Flash Guide Number Calculator
Calculate flash guide number, distance, and aperture relationship for off-camera flash. Enter values for instant results with step-by-step formulas.
Formula
Guide Number = Distance x Aperture (at ISO 100)
The guide number formula relates flash power to the product of flash-to-subject distance and lens aperture at ISO 100. Adjusting for different ISO values multiplies the GN by sqrt(ISO/100). Adjusting for power levels multiplies by sqrt(Power/100). These relationships derive from the inverse square law of light.
Worked Examples
Example 1: Finding Aperture for Portrait at 4 Meters
Problem: A photographer uses a flash with GN 58 (meters) at ISO 100, full power, with a subject at 4 meters. What aperture is needed?
Solution: Guide Number formula: Aperture = GN / Distance\nEffective GN at ISO 100, full power = 58\nAperture = 58 / 4 = f/14.5\nNearest standard f-stop: f/14 or f/16\nAt f/14 the subject will be slightly overexposed (0.05 stops)\nAt f/16 it will be slightly underexposed (0.14 stops)\nUse f/16 and adjust with flash exposure compensation if needed
Result: Required aperture: f/14.5 (use f/16) | Full power at 4 meters
Example 2: Maximum Distance at ISO 400 and f/5.6
Problem: What is the maximum distance for correct exposure with a GN 42 flash (meters) at ISO 400, half power, and f/5.6?
Solution: Base GN: 42 (at ISO 100)\nISO adjustment: sqrt(400/100) = 2.0\nEffective GN at ISO 400: 42 x 2.0 = 84\nPower adjustment (1/2): sqrt(0.5) = 0.707\nAdjusted GN: 84 x 0.707 = 59.4\nMax distance = GN / Aperture = 59.4 / 5.6 = 10.6 meters\nAt full power, max distance would be 84 / 5.6 = 15.0 meters
Result: Max distance: 10.6 meters at 1/2 power | 15.0m at full power
Frequently Asked Questions
What is a flash guide number and what does it mean?
A flash guide number (GN) is a numerical rating that indicates the power output of a flash unit. It represents the relationship between the flash-to-subject distance and the lens aperture needed for correct exposure at a base ISO of 100. A higher guide number means a more powerful flash that can illuminate subjects at greater distances. For example, a flash with GN 58 (in meters) can properly expose a subject at 5 meters at f/11.6, or at 10 meters at f/5.8. Guide numbers are specified either in meters or feet, so always verify which unit system your flash manufacturer uses. The guide number is measured at the flash head's default zoom position, and most modern flashes can increase their effective GN by zooming the flash head to concentrate the beam for telephoto use.
How does ISO affect the guide number and flash range?
ISO and guide number have a square root relationship: doubling the ISO increases the effective guide number by a factor of 1.414 (square root of 2), which means approximately 41% more flash range. At ISO 100 with a GN 58 flash, maximum range at f/4 is 14.5 meters. At ISO 200, effective GN becomes 82, and range extends to 20.5 meters. At ISO 400, effective GN is 116 with 29 meters range. At ISO 800, effective GN is 164 with 41 meters range. Each doubling of ISO effectively adds one stop of flash power. This is why event and wedding photographers often shoot at ISO 800 to 1600, as it dramatically extends their flash working range in large venues like churches and reception halls where subjects may be far from the camera and flash.
What is the relationship between flash power levels and guide number?
Flash power levels follow a square root relationship with guide number and distance. Full power (1/1) uses the rated guide number. Half power (1/2) reduces the GN to 70.7% of full power, cutting maximum distance by about 30%. Quarter power (1/4) reduces GN to 50%, halving the maximum distance. Each halving of power reduces the GN by a factor of 0.707, which equals one stop less light. The practical power levels are: 1/1 = 100% GN, 1/2 = 70.7% GN, 1/4 = 50% GN, 1/8 = 35.4% GN, 1/16 = 25% GN, 1/32 = 17.7% GN, 1/64 = 12.5% GN, 1/128 = 8.8% GN. Understanding this relationship helps you balance flash power with recycle time, as lower power settings produce faster recycle times and more consistent flash output during rapid shooting sequences.
What guide number do I need for different photography scenarios?
Different photography scenarios require different flash power levels. For indoor portraits at 2 to 4 meters distance, a flash with GN 36 to 44 at ISO 100 is generally sufficient, providing apertures of f/9 to f/18 at typical working distances. Event photography in medium-sized venues requires GN 50 to 58 to cover distances of 6 to 10 meters at moderate apertures. Large venue and outdoor fill flash benefits from GN 60 or higher to reach subjects at greater distances. For macro photography, even low-powered flashes with GN 20 are adequate because working distances are measured in centimeters. Wedding photographers typically need flashes with GN 56 to 60 because they must cover large churches and reception halls while bouncing flash off ceilings, which loses approximately 2 stops of light compared to direct flash.
How does flash zoom affect the guide number?
Modern speedlights can zoom their flash heads to match different focal lengths, which concentrates or spreads the light beam and significantly changes the effective guide number. A typical speedlight might be rated at GN 58 at the 105mm zoom setting but only GN 28 at the 24mm setting. This is because zooming the flash head to a narrow beam concentrates all the light into a smaller area, increasing intensity at the center. At wide-angle zoom positions, the light spreads across a larger area, reducing the effective guide number. Manufacturers sometimes advertise the guide number at the most telephoto zoom setting to make their flash appear more powerful. Always check the full zoom-range guide number table in your flash manual. For off-camera flash with modifiers, the zoom setting affects how much light enters the modifier and can impact both light quality and effective output.
What is the inverse square law and how does it affect flash exposure?
The inverse square law states that light intensity decreases proportionally to the square of the distance from the source. When you double the flash-to-subject distance, the light intensity drops to one quarter, requiring two additional stops of exposure compensation. Moving from 2 meters to 4 meters means you lose 2 stops of light, so you need to open your aperture from f/8 to f/4 or increase ISO by 2 stops. This law has important practical implications: subjects at different distances from the flash will have very different exposure levels. A group photo where front row subjects are at 3 meters and back row at 5 meters will have nearly 1.5 stops of exposure difference between them. Moving the flash farther from the group reduces this ratio and creates more even illumination, which is why experienced photographers place their flashes as far back as practical.