Flash Guide Number
Find a flash's effective guide number at a given ISO and distance.
About Flash Guide Number
The guide number is the aperture times the distance, and higher ISO effectively raises it because more light is visible. Guide numbers let you set a correct flash exposure or judge how far a compact flash can reach at a given aperture.
Formula
Flash Guide Number
GN = aperture x distance; scale by sqrt(ISO/100)
- guide number
- Measures flash output; higher means more range
Worked example
With the default values below, Flash Guide Number returns Guide Number at ISO of 40.0. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Aperture (f-number)minimum 0.1
- 8
- Flash-to-subject distanceminimum 0.1
- 5
- ISOminimum 25
- 100
Results
- Guide Number at ISO
- 40.0
- Guide Number at ISO 100
- 40.0
Step by step
- 1Values used
- 2Flash Guide Number
- 3Guide Number at ISO= 40.0
- 4Guide Number at ISO 100= 40.0
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Flash Guide Number calculated?
GN = aperture x distance; scale by sqrt(ISO/100) The guide number is the aperture times the distance, and higher ISO effectively raises it because more light is visible.
Why does Flash Guide Number matter?
Guide numbers let you set a correct flash exposure or judge how far a compact flash can reach at a given aperture.
What values do I need to enter?
This calculator takes 3 inputs: Aperture (f-number), Flash-to-subject distance, ISO. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.