What Is a Flash Guide Number Calculator?
A flash's guide number (GN) is a single number that describes how powerful it is at full output, rated at a specific ISO — almost always ISO 100. It combines the flash's output with the inverse-square falloff of light so you can quickly work out the aperture needed for a correctly exposed subject at a given distance, without a flash meter.
This calculator takes your flash's guide number, your subject distance and your ISO, and returns the aperture you should set — along with a reference table showing the maximum working distance at several common apertures.
How to Read Your Results
Required Aperture
This is the f-stop that should give you a correctly exposed subject at your chosen distance, assuming the flash fires at full manual power with no light modifiers in the way.
ISO-Adjusted Guide Number
Guide numbers are rated at ISO 100. Raising the ISO increases the camera's sensitivity to light, which effectively extends the flash's reach. This calculator scales the guide number by the square root of ISO ÷ 100 to reflect that relationship before dividing by distance.
Real-World Example
A flash rated at GN 40 (in feet, at ISO 100), used at ISO 100 and 10 feet from the subject, needs an aperture of about f/4 — that's 40 ÷ 10 = 4.
| Distance | Aperture (GN 40, ISO 100) |
|---|---|
| 5 ft | ≈f/8.0 |
| 10 ft | ≈f/4.0 |
| 20 ft | ≈f/2.0 |
Doubling the distance from 10 ft to 20 ft cuts the required aperture value in half — from f/4 to f/2 — because light falls off with the square of distance.
Tips for Using Flash Guide Numbers
- Raising your ISO extends your flash's effective range, since the ISO-adjusted guide number grows with the square root of the ISO increase.
- Doubling your distance to the subject requires opening up your aperture by about 2 stops to maintain the same exposure.
- Guide numbers assume a bare flash. Modifiers like softboxes, umbrellas and diffusers absorb and scatter light, which reduces the effective guide number below what's printed in the manual.
Frequently Asked Questions
What is a flash guide number?
A guide number is a rating, usually given at ISO 100, that describes a flash's maximum output. Dividing the guide number by the subject distance gives the aperture needed for a correct exposure at full flash power.
How does ISO affect the guide number?
Guide numbers are rated at ISO 100. Increasing ISO makes the sensor more sensitive to light, which effectively increases the usable guide number — and therefore the flash's working distance or the aperture you can use — following the square root of the ISO ratio.
Why do my measured results differ from the GN calculation?
Guide number math assumes a bare flash firing at full power in average conditions. Light modifiers, bounced flash, room reflections, subject reflectivity and reduced flash power output will all shift your actual results away from the calculated value.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team