Flash Guide Number Calculator

Two everyday inversions: know your distance, divide GN by it for the aperture (GN 36 flash, subject at 4.5 m → f/8); know your aperture, divide GN by f-number for the working distance (f/5.6 → 6.4 m reach). At full power and ISO 100 in metres, that's the whole system. This tool layers on the modifiers — ISO above 100 multiplies GN by √(ISO/100), each halving of power divides it by √2 — so the answer stays one division.

Because GN contains a distance, and light obeys inverse-square: 4× the sensitivity (ISO 400) tolerates 1/4 the light, which arrives at 2× the distance — √4. So ISO 400 doubles your reach, ISO 800 gives 2.8×, and the formula GN_eff = GN × √(ISO/100) follows. The same root governs power settings downward: 1/4 power halves the guide number. Linear thinking ('ISO 400 = 4× reach') is the classic manual-flash exposure error, usually discovered two stops underexposed.

Read the asterisk: makers quote GN at the tightest zoom-head setting (105 mm or 200 mm), where the reflector concentrates the beam — the same flash at the 35 mm spread setting may guide at barely half the headline. Add bounce or a modifier and the packaged GN is fiction entirely: a ceiling bounce typically costs 2–3 stops (GN effectively ÷ 2–2.8). For real work, establish your own GN once: shoot a subject at 3 m, find the aperture that exposes correctly, multiply — that number is yours.

The formula survives; the inputs change: bounce adds path length (flash→ceiling→subject, not flash→subject) and surface absorption, softboxes eat 1–2 stops, and the inverse-square law then runs from the MODIFIER as the effective source. Strobist practice: measure once per setup (the 3-m calibration shot), derive an effective GN for that rig, then the distance/aperture arithmetic works as cleanly as bare flash. Studio photographers skip GN for incident flash meters — same physics, instrument instead of arithmetic.