Flash Guide Number Calculator

Calculate the relationship between a flash's guide number (GN), the f-number and the subject distance. The calculator also corrects for the ISO you shoot at.

What to find

Guide number (GN) at the reference ISO and full power

The number differs with the unit of distance (for the same flash, the GN in feet is about 3.28 times the GN in meters). Enter the GN together with its unit.

Reference ISO of the guide number

ISO

The starting value of 100 is used because the example in the reference is for ISO 100. It is not a value fixed by a standard. Enter the condition given in your flash's manual.

ISO you shoot at

ISO

Distance to the subject

f-number —

Details
Distance (m)	—
Distance (ft)	—
GN (at the ISO you shoot at, in m)	—
GN (at the ISO you shoot at, in ft)	—
GN (at the reference ISO, in m)	—
ISO correction factor	—

A guide number comes with conditions: full power, a reference ISO, and the zoom (coverage) setting. The same flash has a different GN at a different power setting or zoom position, and this calculator does not work out that change (enter the GN for the conditions you use). Reflections from walls and ceilings, ambient light and filters make a real shot differ from the calculation. A guide number is a guide for lighting, not a guarantee of exposure.

Formulas

Guide number (GN) = f-number × distance (the distance is in the same unit as the GN)
f-number = GN ÷ distance, and distance = GN ÷ f-number
Correction for ISO: the GN changes with the square root of the ISO ratio. GN (at the ISO you shoot at) = GN (at the reference ISO) × √(ISO you shoot at ÷ reference ISO). With a reference of ISO 100, ISO 200 gives about 1.414 times the GN (about 41% more) and ISO 400 gives exactly 2 times.

The square root is approximated to 40 decimal places with integer arithmetic. When the ISO ratio is a perfect square (4, 9, 1/4 and so on) the result is exact; otherwise it is labeled a rounded value.

Examples

Conditions	Result
GN 28 m, f/2.8	Distance 10 m
GN 39 m, distance 9.75 m	f/4
GN 28 m (reference ISO 100), shooting at ISO 400, distance 10 m	GN becomes 56 m, giving f/5.6

Meters and feet

Because GN is "f-number × distance", the number differs depending on whether the distance is measured in meters or feet. In the example in the reference, one flash has a GN of 37 in meters and 120 in feet (the feet value is the meters value times 3.28, rounded, so it is not an exact conversion). This calculator converts exactly with 1 ft = 0.3048 m (a definition). Enter the GN together with its unit.

Notes

A guide number is a number with conditions. Full power, the reference ISO and the coverage angle (zoom position) go with it. A manufacturer's stated value is for those conditions.
Changes of the GN with the power setting (1/2, 1/4 and so on) or the zoom position are not calculated. Check the GN for your conditions in the manual and enter it.
Reflections from walls and ceilings, ambient light and filters make a real shot differ from the calculation. A guide number is a guide for lighting, not a guarantee of exposure.
TTL, high-speed sync and combining several flashes are not covered.
This page is based on an explanatory reference (Wikipedia, "Guide number"), confirmed when the Japanese version was made. The definition in a formal standard (ISO or JIS) was not checked.

Frequently asked questions

What is a guide number?

A number that expresses the strength of a flash's light: the product of the f-number and the distance. A flash with a GN of 28 m used at f/2.8 reaches a subject about 10 m away.

Why does the GN go up when I raise the ISO?

A higher ISO needs less light for the same brightness. When the ISO is multiplied by 4 (100 to 400), the GN doubles (the square root of the ISO ratio).

Which reference ISO should I enter?

The ISO given in your flash's manual for its GN. It is often ISO 100, but that is not fixed by a standard.