Flash guide number: converting a feet-rated GN to meters for a metric manual
I bought a used flash unit with its guide number printed only in feet, but every exposure calculation guide I found online used the metric guide number — converting it correctly meant I could actually trust my manual fla
We write the way we argue on the bench: exact factor, one sharp trap, one worked example you can re-check.
I bought a used flash unit with its guide number printed only in feet, but every exposure calculation guide I found online used the metric guide number — converting it correctly meant I could actually trust my manual flash exposure math.
Here is the conversion I wish I had taped to the fridge before that went sideways.
The factor that settles arguments
This is the relationship our converters use. Screenshot it if you have to — just stop re-deriving it from a half-remembered blog post.
1 ft = 0.3048 m
Write that line once at the top of your notes. Every later arithmetic step should point back to it instead of inventing a friendlier number mid-stream.
The mistake that keeps showing up
Mixing a feet-based guide number into a formula expecting a meters-based guide number (or the reverse) without converting — guide number calculations are very sensitive to using consistent units throughout.
I still write the unit next to every intermediate value. It looks pedantic until it catches the one swap that would have shipped wrong.
- Write down the known value in ft (guide number).
- Confirm the target unit is really m, not a similar-looking one.
- Apply the factor: 1 ft = 0.3048 m.
- Round only at the end, to the precision your tool or spec supports.
Worked example
A flash's guide number is rated GN160 (feet) at ISO 100, and you need the metric guide number for a formula.
160 ft (guide number) × 0.3048 = 48.768 m
That's 48.768 m — round only to the precision your tool or spec actually needs.
If you want a second check, invert the conversion and see whether you recover the original quantity within normal rounding.
Quick reference table
| ft (guide number) | Calculation | m |
|---|---|---|
| 1 | 1 × 0.3048 | 0.3048 |
| 2 | 2 × 0.3048 | 0.6096 |
| 5 | 5 × 0.3048 | 1.524 |
| 10 | 10 × 0.3048 | 3.048 |
Use the table for speed once you trust the factor. Do not use it as a substitute for understanding which definition of the unit you are on.
Why the exact number matters
Guide number calculations directly determine manual flash aperture settings — mixing feet and meter guide numbers in the same formula produces a badly wrong exposure.
Precision is not about showing off decimals. It is about making sure the shopping list, the machine setting, and the acceptance check are all describing the same physical quantity.
Pocket recap
Multiply the feet-based guide number by 0.3048 to get meters — GN160 (ft) is about GN48.8 (m), matching typical published specs for that kind of flash.
Keep the converter link nearby for the next time the same debate shows up in a group chat. Re-typing the factor from memory is how the wrong cousin of a unit sneaks back in.
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FAQ
What is the exact ft (guide number) to m factor here?+
1 ft = 0.3048 m
Should I round partway through the calculation?+
No — carry full precision through the multiplication and round only the final answer, to the precision your tool, gauge, or spec actually supports.
What's the single biggest mistake people make with this conversion?+
Mixing a feet-based guide number into a formula expecting a meters-based guide number (or the reverse) without converting — guide number calculations are very sensitive to using consistent units throughout.
This article was written by The Turbo Unit Converter engineering desk (Applied units & field metrology) and last reviewed on 2026-07-16 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.