Flash Guide Number Calculator
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Solve flash exposure with GN = distance × f-number — find GN, distance, or aperture.
Flash exposure
Pick which value to solve for — the other two are used as inputs.
Guide number (GN)
40
10 m at f/4
- Formula
- GN = distance × f-number
Related calculators
Flash guide number
A flash's guide number (GN) rates its power for manual exposure calculations: GN = distance × f-number, using a fixed ISO (usually ISO 100) and often a specific unit of distance (meters or feet — check your flash's manual). A higher GN means a more powerful flash that can properly expose subjects farther away at a given aperture.
Because the formula has three variables, you can solve for whichever one you don't know: divide GN by the f-number to find the maximum flash distance, or divide GN by the distance to find the aperture needed to correctly expose a subject at that distance.
Example: a flash with a guide number of 40 (meters, ISO 100) can properly expose a subject 10 meters away at f/4 (40 ÷ 10 = 4), or a subject 5 meters away at f/8 (40 ÷ 5 = 8) — the same flash power, different apertures for different distances.
Guide number = distance × f-number at ISO 100 — higher GN means more flash power at a given aperture/distance.
Bounce flash off ceilings increases effective distance and softens light — GN math assumes direct flash unless you compensate manually.
Double ISO adds one stop — same flash reaches farther at wider effective aperture equivalent.
Zoom-head flashes change published GN when the reflector narrows — manufacturer GN at 105mm zoom exceeds wide-angle GN at 24mm for the same unit. Match zoom setting to your lens field before trusting distance math. Zoom-head GN changes with reflector position — match flash zoom to lens FOV before comparing manufacturer guide numbers across brands.
High-speed sync (HSS) disables GN math entirely because power spreads across the shutter curtain — this tool assumes standard x-sync direct flash, not HSS outdoor fill at 1/2000 s. High-speed sync invalidates GN math — this assumes x-sync direct flash, not outdoor fill at 1/2000 s shutter speeds. Meter a flash test frame when client proofs depend on the guide-number distance this page predicts.
Default find-GN mode with f/4 at 10 m implies GN 40 (distance × f-number). To expose a subject at f/8 instead at same 10 m, you need GN 80 or +2 stops flash power / ISO. GN 40 at f/4 and 10 m — at f/8 you need GN 80 or +2 stops power/ISO for the same distance on manual flash.
Common questions
Is guide number the same for every ISO?
No — guide numbers are usually published at ISO 100. Raising ISO effectively increases usable flash range or allows a smaller aperture, since higher ISO makes the sensor more sensitive to the same amount of light.
Do I need to know the distance in meters or feet?
Guide numbers are given in either meters or feet depending on the manufacturer — check your flash's specifications and keep your distance units consistent with the guide number's units.
Are guide numbers comparable across brands?
Only at same ISO and zoom head setting — manufacturers measure at different defaults.