash: find your maximum flash-to-subject distance, the aperture you need, or the effective guide number after ISO scaling, modifiers, zoom head and power settings. Everything runs 100% in your browser — no uploads, no tracking.

Max flash distance from guide number

Guide number (rated, ISO 100)

GN units

Aperture (f-number)

ISO

Light modifier (loss)

Flash power

Zoom head position

The guide number formula, explained

A guide number (GN) is the flash industry's practical measure of reach: it links flash power, aperture and distance in one equation, always quoted at ISO 100:

GN = f-number × distance

Rearrange it to solve for whatever you need:

distance = GN ÷ f-number · f-number = GN ÷ distance

Two scaling laws make the formula useful in the real world. First, ISO scaling: doubling the ISO is one stop more sensitivity, so reach grows by √2, not by 2:

GN_effective = GN_rated × √(ISO ÷ 100)

Second, flash power and modifiers scale the same square-root way, because each stop of light is a factor of 2 in energy but only √2 in distance:

GN_effective = GN_rated × √(power fraction) × 2^−loss/2 × zoom factor

That is the inverse-square law hiding inside the guide number: to double your distance you need four times the light (two stops), which is why a modest diffuser or a ceiling bounce eats reach so quickly.

Typical guide numbers by flash

Manufacturer-quoted values at ISO 100 with the zoom head at its longest position. Real numbers vary a little by test conditions — check your manual for the exact figure.

Flash	Guide number (m, ISO 100)	Guide number (ft, ISO 100)
Godox TT685 II / TT600	60	197
Godox V860 III	60	197
Godox AD200Pro (bare bulb)	60	197
Canon Speedlite 600EX II-RT	60	197
Sony HVL-F60RM2	60	197
Nissin i60A	60	197
Nikon SB-5000	34.5	113
Godox V1 (round head)	28	92
Canon Speedlite EL-100	26	85
Typical pop-up / built-in flash	12	39

How modifiers change your effective guide number

Modifier / technique	Typical loss (stops)	GN multiplier
Direct bare flash	0	×1.00
Wide-angle diffuser panel	−1	×0.71
Bounce card / small softbox	−1.5	×0.59
Ceiling bounce (white, 2–3 m)	−2	×0.50
Double bounce / large dark room	−3	×0.35

A single ceiling bounce halves your guide number: a GN 60 flash behaves like a GN 30 flash. That is why event photographers raise the ISO instead of the flash power when bouncing.

Worked examples (step by step)

Example 1 — Max distance: GN 60 at f/8

Start with the formula: distance = GN ÷ f-number.
Insert the numbers: distance = 60 ÷ 8.
Result: 7.5 meters — a GN 60 flash correctly exposes a subject 7.5 m away at f/8, ISO 100, direct flash.

Example 2 — ISO 400 doubles the reach factor

Scale the guide number: GN_eff = 60 × √(400 ÷ 100) = 60 × 2 = 120.
At f/8: distance = 120 ÷ 8 = 15 meters.
Four times the ISO doubled the reach (two stops of light, one √2 per stop).

Example 3 — Required aperture: GN 28 flash, subject at 4 m

Formula: f-number = GN ÷ distance = 28 ÷ 4 = 7.0.
The exact answer is f/7; the nearest 1/3-stop setting on your lens is f/7.1.

Example 4 — Ceiling bounce costs two stops

A −2 stop bounce multiplies the GN by 2^(−2/2) = 0.5: 60 × 0.5 = 30.
Subject at 5 m: f-number = 30 ÷ 5 = 6.0 → the nearest 1/3-stop setting is f/5.6 (5.66 is closer to 6.0 than 6.35 is).
Bouncing turned a 7.5 m flash (at f/8) into a 3.75 m flash at the same aperture.

Example 5 — 1/4 power and feet units

1/4 power: GN_eff = 60 × √(1/4) = 60 × 0.5 = 30 m.
In feet, that same flash is GN 197 ft (60 × 3.28084).
At f/11: 197 ÷ 11 = 17.9 ft of reach at full power, direct, ISO 100.

Example 6 — Wide zoom head

Zooming the head to 24 mm spreads the beam: multiply the GN by ~0.55 → 60 × 0.55 = 33.
At f/5.6: 33 ÷ 5.6 = 5.9 m of reach instead of 10.7 m at the rated zoom position.

Meters vs feet

Guide numbers are unit-bound: a flash rated GN 60 in meters is GN 197 in feet (60 × 3.28084). The formulas work in either unit as long as the GN and the distance use the same unit. This calculator keeps the unit you choose for both the GN input and the distance output.

Guide numbers assume the flash is the dominant light source. They do not account for ambient light — for balancing flash with ambient exposure, see our exposure tools below.

Frequently asked questions

What is a flash guide number?

A guide number is a single number that describes how far a flash can reach: guide number equals the f-number multiplied by the flash-to-subject distance. A higher guide number means a more powerful flash. It is always quoted at ISO 100 and with the zoom head at its longest position.

What units are guide numbers measured in?

Guide numbers are quoted in meters or in feet, and the number changes with the unit: a flash rated GN 60 in meters is GN 197 in feet. Always check which unit your manual uses, because mixing meters and feet gives wrong answers.

Why are guide numbers quoted at ISO 100?

ISO 100 is the reference sensitivity so flashes can be compared fairly. A higher ISO makes the sensor more sensitive, which effectively raises the guide number: at ISO 400 a GN 60 flash behaves like a GN 120 flash.

How do I convert a guide number to a different ISO?

Multiply the rated guide number by the square root of the ISO divided by 100. Doubling the ISO (100 to 200) multiplies reach by 1.41; quadrupling the ISO (100 to 400) doubles the guide number.

Does the zoom head change the guide number?

Yes. The quoted guide number applies at the longest zoom setting, where the beam is narrowest and most concentrated. Zooming the head wider (for example to 24 mm) spreads the light and lowers the effective guide number, roughly halving it at the widest setting.

How much light does a diffuser or bounce cost?

A diffuser panel typically costs about 1 stop of light, which multiplies the guide number by 0.71. A single ceiling bounce costs about 2 stops, which halves the guide number, and a double bounce or large dark room can cost 3 stops.

Guide number vs watt-seconds: which tells me flash power?

Watt-seconds measure the electrical energy the flash stores, while the guide number measures the practical result: how far the light reaches at a given aperture and ISO. Two flashes with the same watt-seconds can have different guide numbers because of reflector and zoom-head design.

Do I still need guide numbers if my flash has TTL?

TTL metering automates exposure, so you do not need guide numbers for everyday shooting. Guide numbers are still useful for planning: checking whether a flash can cover a venue, comparing flashes before buying, and setting up manual multi-flash scenes.

Does high-speed sync change the guide number?

Yes. High-speed sync fires the flash as a rapid series of pulses instead of one burst, which reduces the effective guide number, especially at very fast shutter speeds. Treat HSS as costing roughly 2 stops of reach and test at your actual settings.

Can I use guide numbers with multiple flashes?

Yes. When two identical flashes fire together from the same position, add their guide numbers in quadrature: take the square root of the sum of the squares. Two GN 40 flashes give an effective GN of about 57, which is one stop more light.

Why does doubling the distance need four times the power?

That is the inverse-square law: light intensity falls with the square of the distance. Doubling the distance spreads the same light over four times the area, so you need four times the power, which is exactly two stops, to keep the same exposure.

What guide number do I need for wedding and event photography?

A GN 60 speedlight (meters, ISO 100) is the common standard for events: bounced off a typical ceiling at ISO 800 to 1600 and f/4, it comfortably covers subjects 5 to 10 meters away. For large ballrooms or outdoor night work, a second flash or a strobe with a higher guide number helps.

Related photography tools

Exposure Calculator ND Filter Exposure Suite Astrophotography Exposure Calculator

Free tool by NRTechStudio