Flash Guide Number Calculator

Guide number is metres times f-number at ISO 100 for a direct flash in a room that is not a black hole. Distance = GN / aperture, after scaling GN for ISO. TTL will still lie in a white dress against a dark hall — this is the manual starting distance, then you take a test shot.

Inputs

Units

Results

Subject distance: 5.38m
Guide number at this ISO: 43.0m

Recommended gear

Speedlight flash
Look up the GN in metres at ISO 100 and the zoom you will actually use, not the marketing 200 mm figure.
Typical range: $60–$200
Handheld light meter
A flash meter beats a catalogue GN once the light is off-camera or through a modifier.
Typical range: $150–$400

Calculation method

ISO 100 guide number is the product of distance in metres and f-number for a correct direct-flash exposure. At another ISO, GN scales with the square root of ISO/100 — two stops of ISO (4×) doubles GN. Subject distance is that ISO-scaled GN divided by the taking aperture. Halving distance is two stops more flash, same as inverse square.

Exact formula

GN_ISO = GN100 × √(ISO/100). distance = GN_ISO / N. Limits: Room bounce, zoom head, modifiers, TTL and inverse-square in open space all move the real number; confirm with a test shot.

Fill vs key

For fill, you want the flash below ambient, not at GN distance. Start one to two stops under the calculated full-power distance (closer, or lower power).

High-speed sync

HSS dumps GN. The catalogue number is for sync at or below x-sync. Above that, treat this distance as wildly optimistic.

Frequently asked questions

Feet or metres?

This site uses metres. A GN of 43 m is about 141 ft. Using a feet GN in a metres calculator overstates distance by more than 3× and you will underexpose badly.

Zoom head?

GN is quoted at a zoom setting. At 24 mm the same flash is weaker than at 105 mm. Use the GN for the zoom you set, or meter.

Off-camera through an umbrella?

Throw the catalogue GN away. A shoot-through umbrella is often 1.5–2 stops down. Meter or chimp; this formula is for bare-direct.

Disclaimer — no professional advice, no liability

Manufacturer GNs are often optimistic and assume a bounce-friendly room. Softboxes, high ceilings and outdoors at night will need more power or a closer light. Never treat this as a studio plot.

These calculators, pages and any related content are provided “as is” and “as available”, solely for general educational and informational purposes. They are simplified mathematical estimates, not a light-meter reading, a studio plot or a shooting prescription, and they do not constitute professional photographic, cinematographic or optical advice.

Results can be wrong, incomplete or inapplicable to your camera, lens, sensor, filter, lighting or use case. You must independently verify every figure, assume all risk, and confirm settings on a test shot before a job that can miss the image, damage gear or injure someone.

To the fullest extent permitted by applicable law, the site operator, authors, contributors and affiliates disclaim all warranties, express or implied, and accept no liability for any direct, indirect, incidental, special, consequential or punitive damages, or for property damage, missed shots, injury, death, business interruption or financial loss, arising out of or related to the use of, or reliance on, these tools or their outputs.

Some jurisdictions do not allow certain warranty exclusions or liability limitations, including for personal injury. In those places, our responsibility is limited to the maximum extent permitted by law. If you do not agree, do not use this site.

Flash Guide Number Calculator

Inputs

Results

Recommended gear

Calculation method

Exact formula

Fill vs key

High-speed sync

Frequently asked questions

More in this topic

Exposure Shift Calculator

Exposure Value Calculator

ND Filter Exposure Calculator

Sunny 16 Calculator