What is a flash guide number?

The guide number (GN) describes flash power. It relates aperture and distance: GN = distance (m) × f-number at ISO 100. A GN 58 flash at f/5.6 correctly exposes at 58 ÷ 5.6 = 10.4 metres. Higher GN means more powerful flash. Most speedlights have GN 36-60.

How does ISO affect guide number?

Guide numbers are specified at ISO 100. For other ISOs, multiply the GN by the square root of (ISO/100). At ISO 400, multiply GN by 2. At ISO 1600, multiply by 4. This effectively doubles the usable distance for each 2-stop ISO increase.

Why is my flash underexposing at distance?

Flash follows the inverse square law — doubling the distance requires 4x the power (2 stops). If your subject is beyond the flash range (GN/aperture), increase ISO, open the aperture, or move closer. Also check if you are bouncing the flash, which significantly reduces effective power.

How do I compare flash power between units?

Compare guide numbers at the same ISO and zoom setting. A GN 60 flash is twice as powerful as a GN 42 flash (one stop difference, since 60/42 ≈ 1.4 and one stop = √2 = 1.414). Studio strobes are often rated in watt-seconds (Ws) — roughly, doubling Ws adds one stop.

Flash Guide Number Calculator — GN, Distance & Aperture

Calculate flash guide number, required aperture or maximum distance for correct flash exposure.

Solve ForPick the unknown: aperture, max distance, or the flash's guide number

Guide Number (ISO 100)

Manufacturer GN in metres at ISO 100 (e.g. 58 for a Canon 580EX II at 105mm)

Distance

Flash-to-subject distance

ISO

Shooting ISO — GN scales by √(ISO/100)

How We Calculate This

The guide number formula relates flash power, distance and aperture:

Guide Number Formula

GN = distance (m) × f-number

ISO Adjustment

Adjusted GN = GN₁₀₀ × √(ISO ÷ 100)

Rearranging: aperture = GN ÷ distance, or distance = GN ÷ aperture. Guide numbers assume direct, head-on flash; bouncing or adding a modifier loses 1–3 stops, so reduce GN accordingly with the Light Loss Compensation field.

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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.