Guide number (GN) is the standard rating of flash output, defined as the product of subject distance and f-number that gives a correct exposure at a stated ISO, usually ISO 100. In flash photography, it allows photographers to compare speedlights and strobes and to calculate manual flash exposure without a light meter.
The relationship is simple: GN = distance × f-number. A flash with a guide number of 40 meters at ISO 100 correctly exposes a subject 10 meters away at f/4, or 5 meters away at f/8. Because the rating depends on units, ISO and zoom head position, a guide number is meaningful only when these conditions are stated alongside it.
How the guide number formula works
The formula follows from the inverse square law. Light from a small source spreads over an area that grows with the square of distance, so doubling the distance delivers one quarter of the light, a loss of two stops. Opening the aperture by two stops, from f/8 to f/4, compensates exactly. Distance and f-number therefore change in the same proportion, and their product stays constant for a given flash output.
The three rearrangements of the formula cover all practical questions in manual flash exposure. The table shows each form and when it is used.
| Unknown value | Formula | Typical question |
|---|---|---|
| Aperture | f-number = GN ÷ distance | Which aperture for a subject 5 m away |
| Distance | Distance = GN ÷ f-number | How far can the flash reach at f/8 |
| Guide number | GN = distance × f-number | How powerful is this flash really |
Guide numbers are quoted either in meters or in feet. A rating in feet is about 3.3 times larger than the same rating in meters, so a flash advertised as GN 197 feet is the same unit as GN 60 meters. Comparing manufacturer figures without checking the unit is one of the most common sources of confusion.
Factors that change the effective guide number
The published figure is measured under ideal conditions: direct flash, a specified zoom position, ISO 100 and usually an indoor room where walls contribute some reflected light. Real working conditions often reduce the usable output.
| Factor | Effect on guide number | Reason |
|---|---|---|
| ISO doubled | Multiplied by about 1.4 | One stop more sensitivity equals √2 more distance |
| Power halved | Divided by about 1.4 | One stop less light |
| Zoom head wider | Decreases | Light spreads over a larger angle |
| Zoom head longer | Increases | Light is concentrated into a narrower beam |
| Bounce or modifier | Drops by one to three stops or more | Light travels farther and is absorbed |
| Outdoors at night | Lower than the indoor rating | No reflecting walls or ceiling |
| High-speed sync | Drops sharply | Flash pulses continuously across the shutter travel |
The zoom head effect is especially important for speedlights. Many manufacturers quote the guide number at the longest zoom setting, often 105 mm or 200 mm, which can be one stop or more higher than the output at a 24 mm or 35 mm coverage that photographers actually use.
Guide numbers of typical flash units
Guide numbers allow a rough comparison across flash types, although studio strobes are usually rated in watt-seconds instead. The ranges below are typical at ISO 100 in meters and vary between models and zoom positions.
| Flash type | Typical GN at ISO 100 | Typical use |
|---|---|---|
| Built-in pop-up flash | About 10-13 m | Fill light at close range |
| Compact speedlight | About 20-36 m | Travel, light fill and bounce |
| Full-size speedlight | About 36-60 m | Events, weddings, off-camera use |
| Battery-powered strobe | About 60-80 m and higher | Location portraits, overpowering sun |
Because the guide number scales with the square root of energy, a flash with twice the guide number delivers four times the light, a difference of two stops. A unit rated at GN 60 is therefore about one stop more powerful than one rated at GN 42.
Guide number compared with TTL and metering
Before automatic flash control, the guide number was the primary exposure method. Today TTL flash measures a pre-flash through the lens and sets output automatically, while handheld flash meters measure the actual light at the subject. The guide number remains useful as a planning tool and power reference.
TTL adapts to changing distances but can be fooled by very dark or very bright subjects. Manual flash based on the guide number gives consistent, repeatable results as long as the distance stays fixed, which is why event photographers often prefer it for group shots and interiors where the subject distance is known.
The guide number also helps when choosing equipment. To overpower bright sunlight with a modifier at a few meters, a photographer can estimate the required aperture, subtract the expected light loss of the softbox and see quickly whether a speedlight is sufficient or whether a larger battery strobe is needed. Neither TTL nor a meter can answer this question before the equipment is purchased or packed.
Common mistakes when using guide numbers
Most errors arise from comparing ratings measured under different conditions or from applying the formula to situations it does not describe.
- Ignoring the zoom position in specifications overstates the usable power at wide-angle coverage.
- Mixing meters and feet makes one flash appear more than three times stronger than another.
- Applying the formula to bounced light fails, because the light path and absorption are unknown.
- Measuring distance to the camera instead of the flash gives wrong results with off-camera setups.
- Assuming linear scaling with ISO is incorrect; doubling ISO multiplies the guide number by √2, not by two.
In everyday language
A guide number tells you how strong a flash is. The bigger the number, the farther the flash can reach and still light the subject properly.
It also works like a small calculator. Divide the guide number by the distance to the subject, and you get the aperture to use. A flash with a guide number of 40 and a person 10 meters away means f/4. One simple number connects power, distance and aperture.
Seen in the real world
A wedding photographer lights a large group photo in a dark reception hall. Her speedlight has a guide number of 60 meters at ISO 100 and the 105 mm zoom setting, but she needs the wider 35 mm coverage, where the manual lists about GN 38.
The group stands 6 meters from the flash, and she wants f/5.6 for enough depth of field. At ISO 100 the flash supports about f/6.3, so full power is enough. She sets ISO 200 to gain one stop of reserve, which raises the effective guide number to about 54 and lets her use half power for faster recycling. Every frame of the sequence is exposed consistently.