⚡ Flash Guide Number Calculator

Enter your flash's guide number (GN) and either the subject distance or your aperture to calculate the other value, using the formula GN = Distance × f-number.

Guide Number (GN, meters @ ISO 100)

Solve For

Known Value

One product, rearranged two ways

A guide number is simply the distance-times-aperture product a flash can deliver at full power, so the dropdown just picks which side to divide out:

GN       = distance × f-number
distance = GN / f-number
f-number = GN / distance

The defaults - GN 40, solving for distance, f/8 - give 40 / 8 = 5.0 m. Switch to solving for aperture and put 4 in the known box and you get f/10.0. Guide number already folds in the inverse-square falloff: doubling the distance halves the usable f-number, two stops of light.

ISO, units and power fractions

The number is quoted in metres at ISO 100, and the tool assumes that too - it never scales for sensitivity. Light gain goes with the square root, so multiply the GN yourself before typing it:

ISO: GN × √(ISO / 100). ISO 400 doubles a GN of 40 to 80; ISO 800 takes it to 113.
Power: halving output costs one stop, so ½ power is GN × 0.71, ¼ is GN × 0.5, and 1/8 is GN × 0.35.
Feet: a GN quoted in feet is 3.28× the metric one. Type it and the arithmetic still works, but the answer is in feet while the label still reads m.

Manufacturers quote GN at the flash head's longest zoom setting, which is flattering. At a 35 mm head expect roughly two thirds of it.

Frequently asked questions

What does a guide number of 40 mean?

That the flash correctly lights a subject 5 m away at f/8, 4 m away at f/10, or 7.1 m away at f/5.6 - all at ISO 100 and full power. Any pair whose product is 40 works.

How do I adjust the guide number for a higher ISO?

Multiply by the square root of the ISO ratio. At ISO 800 that is √8 = 2.83, so a GN 40 flash behaves like GN 113 and reaches 14.1 m at f/8 instead of 5 m.

Why does my flash underexpose at the calculated distance?

Bouncing off a ceiling costs two to three stops, which divides the effective GN by 2 to 2.8, and a diffuser dome costs about another stop. Modifiers are not modelled here - derate the GN before you enter it.