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Flash Guide Number Calculator

Determine correct aperture or flash-to-subject distance from guide number and ISO for manual flash photography.

Calculator

GN = Distance × Aperture   |   GNadj = GN100 × √(ISO / 100)

Select what you want to calculate, then enter your values

Common: 24, 36, 43, 58, 60
f/

How It Works

Guide Number Formula

The guide number (GN) is a measure of flash power. It relates aperture and distance:

  • GN = Distance × f-stop
  • Distance = GN / f-stop
  • f-stop = GN / Distance

Guide numbers are typically specified at ISO 100. To adjust for a different ISO:

GNadj = GN100 × √(ISO / 100)

Flash Power Levels

Reducing flash power lowers the effective guide number. Each halving of power reduces the GN by a factor of √2 (about 1.41):

  • 1/1: Full GN
  • 1/2: GN × 0.71
  • 1/4: GN × 0.50
  • 1/8: GN × 0.35
  • 1/16: GN × 0.25

This is because light intensity follows the inverse square law — doubling distance reduces light by 4×.

Examples

  • Portrait: GN 36 (m), f/4, ISO 100 → Max distance = 9 m
  • Event: GN 36 (m), 5 m away, ISO 400 → Aperture ~ f/14.4
  • Macro at 1/16 power: GN 36, 1/16 → Effective GN = 9 → at f/8, distance = 1.1 m

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