CalcFlash Guide Number Calculator
Flash Guide Number Calculator. Free online calculator with formula, examples and step-by-step guide.
Flash Guide Number Calculator: Optimize Your Speedlight Exposure
The flash guide number calculator helps you determine the correct aperture, distance, or flash power setting for perfectly exposed flash photography. Whether you are shooting a wedding reception with a speedlight, lighting a studio portrait with strobes, or using a simple on-camera flash at a family gathering, understanding guide numbers lets you achieve consistent flash exposures without relying on automated TTL metering, which can be fooled by challenging lighting conditions.
Guide Number Formula
GN = Distance × f-stop (at ISO 100)
GNISO = GN100 × √(ISO / 100)
The guide number (GN) represents a flash unit's light output capability. It is typically given in meters (or feet) at ISO 100. The relationship works in any direction: if you know the GN and subject distance, you can calculate the required f-stop. If you know the GN and desired aperture, you can find the maximum effective distance. When changing ISO, the effective GN scales by the square root of the ISO ratio, reflecting the sensor's increased sensitivity to light.
The inverse square law is fundamental to understanding guide numbers: light intensity decreases with the square of distance. This means doubling the distance requires quadrupling the flash power (or opening the aperture by two stops) to maintain the same exposure. Conversely, halving the distance reduces the light reaching the subject by a factor of four, requiring a two-stop aperture reduction. This is why small changes in flash-to-subject distance have dramatic effects on exposure in manual flash photography.
Worked Examples
Example 1: Event Photography with Bounce Flash
You are photographing a reception with a speedlight rated at GN 36 (ISO 100, meters). Your subject is 4 meters away, and you are using ISO 400 to allow some ambient light in the background.
Step 1: Adjust GN for ISO: GN400 = 36 × √(400/100) = 36 × 2 = 72
Step 2: Calculate aperture: f-stop = GN / distance = 72 / 4 = f/18
At f/18, your depth of field will be substantial, making group shots sharp from front to back. However, if you are bouncing the flash off the ceiling (which typically reduces effective power by 1–2 stops), you would open up to approximately f/9–f/13 in practice. This calculation gives you a starting point; adjust based on your test shots and the ceiling height and color.
Example 2: Studio Portrait Setup
Your studio strobe has a guide number of 50 (ISO 100, meters). You want to shoot at f/5.6 for a pleasing portrait depth of field. What is the maximum distance for proper exposure?
Calculation: Distance = GN / f-stop = 50 / 5.6 = 8.9 meters
With a softbox modifier that reduces light output by approximately 1.5 stops, the effective distance decreases to about 6.3 meters. Place your subject and strobe accordingly. If you want to use a shallower aperture like f/2.8 for more background blur, the maximum distance at ISO 100 would be 50 / 2.8 = 17.9 meters with a bare flash, or about 12.7 meters with the softbox. In practice, studio distances are usually 1–3 meters, so you would use the flash at reduced power settings to avoid overexposure.
Common Uses
- Setting up manual flash exposure quickly at events and weddings without relying on TTL metering
- Determining the maximum shooting distance for a given flash unit, aperture, and ISO combination
- Choosing the right aperture for group portraits where depth of field must cover multiple rows of people
- Comparing flash units by converting all guide numbers to a common ISO and distance unit for an apples-to-apples power comparison
- Calculating the effect of light modifiers (diffusers, softboxes, umbrellas) on effective flash power
- Balancing flash exposure with ambient light by calculating the appropriate aperture and distance for fill flash in outdoor portraits
Common Mistakes
- Confusing guide numbers measured in meters with those measured in feet — a GN of 36 (meters) equals approximately GN 118 (feet). Using the wrong unit gives wildly incorrect exposures
- Forgetting to account for light modifiers such as softboxes, umbrellas, or diffusers, which typically reduce effective GN by 1–2 stops
- Relying on the manufacturer's GN without testing in your specific shooting environment — real-world conditions with light-absorbing walls and high ceilings can reduce effective range by 30–50%
- Using guide numbers for bounce flash without accounting for the extra distance (flash-to-ceiling-to-subject) and absorption by the ceiling surface
- Assuming guide numbers apply at all zoom settings on a flash head — the GN is typically specified at the maximum zoom setting (narrowest beam), and wide-angle settings scatter more light, reducing the effective GN
Pro Tip
Create a personal cheat card for your most-used flash setup. Test your flash at several power levels (1/1, 1/2, 1/4, 1/8, 1/16) and record the effective guide number for each. Do this with your most common modifier (softbox, umbrella, or bare) at three typical distances (2m, 4m, 6m). You will discover that half power rarely gives exactly half the GN due to flash tube characteristics — in fact, some speedlights only reduce output by 40% when set to half power. Knowing your real-world GN values eliminates guesswork and wasted test shots on location. Tape the chart to the back of your flash stand or save it as a note on your phone.
Frequently Asked Questions
A flash guide number (GN) is a measure of a flash unit's power output, typically specified at ISO 100. It is defined as GN = distance × f-stop for a given ISO setting. A higher guide number means a more powerful flash that can illuminate subjects at greater distances or allow the use of smaller apertures for greater depth of field.
To use guide numbers for manual flash, divide the GN by your subject distance to find the correct aperture, or divide the GN by your aperture to find the correct distance. For example, with a GN 40 flash at ISO 100 and a subject at 5 meters, set your aperture to f/8 (40 / 5 = 8). This gives you proper exposure without any metering.
The effective guide number increases with ISO by a factor of the square root of the ISO ratio. Doubling ISO from 100 to 200 multiplies the GN by 1.41 (40 becomes 56). At ISO 400, the GN doubles (40 becomes 80). This means higher ISO allows flash to reach further or permits smaller apertures, but at the cost of increased noise in the image.
Manufacturer-rated guide numbers are usually measured in a controlled environment with reflective surfaces that maximize output. Real-world conditions typically reduce the effective GN by 30–50%. Factors include light-absorbing dark walls, ceiling scatter, diffusion modifiers, and battery condition. Always test your flash setup at various power levels to determine your real-world guide numbers.