Flash Guide Number Calculator

Frequently Asked Questions

What is a flash guide number?

A measure of flash power: GN = distance × f-number at a reference ISO. Solve any one from the other two - e.g., aperture = GN ÷ distance - to get correct manual-flash exposure.

How does ISO change the guide number?

GN scales with the square root of ISO: doubling ISO multiplies GN by √2 (~1.41). The calculator adjusts GN between ISO settings for you.

Why are GNs in meters and feet different?

A GN of 38 m equals ~125 ft for the same flash - always note the unit and reference ISO (usually 100) when comparing flashes.

Does the guide number hold with zoom and modifiers?

GN assumes the bare flash at a given zoom head. Wider zoom, diffusers, softboxes, and bounce reduce effective GN, so treat the result as a starting point and chimp the histogram.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

A flash's guide number (GN) expresses its power as the product of distance and aperture at a reference ISO: GN = distance × f−number. Rearranging solves any one variable: distance = GN ÷ f−number, aperture = GN ÷ distance, or GN = aperture × distance. Because doubling ISO doubles sensitivity (one stop), the GN scales as GN₂ = GN₁ × √(ISO₂ ÷ ISO₁). Pick what to solve, the GN's rated ISO, your shooting ISO, and metres or feet.

Worked Example

A speedlight rated GN 40 (m, ISO 100) shooting at f/5.6 reaches 40 ÷ 5.6 ≈ 7.1 m for correct exposure. Raise the camera to ISO 400 (two stops) and the effective GN becomes 40 × √(400÷100) = 40 × 2 = 80, so the same f/5.6 now reaches 80 ÷ 5.6 ≈ 14.3 m. Conversely, to light a subject at 5 m you would need aperture = 40 ÷ 5 = f/8 at ISO 100.

Real-World Use

Guide numbers are the foundation of manual flash before TTL metering existed and are still essential for off−camera strobes, studio packs, and any setup where you want repeatable, predictable output. Use the distance/aperture table to pre−set power for an event, balance flash with ambient by choosing aperture first, or quickly compare two flashes - a unit with double the GN has four times the light output (two stops).

Common Mistakes

Mixing units. A GN in feet is about 3.28× the metre figure for the same flash - always note which the manufacturer quotes.

Trusting the headline GN. It is usually measured at the longest zoom/tele setting; wide zoom heads have a much lower effective GN.

Forgetting ISO. A GN is meaningless without its reference ISO; this tool requires both.

Limitations & Related Tools

The inverse−square model assumes a bare, direct flash in open space. Bouncing, diffusion, modifiers, zoom head position, and reflective surroundings all change real output, often by one to three stops. Manufacturer GNs are frequently optimistic. Use this for a reliable starting point, then confirm with a flash meter or the histogram. Pair with an inverse−square−law or exposure calculator when balancing multiple lights; treat the table as a planning aid rather than an exact exposure.

Flash Guide Number Calculator

Frequently Asked Questions

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