6XC

Flash Guide Number Calculator

Guide number = distance × f-stop. Find the aperture for a distance, the flash range for an aperture, or your flash's GN.

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Working with Guide Numbers

A flash's guide number (GN, at ISO 100 in meters) links power to exposure: GN = distance × f-number. A GN 36 flash at 4.5 m needs f/8; at f/2.8 it reaches 12.8 m. Doubling ISO multiplies effective GN by √2 (×1.41). This manual-flash math still matters for strobes, film shooters and anyone learning off-camera light.

Frequently Asked Questions

My flash says GN 60 — is that in feet or meters?

Marketing numbers are often quoted in feet at a zoomed-in head. Check the unit and zoom setting: GN 60 (ft) is about GN 18 (m) — a very different flash.

How does ISO change flash range?

Range scales with √(ISO/100): at ISO 400 a GN 36 flash behaves like GN 72, doubling every f-stop's reach.

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