A flash guide number describes how much light a flash puts out, and it links three things together: guide number equals the aperture multiplied by the distance to your subject. This calculator lets you solve for whichever one you need. Enter a guide number and a distance to find the aperture that gives a correct flash exposure, or enter a guide number and an aperture to find how far the flash will reach, or enter aperture and distance to work out the guide number of an unknown flash. Guide numbers are quoted at ISO 100, and if you raise the ISO the effective guide number grows, so the tool includes an ISO adjustment. This is invaluable for manual flash work, off camera lighting, and understanding why moving a flash closer brightens a shot so quickly. All math is done in your browser with nothing sent to a server.
A flash guide number describes how much light a flash puts out, and it links three things together: guide number equals the aperture multiplied by the distance to your subject. This calculator lets you solve for whichever one you need.
Yes. Flash Guide Number Calculator is completely free, with no sign-up and no usage limits.
Yes. Flash Guide Number Calculator runs in any modern web browser. There is nothing to download or install.
Yes. Flash Guide Number Calculator runs entirely on your device in your browser, so nothing you enter is uploaded to a server.