Flash Guide Number Calculator

Frequently Asked Questions

What units does the GN use?

Guide numbers can be in metres or feet. They are numerically different for the same flash - select the correct unit to match your working measurements.

Why must I enter the rated ISO?

A GN is meaningless without its reference ISO. Most manufacturers rate at ISO 100, but some older units or studio packs use ISO 25 or ISO 200 as the baseline.

Does TTL flash still use the guide number?

TTL (through-the-lens) metering automatically adjusts flash power, so you rarely need the GN for TTL shooting. Guide numbers matter most in manual mode, off-camera, or when using optical slave triggers.

What if I bounce the flash?

Bounce, diffusion, and modifiers change the real output by one to three stops compared to bare direct flash. Use the GN as a starting point and refine with a flash meter or 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 It Works

A flash's guide number (GN) expresses its power as the product of distance and aperture at a reference ISO: GN = distance x f-number. Rearranging solves any one variable given the other two: distance = GN / f-number, aperture = GN / distance, or GN = aperture x distance.

Because doubling ISO doubles sensor sensitivity (one stop), the effective GN scales as: GN_actual = GN_rated x sqrt(target_ISO / rated_ISO). A speedlight rated GN 40 at ISO 100 has an effective GN of 80 at ISO 400.

This calculator solves whichever variable you choose, scales the GN to your shooting ISO, and produces a distance-versus-aperture reference table so you can plan manual flash power quickly in the field.

Use Cases

Guide number calculations are fundamental to manual flash work:

Off-camera strobe setups where TTL metering is disabled or unavailable

Studio packs with manual output controls

Event photography pre-setting power for a known shooting distance

Balancing flash output with ambient by choosing aperture first, then reading the required distance

Comparing two flashes - a unit with double the GN has four times the light output (two stops more power)

Tips

Always confirm whether a manufacturer's GN is quoted in metres or feet - a GN in feet is roughly 3.28 times larger than the metric figure for the same flash.

The headline GN is usually measured at the flash's longest zoom/telephoto position. Wide-angle diffusion panels dramatically reduce the effective GN.

Bouncing flash off a ceiling or using a softbox reduces the effective GN by one to three stops compared to bare direct flash; meter to confirm.

Use the ISO-scaling feature when shooting at high ISO to take advantage of extra flash reach or to stop down for greater depth of field.

The distance/aperture table lets you plan before arrival - pick your desired aperture and read the maximum subject distance directly.

FAQ