What is a guide number?

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).

How does ISO affect the guide number?

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%.

Why is my flash underexposing at the rated GN?

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.

What is the formula for flash exposure?

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.

PhotoCalcs

Flash Exposure Calculator — Guide Number, Distance & Aperture

Calculate the correct aperture, flash-to-subject distance, or required guide number for flash photography.

Solve For

ISO

Guide Number (ISO 100)

Flash GN in metres at ISO 100

Flash-to-Subject Distance

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

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Last updated: February 2026

All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.