# 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.

- Calculator id: `flash-guide-number` · Category: Photography & Optics (`photography`) · Tool name: `calculate_flash_exposure`
- Canonical page: https://tttkmbb.com/photography/flash-guide-number · This document: https://tttkmbb.com/photography/flash-guide-number.md · JSON definition: https://tttkmbb.com/photography/flash-guide-number.json

## Purpose

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.

**Use when:** 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).

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `guide_number_m` | number | m | required | Manufacturer's guide number in metres at ISO 100 and full power (check the zoom setting it was quoted for). (> 0, max 1000) |
| `iso` | number |  | optional, default 100 | ISO sensitivity setting (100, 200, 400, ...). (min 1, max 10000000) |
| `distance_m` | number | m | optional | Optional distance from flash to subject; returns the aperture to use. At least one of distance_m / aperture_f is required. (> 0, max 1000) |
| `aperture_f` | number |  | optional | Optional f-number in use; returns the maximum distance the flash can light correctly. (min 0.5, max 128) |

## Output

| Field | 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). |

## 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.

## Data Sources

- Wikipedia – Guide number — https://en.wikipedia.org/wiki/Guide_number (reference, retrieved 2026-09-24)
- Wikipedia – Flash (photography) — https://en.wikipedia.org/wiki/Flash_(photography) (reference, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/flash-guide-number?guide_number_m=…`
- `POST https://tttkmbb.com/api/v1/calculate/flash-guide-number` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/flash-guide-number · OpenAPI operationId `calculate_flash_exposure` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "flash-guide-number", "inputs": {…}}`

## Example

- GN 36 flash, ISO 100, subject at 4.5 m: inputs `{"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: inputs `{"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
```

## Limitations

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). 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. All values are computed from the formula above; no measurement or live data is involved.

## 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

- [Exposure Value Calculator](https://tttkmbb.com/photography/exposure-value.md) — Ambient-light exposure value for the same settings.
- [Equivalent Exposure Calculator](https://tttkmbb.com/photography/equivalent-exposure.md) — Trade aperture against ISO once the flash aperture is known.