Flash Guide Number & Multi-Flash Calculator
Plan direct-flash reach from a rated guide number, ISO, manual power, light loss, and identical flashes. It also works backward from the aperture and distance you need.
Effective combined guide number36
- Recommended aperture at entered distance
- 12
- Nearest one-third-stop aperture
- f/11.31
- Maximum distance at target aperture (meters)
- 4.5
- Exposure headroom at target settings (stops)
- 1.17
- Identical flashes required
- 1
- Required ISO 100 full-power guide number per flash
- 24
Guide number and distance are both treated as meters; do not mix meter-rated guide numbers with feet or the reverse.
The multi-flash result assumes equal distances, overlapping coverage, and the same output from every flash.
This guide-number calculation does not apply to high-speed sync.
How to use this calculator
- Enter the ISO 100 guide number for the flash head zoom or coverage angle you will actually use.
- Choose meters or feet, then use that same unit for both the rating and the flash-to-subject distance.
- Set ISO, per-flash manual power, flash count, and any measured or estimated modifier loss.
- Enter the distance and target aperture to compare the available flash capacity with the planned shot.
How flash guide number is calculated
A guide number links a flash, an aperture, and a distance. For direct flash at ISO 100, the basic relationship is G = N × d: guide number G equals f-number N times flash-to-subject distance d. The distance unit is part of the rating. A guide number of 36 m is not interchangeable with a guide number of 36 ft.
This calculator starts with one flash's ISO 100, full-power rating and adjusts it for the selected setup:
Gsingle = G100 × sqrt(ISO / 100) × sqrt(power) × 2^(-loss / 2)
Power is a fraction, so 1/4 power is 0.25. Guide number follows the square root of light energy: doubling ISO raises it by about 1.41 times, while four times the flash energy doubles it. A one-stop modifier loss divides available light by two and guide number by the square root of two.
When identical flashes light the same subject area from roughly equal distances, their illuminances add. Their combined guide number is therefore Gcombined = Gsingle × sqrt(n), where n is the number of flashes. Two equal flashes provide about one extra stop, not twice the guide number. The calculator divides this combined guide number by distance for the recommended aperture, and by the target f-number for maximum distance.
Planning forward and backward
Exposure headroom is H = 2 × log2(Gcombined / (N × d)). A positive result means the selected flashes have capacity beyond the target; a negative result is a shortfall in stops. For reverse planning, the required count is ceil((N × d / Gsingle)^2). The required guide-number row instead solves for the ISO 100, full-power rating each selected flash would need.
The exact recommended f-number is accompanied by the closest one-third-stop f-number, which is normally easier to set on a camera. The exact value remains useful when comparing settings or a flash-meter reading.
What this calculator leaves out
Guide-number ratings assume direct flash under specified test conditions. Actual output changes with zoom position, battery state, color gels, beam pattern, modifier geometry, and the distance from each flash to the subject. The equal-distance multi-flash rule is not reliable when lights have uneven coverage or different paths to the subject.
This tool excludes ambient light, TTL behavior, subject reflectance, and high-speed sync. High-speed sync turns the flash into a pulsed light source during the shutter travel, so ordinary guide-number arithmetic is not an accurate exposure model. Use a test frame or flash meter when the exposure matters.
Worked example
Suppose one speedlight is rated at guide number 36 m at ISO 100 and full power. At 3 m, its direct-flash aperture is 36 / 3 = f/12. If the target is f/8, its maximum direct-flash distance is 36 / 8 = 4.5 m.
Now use ISO 400, 1/4 power, a two-stop modifier loss, and four identical flashes. The ISO adjustment is 2, the power adjustment is 0.5, and the modifier adjustment is 0.5, so one adjusted flash has 36 × 2 × 0.5 × 0.5 = 18 m. Four equal flashes combine to 18 × sqrt(4) = 36 m. At 3 m the recommendation remains f/12, whose nearest one-third-stop setting is f/11.31; at f/8 the setup has about 1.17 stops of headroom.
Common questions
What does a flash guide number mean?
A flash guide number is a direct-flash exposure rating that connects aperture and flash-to-subject distance at a stated ISO. Divide the guide number by distance to find the f-number, or divide it by the f-number to find distance. The guide number is only meaningful when its distance unit, ISO, and flash-head coverage are known.
Should guide number use feet or meters?
Either works, but the guide number and distance must use the same unit. A rating published in meters must be used with meters, and a rating published in feet must be used with feet. Convert one before calculating rather than mixing them.
How does reducing flash power affect guide number?
Guide number changes with the square root of the power fraction. Half power reduces guide number by about 0.71 times, or one stop in aperture terms; quarter power reduces it to half. This is why a flash can lose reach quickly when reduced for a wider aperture or faster recycle.
Why do two flashes add only about one stop?
Two equally bright flashes double illuminance, which is one stop more light. Because guide number tracks the square root of illuminance, doubling the flashes multiplies guide number by the square root of two, not by two. Four equal flashes double guide number and give two stops more light.
Why does this calculator not apply to high-speed sync?
High-speed sync emits a series of pulses while the shutter slit crosses the sensor, rather than one short ordinary flash burst. Its effective output depends on shutter speed, camera, flash, and coverage. Use the flash maker's high-speed-sync data or a meter reading instead of a normal guide number.