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.
Calculator
Adjust values & calculateDistance by Aperture
Distance by Power Level
Formula
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.
Last reviewed: December 2025
Worked Examples
Example 1: Finding Aperture for Portrait at 4 Meters
Example 2: Maximum Distance at ISO 400 and f/5.6
Background & Theory
The Flash Guide Number Calculator applies the following established principles and formulas. Computers represent all information using binary, a base-2 number system consisting solely of the digits 0 and 1, each called a bit. Because long binary strings are unwieldy, programmers routinely use octal (base 8) and hexadecimal (base 16) as compact shorthand. Converting between bases follows a consistent algorithm: divide the source number repeatedly by the target base, collecting remainders in reverse order. Hexadecimal digits A through F represent the values 10 through 15, allowing a single character to encode four binary bits, making it the preferred notation for memory addresses, color codes, and bytecode. Bitwise operations manipulate individual bits within integers. AND produces a 1 only when both input bits are 1, making it useful for masking. OR produces a 1 when either bit is 1 and is used for combining flags. XOR flips bits that differ, enabling simple toggle logic and efficient swap algorithms. NOT inverts every bit (one's complement), while left and right shifts multiply or divide by powers of two in constant time. Data storage units ascend in binary multiples of 1024: 8 bits form one byte, 1024 bytes form one kibibyte (KiB), 1024 KiB form one mebibyte (MiB), and so forth. Hard-drive manufacturers historically use decimal prefixes (1 KB = 1000 bytes), creating the persistent confusion between binary and decimal interpretations of the same label. The IEC standardized the binary prefixes KiB, MiB, GiB, and TiB in 1998 to resolve this ambiguity. Network bandwidth is measured in bits per second (bps), most commonly megabits per second (Mbps) or gigabits per second (Gbps). A 100 Mbps connection transfers 100 million bits every second, equating to roughly 12.5 megabytes per second. IP subnet masks define network boundaries; CIDR notation appends a prefix length (e.g., /24) to an address, indicating how many leading bits are fixed. A /24 subnet contains 256 addresses with 254 usable hosts. Algorithm efficiency is described using Big-O notation, which characterises the worst-case growth of time or space relative to input size. O(1) is constant, O(log n) is logarithmic (binary search), O(n) is linear, and O(nยฒ) is quadratic. Cryptographic hash functions like SHA-256 produce a fixed 256-bit (32-byte) digest regardless of input length. File compression algorithms exploit statistical redundancy to reduce storage footprint, and compression ratio equals the original file size divided by the compressed size.
History
The history behind the Flash Guide Number Calculator traces back through the following developments. The conceptual foundation of modern computing traces back to Charles Babbage, whose Analytical Engine design of 1837 introduced the idea of a general-purpose mechanical computer with separate storage and processing units, including what he called the Store and the Mill. Ada Lovelace wrote what many consider the first algorithm intended for machine execution while annotating a translation of Luigi Menabrea's account of Babbage's work, also recognising the machine's potential to manipulate symbols beyond mere numbers. George Boole published "The Laws of Thought" in 1854, formalising a two-valued algebra of logic that would later map perfectly to electrical circuits. It remained largely a mathematical curiosity until Claude Shannon's landmark 1937 master's thesis demonstrated that Boolean algebra could describe switching circuits, laying the theoretical groundwork for all digital electronics. Shannon's 1948 paper "A Mathematical Theory of Communication" defined the bit as the fundamental unit of information and established information theory as a rigorous discipline. The same year, the transistor was invented at Bell Labs by Bardeen, Brattain, and Shockley, eventually replacing vacuum tubes and enabling miniaturisation at scale. ENIAC, completed in 1945, was one of the first general-purpose electronic computers, occupying 1800 square feet and consuming 150 kilowatts of power while performing roughly 5000 additions per second. The ASCII standard was ratified in 1963, assigning 7-bit codes to 128 characters and enabling interoperability between computers from different manufacturers. Through the 1970s, the microprocessor consolidated an entire CPU onto a single chip; Intel's 4004 in 1971 marked the beginning of this trend. The Apple II launched in 1977 and the IBM PC in 1981 brought computing to homes and offices, triggering a mass-market software industry. Tim Berners-Lee proposed the World Wide Web in 1989 and launched the first website in 1991 at CERN, transforming the internet from an academic and military network into a global information infrastructure. Mobile computing accelerated through the 2000s with smartphones integrating powerful processors, wireless networking, and GPS into pocket-sized devices, extending computation into every facet of daily life and cementing TCP/IP as the universal communications fabric.
Frequently Asked Questions
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.
References
Reviewed by Daniel Agrici, Founder & Lead Developer ยท Editorial policy