⚡ Flash Guide Number Calculator
Enter a flash guide number and either the distance or aperture to get the correct exposure setting, adjusted for ISO.
Aperture at 5 m
f/11.2
Guide number ties flash power to aperture and distance: GN = f-number × distance. So aperture = GN ÷ distance and reach = GN ÷ f-number, using the flash\'s rated GN (quoted at ISO 100 in metres or feet). Raising ISO increases effective GN by √(ISO ÷ 100) — ISO 400 doubles it. This is for direct, manual flash; bounced or diffused light needs more power. 🔒 In your browser.
How the flash guide number calculator works
A flash's guide number (GN) captures its power as a simple product: GN = f-number × distance, at ISO 100. Rearranged, the aperture for a correct exposure is GN ÷ distance, and the maximum reach at a chosen aperture is GN ÷ f-number. Because film/sensor sensitivity scales the effective power, raising the ISO multiplies the effective guide number by the square root of the ISO ratio — ISO 400 doubles it. The tool solves for whichever value you need and applies the ISO adjustment.
Guide numbers assume direct, undiffused flash in the rated distance unit (metres or feet — check which). Bouncing off a ceiling, using a diffuser or shooting a dark room all lose light and need a wider aperture or more power. Manufacturers often quote GN at the flash's longest zoom setting, which flatters the number, so treat it as a starting point and confirm with a test shot.
Frequently asked questions
What is a flash guide number?
A measure of flash power: guide number (GN) = f-number × distance for a correct exposure at ISO 100. A higher GN means a more powerful flash that reaches farther or allows a smaller aperture.
How do I calculate flash aperture?
Aperture = guide number ÷ distance. With a GN of 56 (metres, ISO 100) and a subject 5 m away, the aperture is 56 ÷ 5 ≈ f/11. Move closer and you stop down; move away and you open up.
How does ISO affect the guide number?
Effective GN scales with the square root of the ISO ratio: GN × √(ISO ÷ 100). So doubling ISO from 100 to 200 multiplies GN by about 1.4, and ISO 400 doubles it — letting the same flash reach farther or use a smaller aperture.
Are guide numbers in metres or feet?
Both are used, so always check which your flash is rated in — a GN of 56 in metres is far more powerful than 56 in feet. This calculator keeps the distance in whichever unit you choose, matching the guide number.
Why is my flash exposure off from the guide number?
Guide numbers assume direct flash at a rated zoom. Bouncing, diffusing, or a wide zoom setting all reduce the light reaching the subject, so you'll need a wider aperture or higher ISO than the raw GN math suggests. Use it as a starting point and confirm with a test frame.