Flash Guide Number Calculator: Flash Range and Aperture
A flash guide number (GN) is a single figure that describes how powerful a flash unit is. It encodes the relationship between the flash-to-subject distance and the aperture needed for a correct exposure at ISO 100. The fundamental formula is GN = aperture (f-number) multiplied by distance (in metres or feet). Rearranging gives two practical tools: divide the guide number by the aperture to find the maximum flash range, or multiply the aperture by the distance to find the guide number required for a shot. At ISOs above 100 the sensor gains additional sensitivity, so the effective guide number scales by the square root of the ISO ratio: effective GN = GN_100 x sqrt(ISO / 100). At ISO 400 the effective guide number doubles, meaning you can either shoot from twice the distance or close down the aperture by two stops. This calculator handles both modes: enter a guide number and aperture to find flash range, or enter distance and aperture to find the guide number you need. Switch between metres and feet using the units selector. All ISO adjustments are applied automatically.
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How the guide number formula works
The guide number formula is derived from the inverse square law of light combined with the exposure equation. At ISO 100 and full manual flash power:
GN (ISO 100) = aperture (f-number) x distance
Max distance = GN_100 x sqrt(ISO / 100) / aperture
Required GN (ISO 100) = aperture x distance / sqrt(ISO / 100)
Effective GN at any ISO = GN_100 x sqrt(ISO / 100)
Worked example: find flash distance
Flash GN 50 m at ISO 100, aperture f/5.6, ISO 400:
- ISO gain factor = sqrt(400 / 100) = sqrt(4) = 2.00
- Effective GN at ISO 400 = 50 x 2.00 = 100.00 m
- Max flash distance = 100.00 / 5.6 = 17.86 m
Worked example: find required guide number
Subject at 5 m, aperture f/8, ISO 200:
- ISO gain factor = sqrt(200 / 100) = sqrt(2) = 1.41
- Required GN at ISO 100 = (8 x 5) / 1.41 = 40 / 1.41 = 28.37 m
Common flash guide numbers at ISO 100
| Flash unit | GN (metres, ISO 100) | Notes |
|---|---|---|
| Canon 600EX-RT II | 60 m | At 200 mm zoom head position |
| Canon 430EX III-RT | 43 m | At 105 mm zoom head position |
| Canon 270EX II | 27 m | Compact speedlite |
| Nikon SB-910 | 34 m | At 35 mm zoom position |
| Nikon SB-700 | 28 m | At 35 mm zoom position |
| Sony HVL-F60RM2 | 60 m | At 105 mm zoom position |
| Godox V1 | 60 m | At 200 mm zoom position |
| Built-in camera flash (typical) | 12 m | Varies by camera model |
Guide numbers and zoom head positions vary between manufacturers and model variants. Always consult the official specification sheet for the exact flash unit you are using. The figures above are representative values drawn from manufacturer documentation.
Practical notes for manual flash exposure
The guide number formula assumes the flash is the dominant light source and the subject is in an average reflectance environment. In practice, a very dark or very light subject will require compensation: open up half to one stop for dark subjects, and close down half to one stop for highly reflective or white subjects.
Room size also matters. In a small room with light-coloured walls, reflected light from the walls and ceiling adds to the flash illuminance, meaning the effective guide number is higher than the rated value. In a large dark hall, the effective guide number may be lower than rated because there is little fill from room surfaces.
When using flash at a reduced power fraction (e.g. 1/4 power), multiply the rated guide number by the square root of the power fraction. At 1/4 power: GN_effective = GN_full x sqrt(1/4) = GN_full / 2. At 1/16 power: GN_effective = GN_full / 4.
Flash guide number calculator: frequently asked questions
What is a flash guide number?
A guide number (GN) is a measure of the power output of a photographic flash unit. It encodes the relationship between the flash-to-subject distance and the aperture required for a correct exposure at a given ISO. The defining formula is: GN = aperture (f-number) x distance. Guide numbers are usually quoted in metres at ISO 100. A higher guide number means a more powerful flash that can correctly expose subjects at greater distances or smaller apertures. For example, a flash with GN 60 m at ISO 100 can expose a subject at 10 m using f/6, or at 5 m using f/12.
How does ISO affect flash range?
Guide numbers are specified at ISO 100 by convention. At higher ISO settings the sensor is more sensitive, so the effective guide number increases. The relationship is: effective GN = GN_100 x sqrt(ISO / 100). At ISO 400 the effective GN is twice the ISO 100 value (sqrt(4) = 2), giving twice the range at the same aperture. At ISO 800 the effective GN is 2.83 times the ISO 100 value. This calculator adjusts automatically for the ISO you select.
What is the inverse square law for light?
The inverse square law states that the intensity of light from a point source is inversely proportional to the square of the distance from the source. If you double the distance to your subject, the flash illuminance falls to one quarter, not one half. In practical terms, this means that halving the distance to your subject allows you to close down the aperture by two full stops, or reduce flash power by a factor of four. The guide number formula (GN = aperture x distance) already incorporates the inverse square law: the GN is constant at a given ISO, so as distance increases, aperture must decrease proportionally.
How do I read flash guide number specifications?
Flash manufacturers quote guide numbers in metres (m) or feet (ft) at ISO 100, and sometimes at the maximum zoom head position (e.g. 105 mm coverage). Always check the units and the zoom setting. A Canon 600EX-RT is rated GN 60 m at ISO 100 and 200 mm zoom; at the widest zoom (20 mm) the GN drops considerably. If a guide number is quoted in feet, divide by 3.281 to convert to metres. When comparing flashes, ensure you are comparing like-for-like: the same ISO and zoom head position.
What is the difference between TTL and manual flash?
TTL (Through The Lens) flash metering uses the camera sensor to measure the pre-flash reflected light and automatically sets the flash power for a correct exposure. Manual flash requires you to set the power level yourself, typically as a fraction of full power (1/1, 1/2, 1/4, 1/8, and so on). Manual flash is more consistent and predictable, which is why guide number calculations apply most directly to manual mode: you dial in the power, apply the GN formula to determine distance or aperture, and get a reliable exposure. TTL is convenient for run-and-gun shooting but the exposure can vary between frames.
Sources
- Wikipedia: Guide number.
- B&H Photo: Guide Numbers Demystified.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. General information only.