How to use it
- Choose What to find: the guide number, the distance or the aperture.
- Choose the Unit of distance and guide number and type the ISO of your settings.
- Type the Guide number at ISO 100 from the manual for your zoom, and the Distance to the subject or the Aperture (f-number), whichever the mode needs.
- Read the Result, and check the Guide number at this ISO against the flash output you set.
- Use the table for the distance at other apertures at the same ISO.
How the guide number works
The guide number (GN) rates how much light a flash puts out: the larger the number, the greater the output. It is quoted at ISO 100, and the distance times the f-number gives it. The Nikon SB-800 manual writes the relation with the ISO sensitivity factor: at ISO 400 the factor is 2, so a flash at 4 m and f/8 has a guide number at ISO 100 of 4 × 8 ÷ 2 = 16.
Read the other way, the guide number at the ISO of your settings divided by the distance gives the aperture, and divided by the aperture it gives the distance.
GN at ISO 100 = distance × f-number ÷ ISO factor
f-number = GN at ISO 100 × ISO factor ÷ distance
distance = GN at ISO 100 × ISO factor ÷ f-number
ISO factor = √(ISO ÷ 100)The ISO factor
The manual's table gives the factors ×0.5 at ISO 25, ×1 at ISO 100, ×1.4 at ISO 200, ×2 at ISO 400, ×2.8 at ISO 800 and ×4 at ISO 1600. These are the square roots of ISO ÷ 100, rounded to two places, so this calculator uses the root itself. Each doubling of the ISO multiplies the guide number by about 1.4, and four times the ISO doubles it.
What the calculator does not cover
- The zoom position of the flash head: the guide number changes with it, and the manual prints one figure for each zoom position.
- Accessories such as a diffusion dome or a wide-flash adapter: the manual gives separate guide numbers when one is fitted.
- Mixing units: a guide number in metres gives a distance in metres, and the same guide number is not a distance in feet.
Worked example: guide number 32 at ISO 100, shot at ISO 400 and 4 m
The factor at ISO 400 is √4 = 2, so the guide number at this ISO is 32 × 2 = 64. The aperture for 4 m is 64 ÷ 4 = 16, which is f/16. The Nikon SB-800 manual uses the same example, with a guide number of 32 m at the 28 mm zoom position, and it gives the aperture 16.
The table lists the distance at each aperture with the same guide number at ISO 400: at f/8 the flash lights 8 m.
| Aperture | Distance at ISO 400 (m) |
|---|---|
| f/2.8 | 22.86 |
| f/4 | 16.00 |
| f/5.6 | 11.43 |
| f/8 | 8.00 |
| f/11 | 5.82 |
| f/16 | 4.00 |
Frequently asked questions
What does the guide number mean?
The guide number is a rating of how much light a flash puts out. The larger the number, the more light, and it is quoted at ISO 100 in metres or in feet, as the flash manual states.
Why do I need both the distance and the aperture?
Distance multiplied by the f-number gives the guide number, so any two of the three values give the third. Knowing the guide number and one of the other two is enough to set the flash for your shot.
Why does a higher ISO give a longer flash distance?
A higher ISO makes the sensor more sensitive, and the guide number at that ISO rises with the square root of ISO ÷ 100. At the same aperture the flash then lights a longer distance, and at the same distance it allows a smaller aperture.
Can I use feet instead of metres?
Yes, if the guide number is rated in feet. A flash gives different numbers in feet and in metres, so use the unit printed in the manual and choose the same unit here.
Why does the zoom setting change the guide number?
The manual gives one guide number for each zoom-head position, and the figure is not the same at every zoom. Use the figure for the zoom you shoot at, because the number is only valid at that position.
Sources
Figures and text last checked on Thursday, November 12, 2026.