PhotoCalcs
Flash Exposure Calculator — Guide Number, Distance & Aperture
Calculate the correct aperture, flash-to-subject distance, or required guide number for flash photography.
How We Calculate This
Flash exposure is governed by the guide number equation:
GN = Distance × f-number (at ISO 100, in metres)
This can be rearranged to solve for any variable:
- Aperture: f = GN / distance
- Distance: d = GN / f-number
- Guide Number: GN = f × d
For ISO other than 100, the effective guide number is adjusted: GN_adj = GN × √(ISO / 100)
Frequently Asked Questions
A guide number (GN) is a measure of flash power, expressed in metres (or feet). It relates flash power to aperture and distance: GN = distance × f-number, at ISO 100. A higher GN means a more powerful flash. A typical speedlight has a GN of 36-60 (metres, ISO 100).
Increasing ISO effectively increases the guide number. The adjusted GN = base GN × sqrt(ISO/100). Doubling the ISO (one stop) increases the effective GN by a factor of 1.41 (sqrt(2)), which extends your flash range by about 40%.
Guide numbers are typically measured in ideal conditions (bare flash, white room). Modifiers like softboxes, umbrellas, and bounce techniques reduce effective output by 1-3 stops. Also, the GN rating may be at telephoto zoom, not wide angle.
The fundamental formula is GN = distance × f-number (at ISO 100). Rearranged: f-number = GN / distance, or distance = GN / f-number. Adjust the GN for ISO before applying the formula.
Related Calculators
EV Calculator
Calculate Exposure Value from your camera settings or convert between EV, lux and foot-candles. Ideal for matching light meter readings. Free calculator.
Sunny 16 Calculator
Estimate correct exposure settings using the Sunny 16 rule for outdoor photography. Select lighting conditions for instant aperture and shutter recommendations.
Exposure Triangle Calculator
Balance aperture, shutter speed and ISO to achieve correct exposure. Adjust one setting and see how the others compensate. Free online calculator.
Zone System Calculator
Map scene luminance values to Ansel Adams Zone System zones from 0 to X. Visualise tonal placement for precise black-and-white exposure control.
Long Exposure Calculator
Calculate adjusted shutter speed when using ND filters. Supports stacked filters, reciprocity correction for film and timer countdown display. Free online calculator.
Was this calculator helpful?
Last updated: February 2026
All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.