How to use the flash guide number calculator
- Choose what to find: the flash distance, the f-number or the guide number.
- Enter the other two quantities, the ISO and the flash power. Choose meters or feet for the distance.
- Read the answer and the guide number that applies at your settings.
- Use the table to see how far the flash reaches at each full-stop aperture from f/1.4 to f/16.
Formula
guide number = flash distance x f-number
guide number at ISO and power = rated guide number x sqrt(ISO / 100) x sqrt(power share)
distance = guide number / f-number
f-number = guide number / distance
A rated guide number is quoted for ISO 100 at full power. Halving the power divides the guide number by the square root of 2. Quadrupling the ISO doubles it.
A guide number of 40
A flash rated at guide number 40 (meters, ISO 100, full power) at f/4 reaches 40 / 4 = 10.00 m, which is 32.8 ft. At half power the guide number becomes 40 x 0.7071 = 28.28 and the reach at f/4 is 7.07 m.
A guide number of 36 at ISO 400 becomes 36 x 2 = 72, so at f/5.6 the flash reaches 12.86 m. For a subject 5 m away at f/4, ISO 200 and a quarter power, the rated guide number needed is 20 / (1.4142 x 0.5) = 28.3.
The idea behind the guide number
Light from a flash spreads out, so the light on the subject falls with the square of the distance. The aperture controls the light that reaches the sensor in the opposite way, and the f-number changes by about 1.4 for each stop. Together these give the simple rule that distance times f-number is a constant for a flash, which is the guide number. Every halving of the power cuts the light in two and therefore shrinks the reach by the square root of two.
Because the rule is easy to rearrange, the tool lets you solve for whichever of the three you need. To see what the aperture does to sharpness, use the depth of field calculator, and for matching exposure at other settings see the exposure equivalent settings calculator.
Where the number misleads
Guide numbers are rated for one zoom setting and an ordinary indoor room. A zoomed flash head concentrates the light and reaches farther, a wide diffuser spreads it, a dark room reflects less, and bounced light travels a longer path. Treat the result as a starting point and check a test frame. The field of view calculator helps match the flash zoom to the lens.
Assumptions and limits
- The guide number you enter is for ISO 100 and full power, in the distance unit you choose.
- The flash light is not modified by a diffuser, bounce or zoom change beyond what the rating states.
- Power steps are full-power fractions from 1/1 to 1/128, and the ISO can be any value from 6 to 409,600.
- Distances are measured from the flash to the subject, and the f-number can be any value from 0.7 to 128.
Frequently asked questions
Why does the guide number depend on ISO?
A higher ISO makes the sensor more sensitive, so less light is needed. Quadrupling the ISO is the same as multiplying the light by four, so the guide number doubles.
Is the guide number in meters or feet?
Both are used, and the rating always says which. A guide number of 131 in feet is about 40 in meters. The page converts for you in the table.
Does it work for studio strobes?
Studio lights are often rated in watt-seconds and used with a meter, but the same relation holds if you know a guide number, which you can find from a test by solving for it here.
How accurate is this?
It is a fair first guess in an ordinary room. Walls, ceiling color, zoom and diffusion can change the result by a stop or more, so confirm with a test shot.