What is a flash guide number?

A guide number (GN) is a standardized measure of a flash unit's power output. It encodes the relationship between subject distance and aperture needed for correct exposure at a given ISO. Higher GN = more powerful flash.

What ISO does the guide number assume?

Most manufacturers rate GN at ISO 100. For every doubling of ISO sensitivity (one stop), multiply the GN by √2 (~1.4). For example, a GN 40 at ISO 100 becomes GN 56 at ISO 200.

How do I convert a GN from meters to feet?

Multiply the meter-based GN by 3.2808 to get the feet-based equivalent. For example, GN 40 (meters) equals approximately GN 131 (feet).

Does zoom position affect the guide number?

Yes. Flash units with a zoom head have higher GN at longer focal lengths because the beam is concentrated. Manufacturers typically publish GN at 105mm zoom. Check your flash manual for GN at your actual zoom setting.

Why does distance matter so much?

Flash obeys the inverse-square law: doubling the distance cuts the light to one quarter (2 stops less). The GN formula accounts for this — aperture must open by one stop each time distance doubles.

Flash Guide Number Calculator

Name: Flash Guide Number Calculator
Availability: InStock
Author: Nham Vu

Calculate flash guide number, aperture, or subject distance using the GN = distance × aperture formula.

Calculate

Solve for

Unit system

Guide Number (at ISO 100, meters)

Aperture (f-number)

Subject Distance (meters)

ISO note: Guide numbers are rated at ISO 100. Multiply GN by √(ISO/100) for other ISOs. Example: GN 40 at ISO 400 → GN 40 × 2 = 80.

Result

Enter values to see the result

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Summary

Calculate flash guide number, aperture, or subject distance using the GN = distance × aperture formula.

How it works

Select which value you want to calculate: Guide Number, Aperture, or Distance.
Enter the two known values in the input fields.
Choose your unit system — meters or feet.
The result appears instantly below along with the full formula breakdown.
Use the ISO note to adjust: GN roughly doubles for every 2 stops of ISO increase.

Use cases

Determine the correct aperture for a given flash and subject distance.
Find how far your flash can properly expose a subject at a specific f-stop.
Compare flash power between different flash units using their guide numbers.
Plan studio lighting setups before a shoot.
Understand manual flash exposure when automatic TTL is unavailable.
Convert published guide numbers between meters and feet.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu