Flash Guide Number Calculator — Correct Flash Exposure Every Time
Calculate flash guide number, correct aperture for flash distance, or subject distance from guide number and aperture. Master manual flash in any situation.
Auto flash modes do a decent job for snapshots, but if you're doing portrait work, event photography, or anything with off-camera flash, understanding the guide number puts you in control. It's the only number you need to dial in manual flash exposure.
Use the flash guide number calculator on CalcHub to find the right aperture or distance for any flash setup.
What the Guide Number Means
The guide number (GN) is a single number that describes a flash unit's power. It encodes the relationship between aperture and subject distance at a specific ISO:
GN = f-stop × distance
Or rearranged:
- f-stop = GN ÷ distance
- distance = GN ÷ f-stop
A flash with GN 40 (meters, ISO 100) at a subject 4 meters away needs f/10 for correct exposure. Move the subject to 2 meters, and f/20 is correct. Move them to 8 meters, and f/5.
How to Use the Calculator
- Select what you want to find: aperture, distance, or guide number
- Enter the other two known values
- Get the result instantly
Guide Number Reference Table
| Flash Power | GN (meters, ISO 100) | Aperture at 3m | Aperture at 5m |
|---|---|---|---|
| Small on-camera | 20 | f/6.7 | f/4 |
| Mid-range speedlight | 40 | f/13.3 | f/8 |
| Full-size speedlight | 58 | f/19.3 | f/11.6 |
| Studio monolight (200Ws) | ~80 | f/26.7 | f/16 |
| Large studio strobe (1000Ws) | ~160 | f/53 | f/32 |
ISO Adjustment
Guide numbers scale with ISO. The formula: GN at new ISO = GN at ISO 100 × √(new ISO ÷ 100).
Doubling ISO doubles the effective GN (since √2 × GN = GN × 1.41, but in stops that's one full stop). In practice:
- ISO 100 → GN as rated
- ISO 400 → GN × 2 (2 stops more sensitive)
- ISO 1600 → GN × 4
Bounce Flash and GN
When you bounce flash off a ceiling or wall, the light travels further before reaching the subject, and some is absorbed by the surface. A simple rule of thumb: bouncing off a white ceiling from a typical room height costs about 1.5–2 stops of light. Increase your aperture by that much or raise ISO accordingly. The GN calculation handles the direct flash path — bounce requires extra experimentation or a flash meter.
What's a realistic guide number for entry-level speedlights?
Most entry-level speedlights (like the Canon 430EX or Nikon SB-500) have guide numbers in the GN 28–43 range at ISO 100. High-end speedlights (Canon 600EX-RT, Nikon SB-900) reach GN 58–60. Guide numbers are sometimes stated at the flash's maximum zoom position (usually 105mm) and will be lower at wider flash coverage.
How do I find my flash's guide number if it's not in the manual?
Set camera to ISO 100, position your subject at exactly 1 meter, fire a test shot at f/1.0 (or calculate back from whatever aperture gives correct exposure). The correct aperture at 1 meter = the guide number in meters. Alternatively: correct exposure aperture × subject distance in meters = guide number.
Does guide number apply to TTL flash too?
Guide number is specifically for manual flash operation. TTL (through-the-lens) flash measures actual reflected light and adjusts automatically. Understanding GN still helps you understand why TTL sometimes gets it wrong and how to compensate with flash exposure compensation.
Related Calculators
- Exposure Calculator — balance full exposure triangle
- Depth of Field Calculator — how aperture affects sharpness at any flash distance
- Shutter Speed Calculator — sync speed and flash considerations