Flash Guide Number Calculator
Calculates the correct aperture to use given a flash guide number and subject distance, or vice versa. Indispensable for manual flash photography when you need precise exposure control without test shots.
About this calculator
A flash's Guide Number (GN) encodes its maximum power output at a reference ISO (usually 100). The relationship between guide number, aperture, and flash-to-subject distance follows the simple formula: GN = f-number × distance, or rearranged, f-number = GN / distance. This stems from the inverse-square law of light: doubling the distance quarters the light intensity. Guide numbers are typically given in meters or feet at ISO 100; when shooting at a different ISO, adjust GN by multiplying by √(ISO/100). For example, at ISO 400 multiply GN by 2. Knowing the GN lets you set aperture confidently for any distance without relying on the camera's automatic flash metering.
How to use
Your flash has a guide number of 40 (meters, ISO 100) and your subject is 5 m away. Apply the formula: f-number = GN / distance = 40 / 5 = f/8. Set your lens to f/8 for a correct flash exposure at that distance. If you move the subject to 2.5 m, the new aperture would be 40 / 2.5 = f/16. Conversely, if your lens is fixed at f/5.6, the maximum distance for correct exposure is 40 / 5.6 ≈ 7.1 m.
Frequently asked questions
What is a flash guide number and how is it used in photography?
A guide number is a single value that expresses a flash unit's power at a stated ISO and zoom setting, usually ISO 100 and the flash head at its widest or a standard angle. It lets you calculate the correct aperture for any flash-to-subject distance using the formula f-number = GN / distance. Higher guide numbers indicate more powerful flashes capable of illuminating subjects at greater distances or with smaller apertures. Manufacturers measure GN in either meters or feet, so always confirm the unit before using it in calculations.
How does changing ISO affect the effective flash guide number?
Guide numbers are calibrated at ISO 100. When you shoot at a higher ISO, the sensor is more sensitive and effectively amplifies the flash's reach. The adjusted GN equals the rated GN multiplied by √(ISO / 100). At ISO 400 (four times ISO 100), the effective GN doubles because √4 = 2. This means you can either use a smaller aperture for more depth of field, or reach a subject twice as far away with the same aperture — a useful trick when shooting in large dark spaces.
Why does distance have such a large effect on flash exposure?
Flash follows the inverse-square law: intensity falls off proportional to the square of the distance. Doubling the flash-to-subject distance reduces the light landing on the subject to one quarter, requiring two full stops of compensation. This is why the guide number formula uses a linear distance but the resulting aperture changes by a square-root relationship — the math already accounts for the inverse-square falloff. In practical terms, moving a subject from 2 m to 4 m means you need to open up two full stops (e.g., from f/8 to f/4) to maintain the same exposure.