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Flash guide-number distance
Estimate nominal direct-flash distance from an ISO-100 guide number, aperture and ISO.
Guide number at ISO 100 (m): 0.1–200; Aperture (f-number): 0.7–64; ISO sensitivity: 25–25600
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Flash guide-number distance
Guide number at ISO 100 (m): 36 Aperture (f-number): 4 ISO sensitivity: 400
- Nominal distance (m)
- 18.000000
- Nominal distance (ft)
- 59.055118
Distance in metres = GN100 × √(ISO / 100) / f-number; feet = metres / 0.3048. Enter a metre-based ISO-100 guide number 0.1–200, f-number 0.7–64 and ISO 25–25600, allowing fractions. Use the guide number for the selected flash power and head zoom; do not enter a feet-based value or an already ISO-adjusted guide number. This nominal model assumes direct flash at normal synchronization with the stated beam coverage. It is not a measured exposure or a guaranteed maximum usable range. Bounce, modifiers, high-speed sync, TTL behavior, subject reflectance, ambient light, lens transmission and equipment tolerances can change exposure. It does not calculate recycle time, flash energy or safe operating limits. Check the flash manual and make a test exposure. Display uses six decimals without intermediate rounding.
Methodology
How it works
Distance in metres = GN100 × √(ISO / 100) / f-number; feet = metres / 0.3048. Enter a metre-based ISO-100 guide number 0.1–200, f-number 0.7–64 and ISO 25–25600, allowing fractions. Use the guide number for the selected flash power and head zoom; do not enter a feet-based value or an already ISO-adjusted guide number. This nominal model assumes direct flash at normal synchronization with the stated beam coverage. It is not a measured exposure or a guaranteed maximum usable range. Bounce, modifiers, high-speed sync, TTL behavior, subject reflectance, ambient light, lens transmission and equipment tolerances can change exposure. It does not calculate recycle time, flash energy or safe operating limits. Check the flash manual and make a test exposure. Display uses six decimals without intermediate rounding.
Method and assumptions documented by Techrider.live
2026-11-01
Frequently asked questions
Can I use this for bounce flash or high-speed sync?
No. Those change effective output and light path. Use the equipment’s specific guidance and a meter or test image rather than treating this direct-flash estimate as measured exposure.