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Flash Guide Number Calculator

Scales a flash's guide number (quoted at ISO 100) to the working ISO with GN × √(ISO/100) and solves the guide-number equation GN = distance × f-number for the required aperture at a distance or the maximum flash range at an aperture.

When to use

You are using a flash in manual mode and need the f-number for a subject distance, or the distance a flash can reach at a chosen aperture and ISO.

Do not use when: The flash is in TTL/automatic mode, is bounced or heavily diffused (the guide number no longer applies), or you need ambient exposure settings (use exposure-value).

Formula

GN_eff = guide_number_m × √(iso / 100); required_aperture_f = GN_eff / distance_m; max_distance_m = GN_eff / aperture_f

Guide numbers follow the inverse-square law for direct, full-power flash in an average room; outdoors at night or in large dark halls the effective GN is 1/2 to 1 stop lower, and a fractional power setting of 1/n reduces GN by √n.

Inputs

Parameter	Type	Unit	Required	Description
`guide_number_m`	number	m	yes	Manufacturer's guide number in metres at ISO 100 and full power (check the zoom setting it was quoted for). Range: > 0, ≤ 1000
`iso`	number		default 100	ISO sensitivity setting (100, 200, 400, ...). Range: ≥ 1, ≤ 10000000
`distance_m`	number	m	no	Optional distance from flash to subject; returns the aperture to use. At least one of distance_m / aperture_f is required. Range: > 0, ≤ 1000
`aperture_f`	number		no	Optional f-number in use; returns the maximum distance the flash can light correctly. Range: ≥ 0.5, ≤ 128

Outputs

Output	Type	Unit	Description
`effective_guide_number_m`	number	m	guide_number_m × √(iso / 100).
`guide_number_ft`	number	ft	Effective guide number in feet (÷ 0.3048).
`iso_gain_stops`	number	stops	log2(iso / 100): stops gained over ISO 100.
`required_aperture_f`	number		Effective GN / distance_m (only when distance_m is given).
`nearest_full_stop_aperture`	string		Closest standard f-stop to the required aperture (only when distance_m is given).
`max_distance_m`	number	m	Effective GN / aperture_f (only when aperture_f is given).
`max_distance_ft`	number	ft	Maximum distance in feet (only when aperture_f is given).

Example

GN 36 flash, ISO 100, subject at 4.5 m: {"guide_number_m":36,"iso":100,"distance_m":4.5} → {"effective_guide_number_m":36,"required_aperture_f":8,"nearest_full_stop_aperture":"f/8","guide_number_ft":118.1}

GN 36 flash at ISO 400, shooting at f/16: {"guide_number_m":36,"iso":400,"aperture_f":16} → {"effective_guide_number_m":72,"iso_gain_stops":2,"max_distance_m":4.5,"max_distance_ft":14.76}

GET https://tttkmbb.com/api/v1/calculate/flash-guide-number?guide_number_m=36&iso=100&distance_m=4.5

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/flash-guide-number (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/flash-guide-number · Markdown: https://tttkmbb.com/photography/flash-guide-number.md · JSON definition: https://tttkmbb.com/photography/flash-guide-number.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="flash-guide-number"
OpenAPI operationId: calculate_flash_exposure
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – Guide number (reference)
Wikipedia – Flash (photography) (reference)

FAQ

Why does ISO scale the guide number by a square root?

The guide number is a product of distance and f-number; ISO 400 admits 4× (2 stops) more light, which equals 2× the distance (inverse-square law) or 2× the f-number, so GN grows by √4 = 2.

Which guide number do I enter for a zoom flash?

The one for the zoom position you use: makers quote the highest figure (e.g. GN 60 at 200 mm), while the wide setting may be GN 25–30. Enter the value for your zoom, at ISO 100 and full power.

Related calculators

Exposure Value Calculator — Ambient-light exposure value for the same settings.
Equivalent Exposure Calculator — Trade aperture against ISO once the flash aperture is known.