About guide numbers
A flash's guide number (GN) is the classic way manufacturers rate manual flash output: Guide Number = f-stop x distance, at a stated reference ISO (almost always ISO 100) and, for zoom-head flashes, a stated zoom setting. Rearranging that formula lets you solve for whichever value you need — the farthest distance a given aperture will properly expose, or the aperture required at a given distance. Guide numbers also scale with sensitivity as the square root of the ISO ratio, so this calculator adjusts your flash's rated guide number to whatever ISO you're actually shooting at before doing the distance/aperture math. Pair this with our exposure triangle calculator if you also need to work out how ambient light and flash exposure interact.
How the calculation works
Guide number (GN) equals f-number × distance. Knowing GN, distance = GN ÷ f-number and f-number = GN ÷ distance. Manufacturers usually publish GN at ISO 100, so when you change ISO the calculator uses GN × √(ISO ÷ 100).
GN and distance are handled in the same unit, either meters or feet. One meter is about 3.28 feet.
Worked examples
- A flash with GN 58 (m, ISO 100) used at f/8 can properly light a subject up to about 7.25 m away.
- Raising the same flash to ISO 400 doubles GN to about 116, so at f/8 it reaches about 14.5 m. Four times the ISO but only twice the GN is the square-root relationship.
- With a GN 60 (m, ISO 100) flash and a subject 5 m away, the aperture needed is f/12. At ISO 400 you need f/24.
- Going from ISO 100 to 200 multiplies GN by about 1.41, so GN 58 becomes about 82.
- A flash rated GN 190 (feet, ISO 100) used at f/5.6 reaches about 33.9 ft.
When to use it
It helps when using flash in manual mode or with an off-camera strobe, to judge at which aperture and distance you get enough light. It is also good for building a reference chart ahead of time for events where distances change often.
Limits and common mistakes
- A zoom-head flash has a different GN at each zoom position. Enter the GN for the zoom position you are actually using.
- Published GN is measured under ideal conditions. Room reflectivity, subject size and diffusers can make real results differ a lot.
- Bouncing off a ceiling or wall, or using a diffuser or softbox, reduces the light reaching your subject. You may need a wider aperture or a closer distance than the result suggests.
- Lowering flash power lowers GN. If you entered the full-power GN, assume a shorter distance at fractional power (1/2, 1/4 and so on).
- Shutter speed does not affect this calculation, but going above your flash sync speed can block part of the frame.
Guides that pair well with this calculator
Guide numbers are manufacturer-published ratings measured under controlled conditions — real-world results vary with room reflectivity, subject size, flash zoom setting, and diffusers or modifiers.
Frequently Asked Questions
Where do I find my flash's guide number?+
It's in the flash's manual or manufacturer spec sheet, usually listed at ISO 100 for a specific zoom head position (often 35mm or 50mm equivalent). If your flash has a zoom head, the guide number changes at each zoom setting — use the value for whatever zoom position you're actually shooting at.
Why does the guide number change with ISO?+
A higher ISO makes the sensor more sensitive to the same amount of light, which is equivalent to boosting the flash's effective reach — but only by the square root of the ISO ratio, not proportionally. Doubling ISO increases the usable guide number (and distance) by about 41%, not 100%.
Does this account for bounce flash or light modifiers?+
No — guide numbers assume direct, undiffused flash in open conditions. Bouncing the flash off a ceiling or wall, or adding a diffuser/softbox, reduces the effective light reaching your subject, so you'll typically need a wider aperture or closer distance than this calculator suggests.