About the Flash Guide Number Calculator
This flash guide number calculator solves the classic manual-flash equation in any direction. Enter your flashgun’s guide number (as quoted at ISO 100), the ISO, the power setting and the aperture to get the distance at which the flash gives a correct exposure — or enter a distance to find the aperture, or a measured distance and aperture to work out the guide number of an unknown flash.
It is useful when shooting manual flash at events and weddings, setting up off-camera speedlights, comparing flash units before buying, or checking whether a small flash can reach the back of a room. Guide numbers are quoted in metres or feet; pick the unit that matches your flash’s specification sheet, and all distances will be shown in that unit.
Guide numbers assume direct, bare flash on a subject in a typical indoor room. Bounce, umbrellas and softboxes lose light, so add an estimated loss in stops for modifiers. Manufacturers also quote guide numbers at a particular zoom-head setting (often 105 mm or 200 mm), so a wider zoom setting gives a lower real guide number.
How to use the flash guide number calculator
- 1Choose whether to solve for distance, aperture or guide number.
- 2Enter the flash’s guide number at ISO 100 and choose metres or feet to match its spec.
- 3Set the ISO, flash power and any modifier or bounce loss.
- 4Enter the aperture or the distance (or both when solving for guide number).
- 5Read the answer and check the chart to see how far each power level reaches.
Formula and method
A flash’s guide number is the product of the flash-to-subject distance and the f-number that gives a correct exposure at ISO 100 and full power. Because flash light falls off with the square of distance (the inverse-square law), doubling the distance needs two more stops of light, which is exactly what doubling the f-number provides.
Changing ISO or power changes the amount of light the exposure needs or receives linearly, but the guide number scales with distance, so it changes by the square root: ISO 400 gives twice the ISO 100 guide number, and 1/4 power gives half. Modifier loss in stops works the same way — each stop lost divides the guide number by √2. Distance = GN ÷ aperture and aperture = GN ÷ distance follow directly.
- GN₁₀₀
- Rated guide number at ISO 100 and full power (m or ft)
- GN_eff
- Effective guide number at your ISO, power and modifier
- power
- Power fraction (1 = full, 0.5 = 1/2 …)
- loss
- Light lost to bounce or modifiers, in stops
Worked examples
GN 36 m speedlight at ISO 200, f/8
Raising ISO from 100 to 200 multiplies the guide number by √2, from 36 to 50.9. At f/8 the flash correctly exposes a subject 50.9 ÷ 8 = 6.36 m away. A GN of 36 m is the same flash as GN 118 ft.
Which aperture at 10 m with 1/4 power?
Quarter power halves the guide number (√0.25 = 0.5), so a GN 60 flash acts like GN 30. At 10 m the aperture is 30 ÷ 10 = f/3, which you would set as the nearest marked stop, f/3.2.
Measure an unknown flash through an umbrella
A correct exposure at 15 ft and f/5.6 means an effective GN of 84 ft. Dividing out ISO 400 (×2) and one stop of umbrella loss (×0.707) gives a bare-flash guide number of about 59.4 ft at ISO 100, or 18.1 m.
Frequently asked questions
What is a flash guide number?+
A guide number (GN) describes a flash’s power: it equals the subject distance multiplied by the f-number that gives correct exposure at ISO 100. A GN 36 m flash correctly exposes a subject at 9 m at f/4, or 4.5 m at f/8.
How do I convert a guide number from metres to feet?+
Multiply the metric guide number by 3.28 to get feet, or divide a feet guide number by 3.28 to get metres. A GN 60 m flash is about GN 197 ft. Always check which unit a manufacturer uses before comparing flashes.
How does ISO change the guide number?+
The guide number scales with the square root of the ISO ratio. Doubling ISO multiplies it by 1.41, and quadrupling ISO (for example, ISO 100 to 400) doubles it, which doubles the distance the flash can reach at the same aperture.
Why does my flash not reach as far as the guide number says?+
Guide numbers are usually quoted at the longest zoom-head setting and assume a room with reflective walls. A wider zoom setting, an outdoor location with no bounce, a diffuser or a bounce card all reduce the effective guide number, sometimes by two stops or more.
Does shutter speed affect flash exposure?+
Not for the flash itself — the flash burst is far shorter than normal shutter speeds, so aperture, ISO, power and distance control flash exposure. Shutter speed controls the ambient light in the frame, up to the camera’s flash sync speed.