Flash Guide Number Calculator
Guide number = distance × f-number (at ISO 100). Pick what to solve for and this calculator handles the ISO scaling for you.
How guide numbers work
A guide number describes flash power: GN = distance × f-number for a correct exposure at ISO 100. Doubling ISO multiplies the effective guide number by √2 (about 1.4). A GN 36 flash at ISO 100 exposes a subject 4.5 m away correctly at f/8, or 9 m away at f/4.
Frequently asked questions
Is the guide number in meters or feet?
Both conventions exist. This tool works in meters; a guide number quoted in feet is about 3.28× larger. Check whether your flash's spec sheet says m or ft at ISO 100.
Why does my zoom head change the guide number?
Flash zoom concentrates light into a narrower beam. Manufacturers usually quote the guide number at the longest zoom setting, so real-world power at wide settings is lower.
Do bounce and modifiers change the math?
Yes. Bouncing off a ceiling or adding a softbox costs light (often 2–3 stops) and increases the effective distance. Guide-number math applies cleanly only to direct, bare flash.
Last updated: 2026-07-12