Flash exposure calculator
Enter your flash\'s guide number, ISO and any two of aperture, distance and guide number - the calculator solves the missing value using guide-number maths.
Flash setup
The power rating printed on the flash, normally in metres at ISO 100 and full power.
Leave blank to solve for aperture.
Leave blank to solve for distance.
Leave exactly one of guide number, aperture or distance blank to solve it - or fill all three to check consistency.
Result
Solved flash setting
Direct flash at full power - no diffusers or bounce.
How it works
Flash exposure follows the guide number relationship. First scale the flash’s power to your ISO, then the aperture and distance trade off against each other:
N is the f-number, d the subject distance in the same unit as the guide number, and the square-root term scales the flash for ISO settings other than 100.
Example
A flash rated GN 32 at ISO 100. At f/4 and ISO 100 its reach is 32 ÷ 4 = 8 m. Bump the ISO to 400 and the effective GN becomes 32 × √4 = 64, so the same f/4 now reaches 16 m - but raise the ISO and noise rises with it.
Assumptions to remember
- The quoted GN assumes full power and a bare head pointing straight at the subject.
- Bounce, umbrellas, softboxes and gels can cost anywhere from one to four stops - meter or test rather than trusting the number blindly.
- Speedlight zoom and TTL modes change effective reach; use the GN of your zoom setting when you know it.
Related tools
Once the flash sets your aperture, keep the ambient exposure in balance with the exposure triangle calculator. For the ambient half of a fill-flash shot, see the shutter speed calculator.
Frequently asked questions
What is a guide number?
The guide number (GN) describes a flash's reach at ISO 100, usually in metres. Multiply it by sqrt(ISO/100) for the effective power at other ISO settings, then divide by aperture for reach: distance = GN·sqrt(ISO/100) / f-number.
What aperture do I need at a given flash distance?
Use N = GN·sqrt(ISO/100) / distance. For example a flash with GN 32 (m, ISO 100) at 8 m needs 32/8 = f/4 at ISO 100.
Why does raising ISO increase my flash reach?
Because the sensor becomes more sensitive, the same flash output lights a greater distance. The effective guide number scales with the square root of ISO: GN·sqrt(ISO/100). Doubling ISO adds roughly one stop of reach.
Does the guide number assume any modifiers?
Yes - the quoted GN assumes the bare flash head pointing directly at the subject at full power. Diffusers, bounce, umbrellas and zoom settings all cut light, so treat any calculated aperture or distance as a starting point and adjust.