What is a flash guide number?

The guide number (GN) describes flash power. It relates aperture and distance: GN = distance (m) x f-number at ISO 100. A GN 58 flash at f/5.6 correctly exposes at 58/5.6 = 10.4 metres. Higher GN means more powerful flash. Most speedlights have GN 36-60.

How does ISO affect guide number?

Guide numbers are specified at ISO 100. For other ISOs, multiply the GN by the square root of (ISO/100). At ISO 400, multiply GN by 2. At ISO 1600, multiply by 4. This effectively doubles the usable distance for each 2-stop ISO increase.

Why is my flash underexposing at distance?

Flash follows the inverse square law — doubling the distance requires 4x the power (2 stops). If your subject is beyond the flash range (GN/aperture), increase ISO, open the aperture, or move closer. Also check if you are bouncing the flash, which significantly reduces effective power.

How do I compare flash power between units?

Compare guide numbers at the same ISO and zoom setting. A GN 60 flash is twice as powerful as a GN 42 flash (one stop difference, since 60/42 ≈ 1.4 and one stop = √2 = 1.414). Studio strobes are often rated in watt-seconds (Ws) — roughly, doubling Ws adds one stop.

PhotoCalcs

Flash Guide Number Calculator — GN, Distance & Aperture

Calculate flash guide number, required aperture or maximum distance for correct flash exposure.

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Guide Number (ISO 100)

Flash GN at ISO 100

Distance

Flash to subject

ISO

Camera ISO setting

How We Calculate This

The guide number formula relates flash power, distance and aperture:

Guide Number Formula

GN = distance (m) x f-number

ISO Adjustment

Adjusted GN = GN_100 x √(ISO / 100)

Rearranging: aperture = GN / distance, or distance = GN / aperture.

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Last updated: February 2026

All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.