Flash Guide Number Calculator: Aperture, Distance and ISO
This flash guide number calculator applies your flash’s guide number: enter guide number, ISO, flash power and distance and you get the right aperture plus the range at every aperture.
Example: Guide number (at ISO 100, full power) 40 · Unit of the guide number: Metres · Camera ISO: 100 · Flash power: 1/1 (full) · Distance to subject 5 → Aperture needed: Aperture f/8 (nearest standard stop f/8) at 5 m and ISO 100. Source: Wikipedia – Guide number (guide number = f-number × distance). Updated: .
How it is calculated
With a guide number of 40, the flash reaches exactly 5 metres at f/8: guide number = f-number × distance. The larger the guide number, the farther or the more stopped down you can flash.
How it is calculated
The guide number (GN) is quoted for ISO 100. Flash light falls off with the square of the distance, and the aperture also acts quadratically, so the product of f-number and distance stays constant. At a higher ISO the guide number grows with the square root of ISO / 100. At half flash power it drops by a factor of √2, at a quarter to one half.
What it is useful for
- Finding the aperture for a given distance when flashing manually.
- Comparing the range of flashes before buying or using them.
- Saving battery with partial power and still exposing correctly.
Frequently asked questions
How do you calculate aperture from guide number?
F-number = guide number divided by distance. With a guide number of 40 and 5 m that gives f/8, at 10 m only f/4.
How far does a flash with guide number 40 reach?
About 10 m at f/4, about 5 m at f/8 and roughly 14 m at f/2.8 (at ISO 100 and full power).
How does ISO change the guide number?
The guide number grows with the square root of ISO / 100: it doubles at ISO 400 and quadruples at ISO 1600.
How do I account for partial power?
The guide number drops with the square root of the power fraction: at 1/4 power the flash has only half the guide number.
Sources and legal basis
As of: · Editorial policy
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