Flash guide number calculator
The guide number links flash distance and aperture: GN = distance × f-number. Choose what to solve for, enter the others and the ISO, and the calculator does the rest.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose whether to find the distance, the aperture or the guide number.
- Enter the values you know, using the guide number at ISO 100 from your flash’s manual.
- Set the ISO and read the answer.
How it works
At ISO 100, GN = distance × f-number. So distance = GN ÷ f-number and aperture = GN ÷ distance. For another ISO the guide number is multiplied by √(ISO ÷ 100). The tool does not convert between metres and feet for the guide number itself, so use a guide number quoted in the same unit as the distance.
Worked example
A flash with GN 40 (metres, ISO 100) at f/8: distance = 40 ÷ 8 = 5 m. At ISO 400 the effective GN is 80, so at f/4 the flash reaches 80 ÷ 4 = 20 m.
Tips
- Halving the distance lets you stop down by two stops, or lower the flash power.
- Bounced or diffused flash loses power; test a shot and adjust.
Frequently asked questions
What is a flash guide number?
A number that tells how powerful a flash is: the distance multiplied by the f-number needed for a correct exposure at a given ISO, usually ISO 100 at full power.
Does the guide number use metres or feet?
Either, so check the unit on the flash. A guide number of 40 in metres is about 131 in feet. The result of this tool is in the same unit as the distance, so enter a guide number quoted in that unit.
How does ISO change the guide number?
Doubling the ISO multiplies the guide number by √2, so ISO 400 doubles it compared with ISO 100. The calculator scales it by √(ISO ÷ 100).
Are manufacturer guide numbers accurate?
They are measured at a particular zoom head setting, at full power and often in ideal conditions. Reflective walls help, and wide diffusers and bounce reduce the usable distance, so treat results as a starting point.