The Guide Number (GN) is a measure of a flash unit's power output. It represents the maximum distance (in meters or feet) at which the flash can properly illuminate a subject at a given aperture (f-stop) and ISO. The standard formula is: GN = Distance × f-number (at ISO 100, full power). A higher GN means a more powerful flash. For example, a flash with GN 40 (meters, ISO 100) can illuminate a subject at 10 meters when using f/4, or at 5 meters when using f/8.

ISO significantly impacts effective flash range. The effective GN adjusts by a factor of √(ISO/100). Doubling the ISO (e.g., from 100 to 200) multiplies the effective GN by √2 ≈ 1.41, giving you about 41% more reach. At ISO 400, the effective GN doubles (√4 = 2). This means a GN 40 flash at ISO 400 behaves like a GN 80 flash at ISO 100. This calculator automatically adjusts for ISO changes.

Reducing flash power decreases the effective GN by the square root of the power fraction. At 1/2 power, effective GN = GN × √(1/2) ≈ GN × 0.707. At 1/4 power, GN is halved (√(1/4) = 0.5). At 1/16 power, GN drops to 1/4 of full power. This is why you'll often shoot at wider apertures or closer distances when using reduced flash power for faster recycle times or subtle fill light.

GN values are typically given in meters (ISO standard) or feet. A flash rated GN 40 (meters) equals approximately GN 131 (feet), since 1 meter ≈ 3.28 feet. The relationship is: GN(ft) = GN(m) × 3.28. Always check which unit system your flash manufacturer uses. Most European and Asian brands use meters, while some older American specifications use feet. This tool lets you switch units to match your preference.

Several factors affect real-world flash performance: light modifiers (softboxes, umbrellas reduce output by 1-3 stops), bounce flash (ceilings/walls absorb light), ambient light (can overpower flash in bright conditions), and manufacturer GN ratings (often measured at maximum zoom, which overstates real-world output at wider zoom settings). Treat calculated values as starting points, and always test with your specific setup. The GN formula assumes point-source light in a dark environment with no modifiers.

Check your flash unit's manual or specifications. GN is usually listed at ISO 100 with the flash head zoomed to its maximum telephoto setting (e.g., 105mm or 200mm). Common GN ranges: built-in camera flashes ~8-14, mid-range speedlites ~24-40, professional speedlites ~50-65, and studio strobes ~60-100+. If you can't find the GN, you can calculate it yourself: set up a controlled test at a known distance with a fixed aperture and ISO, then use GN = Distance × f-number ÷ √(ISO/100).

Standard full f-stops are: f/1.0, f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22, f/32. Each full stop halves or doubles the light reaching the sensor. For flash photography, wider apertures (smaller f-numbers like f/2.8) allow greater flash reach, while narrower apertures (like f/11) reduce effective distance. The relationship is linear: doubling the f-number (e.g., f/4 to f/8) halves the effective flash distance. Use the calculator's aperture datalist to quickly select common values.