Calculating your aperture (f-number) is straightforward once you know the Flash Guide Number formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Flash Guide Number Calculator.
What is Flash Guide Number?
The Flash Guide Number calculation tells you your aperture (f-number) from a few simple inputs. The figure you are solving for here is the aperture (f-number).
The Flash Guide Number formula
The core formula is:
Aperture (f-number) = Guide number (GN) ÷ Distance to subjectHere is what each input means:
- Guide number (GN) — a number. Example: 40.
- Distance to subject — a value measured in m. Example: 5 m.
How to calculate it step by step
- Write down the guide number (gn) (for example, 40).
- Write down the distance to subject (for example, 5 m).
- Apply the formula above to get your aperture (f-number).
- Double-check the result with the Flash Guide Number Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Guide number (GN) | 40 |
| Distance to subject | 5 m |
| Aperture (f-number) | 8.0 |
With guide number (gn) of 40 and distance to subject of 5 m, the aperture (f-number) works out to 8.0.
Example 2
With guide number (gn) of 80 and distance to subject of 5 m, the aperture (f-number) works out to 16.0.
| Result | Value |
|---|---|
| Aperture (f-number) | 16.0 |
Example 3
With guide number (gn) of 20 and distance to subject of 5 m, the aperture (f-number) works out to 4.0.
| Result | Value |
|---|---|
| Aperture (f-number) | 4.0 |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
Prefer not to do the maths by hand? — the Flash Guide Number Calculator does it instantly, for free, with the formula and a worked example built in.
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