Flash Guide Number Calculator
Calculate flash power, aperture, and distance
Flash Exposure Calculator
Common Flash Guide Numbers
| Flash Type | GN (m) | GN (ft) |
|---|---|---|
| Built-in (crop) | 12 | 39 |
| Built-in (FF) | 15 | 49 |
| Speedlight (mid) | 36 | 118 |
| Speedlight (pro) | 58 | 190 |
| Studio Strobe | 60-80 | 200-260 |
Flash Guide Number — Field Notes
GN units: meters vs. feet confusion
Manufacturers often quote guide numbers in feet in US marketing materials and meters in European specs — a GN 36 in meters is roughly GN 118 in feet. Always confirm the unit before plugging numbers into the formula. When a spec sheet doesn't specify, the higher number is almost always feet. Using the wrong unit will leave you two-and-a-half stops off, meaning badly underexposed subjects even at full power.
Inverse-square law shrinks range fast
Moving a flash from 2 meters to 4 meters doesn't halve the light — it quarters it (two stops). This is brutal for group shots where people stand at different depths: a subject 3 meters away gets four times as much light as one at 6 meters. The fix is to position the flash much farther back and compensate with power, so the depth variation becomes a smaller fraction of total distance.
TTL diverges from GN math when bouncing
Guide-number calculations assume direct, unobstructed flash-to-subject distance. Bouncing off a white ceiling adds 1.5-2 stops of light loss and changes the effective distance dramatically — your GN calculation becomes nearly useless. TTL handles bounce reasonably well because it actually measures return light. Use manual flash with direct flash and GN math; use TTL when bouncing, feathering, or working with modifiers where the geometry changes constantly.
Frequently Asked Questions
How does flash guide number work to calculate exposure?
Guide number equals the subject distance in meters multiplied by the f-number. Rearrange the formula to find what you need: aperture equals GN divided by distance, or distance equals GN divided by aperture. A flash with GN 36 at ISO 100 placed 3 meters from a subject needs f/12. Raise ISO to 400 and the effective GN doubles to roughly 72.
How does ISO change the effective guide number of my flash?
The effective guide number scales with the square root of the ISO ratio relative to ISO 100. At ISO 400, multiply the base GN by the square root of 4, which is 2, doubling the effective GN. This means the same flash reaches twice the distance or allows two stops smaller aperture at ISO 400 compared to ISO 100.
What guide number do I need to light a subject 5 meters away at f/5.6?
You need a guide number of at least 28 (5 meters times f/5.6) at your working ISO. Mid-range speedlights typically list GN 36-42 at ISO 100, which comfortably covers this distance. If shooting at ISO 400, a GN 20 flash becomes effectively GN 40, also sufficient.
Why does my flash consistently underexpose even when I follow the guide number formula?
The most common cause is that manufacturers measure guide numbers at a high zoom position such as 105mm, where light is tightest. At the default 35mm or 50mm zoom head position the actual power is 20-40% lower than the advertised GN. Also, light-colored bounce surfaces are assumed; in dark rooms or outdoors with no reflectors the guide number is effectively reduced further.
Can I use guide numbers to mix two flashes for balanced light?
Yes. Place each flash so its guide number divided by the subject distance equals the same aperture value. If one flash has GN 58 at 4 meters (f/14.5) and a second fill light has GN 36, place it at about 2.5 meters to also produce approximately f/14.5. This gives equal exposure from both lights before you add any intentional lighting ratio.