Flash Guide Number Calculator

Enter your aperture, subject distance, and ISO to calculate your flash guide number, maximum reach, and the flash power required for a correct exposure.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Set Aperture
Enter the f-stop setting on your lens (e.g., f/8, f/11). This controls light entering the camera.
2
Input Subject Distance
Enter the distance in meters from your flash to the subject. This is key to exposure calculation.
3
Specify ISO Sensitivity
Enter your camera's ISO setting. ISO 100 is the standard reference for guide numbers.
4
Review Your Results
The calculator will display the guide number at ISO 100, the guide number at your current ISO, max flash distance, and effective aperture.

Example Calculation

A photographer wants to determine the guide number of their flash for a portrait shot at f/8, with the subject 3 meters away, using ISO 100.

Aperture (f-number)

f/8

Subject Distance (m)

ISO Sensitivity

100

Results

24.0 m

Tips

ISO and Guide Number

Remember that doubling your ISO (e.g., from 100 to 200) increases your effective guide number by a factor of 1.4 (square root of 2), extending your flash's reach.

Inverse Square Law

The guide number relationship is based on the inverse square law. If you double the distance, you need to open your aperture by two stops to maintain the same exposure (e.g., f/8 to f/4).

Use for Manual Flash

The guide number is most useful for manual flash photography. It allows you to quickly calculate settings without relying on TTL (Through-The-Lens) metering, especially with older or simpler flash units.

Mastering Flash Exposure with the Flash Guide Number Calculator

The Flash Guide Number Calculator is an indispensable tool for photographers seeking precise control over their flash exposure. It helps calculate the flash guide number at any ISO, determine the maximum subject distance, and find the effective aperture needed for perfect illumination. Understanding these metrics is fundamental for manual flash control and mastering light. For instance, a common portrait setup at f/8 with the subject 3 meters away at ISO 100 would require a Guide Number of 24 meters, indicating a moderate power flash.

Demystifying the Guide Number: Your Flash's Power Rating

The Guide Number (GN) is a crucial specification for any flash unit, serving as a standardized measure of its light output power. It represents the flash's ability to correctly expose a subject at a given distance when the camera is set to f/1 and ISO 100. While no one actually shoots at f/1, the GN allows for simple calculation: dividing the GN by the flash-to-subject distance yields the correct aperture. For example, a speedlight might have a GN of 20-60 meters, while powerful studio strobes can boast GNs of 60-100 meters or more. This metric simplifies manual exposure calculations, helps photographers compare the output of different flash units objectively, and is vital for planning lighting setups where precise light falloff and coverage are critical.

The Guide Number Formula Explained

The Flash Guide Number Calculator uses a fundamental relationship in photography that connects aperture, distance, and flash power (represented by the Guide Number). This formula allows photographers to determine one unknown variable if the other two are known.

The core formulas are:

Guide Number (at ISO 100) = Aperture (f-number) × Subject Distance (m)
Guide Number (at Current ISO) = Guide Number (at ISO 100) × sqrt(Current ISO / 100)
Max Distance = Guide Number (at Current ISO) / Aperture (f-number)
Effective Aperture = Guide Number (at Current ISO) / Subject Distance (m)

Here, Aperture is your f-stop setting, Subject Distance is in meters, and Current ISO is your camera's sensitivity. The sqrt(Current ISO / 100) factor accounts for the change in sensor sensitivity.

💡 Understanding how aperture affects your flash calculations is similar to how it impacts depth of field. Our Depth of Field (DoF) Calculator helps you visualize how much of your scene will be in focus.

Example: Calculating Guide Number for a Group Shot

A photographer is planning a group shot outdoors and needs to ensure their flash has enough power. They intend to use an aperture of f/8 and position the flash 3 meters away from the group. The camera is set to ISO 100.

Aperture (f-number): f/8
Subject Distance (m): 3 m
ISO Sensitivity: 100

Using the formulas:

Guide Number (at ISO 100): 8 × 3 = 24 m
Guide Number (at Current ISO): 24 × sqrt(100 / 100) = 24 m

The calculator determines a Guide Number at ISO 100 of 24.0 m. This tells the photographer that their flash unit needs to have a Guide Number of at least 24 (at ISO 100) to properly expose the group at f/8 from 3 meters away. If their flash has a lower GN, they would need to increase the ISO, use a wider aperture, or move the flash closer to the subjects.

💡 Just as Guide Number helps you assess flash power, comparing depth of field across different sensor sizes can inform your equipment choices. Explore our Depth of Field by Sensor Size Comparison Calculator to see the impact.

Comparing Guide Numbers Across Different Flash Systems

Guide numbers provide a universal benchmark for evaluating and comparing the power output of various flash systems. For on-camera speedlights, which are often compact and battery-powered, typical guide numbers at ISO 100 range from 20 to 40 meters. These units are versatile for events and on-the-go portraiture but have limited reach for large spaces. Portable off-camera flashes, which are slightly more powerful and often include larger battery packs, tend to have guide numbers in the 40 to 60-meter range, offering better light output for outdoor portraits or small groups. At the top end, studio monolights and pack-and-head systems can boast guide numbers of 60 meters or more, reaching up to 100 meters or higher for high-power units. These are designed for professional studio work, capable of illuminating large sets or overpowering direct sunlight, providing significant headroom for creative lighting setups. Understanding these ranges helps photographers select the appropriate flash for their specific shooting environment and desired lighting effect.

Frequently Asked Questions

What is a flash guide number and how is it used?

A flash guide number (GN) is a rating of a flash unit's power, typically expressed in meters or feet at ISO 100. It's used to calculate the correct aperture for a given flash-to-subject distance, or vice versa, using the formula: Aperture = Guide Number / Distance. This allows photographers to manually set their flash and camera for proper exposure without relying on automatic metering, providing precise control, particularly in studio or off-camera flash setups.

How does changing ISO affect the effective guide number?

Changing ISO directly affects the effective guide number by altering the camera's sensitivity to light. The guide number is usually specified at ISO 100. For every doubling of the ISO, the effective guide number increases by approximately 1.4x (the square root of 2). For example, a flash with a GN of 30m at ISO 100 would have an effective GN of approximately 42m at ISO 200, allowing for wider apertures or greater working distances.

Can a guide number be used to determine maximum flash distance?

Yes, a guide number can be used to determine the maximum effective flash distance. By rearranging the guide number formula (Distance = Guide Number / Aperture), you can calculate how far your flash can illuminate a subject at a specific aperture setting and ISO. This is particularly useful for planning outdoor shoots, large group photos, or event photography where the flash's reach is a critical factor in achieving proper exposure across the scene.