Flash Guide Number Calculator
Calculate flash exposure using GN = aperture x distance.
Find the f-stop for any flash-to-subject distance and ISO for studio strobes and hot shoe speedlights.
A flash guide number (GN) is the standardized rating that describes a camera flash unit’s maximum power output. It defines the relationship between flash distance and aperture needed for a correctly exposed subject at ISO 100.
Core formula: Guide Number = Distance × Aperture (f-number) Aperture = Guide Number ÷ Distance Distance = Guide Number ÷ Aperture
ISO adjustment: Effective GN at ISO X = GN(ISO 100) × √(ISO X ÷ 100)
What each variable means:
- Guide Number (GN) — a fixed value published by the flash manufacturer (in meters or feet at ISO 100). Higher GN = more powerful flash. A GN of 58m/ISO 100 is a professional-grade speedlight.
- Distance — the distance from flash to subject in meters or feet. As distance doubles, light intensity drops to one-quarter (inverse square law).
- Aperture (f-stop) — the lens opening. Wider aperture (lower f-number) allows more light, meaning the flash covers greater distances.
- ISO — sensor sensitivity. Doubling ISO effectively doubles the GN (one f-stop gain). The formula uses the square root because aperture is on a square root scale.
Inverse square law: Flash intensity falls off rapidly with distance. If a subject at 2m is correctly exposed, at 4m (double the distance) only 1/4 of the light reaches the subject, requiring 2 stops more exposure.
Common flash guide numbers (ISO 100, meters):
| Flash Model | GN (meters) | Power Level |
|---|---|---|
| Built-in camera flash | GN 12–16 | Very limited |
| Entry speedlight | GN 24–30 | Adequate for nearby subjects |
| Mid-range speedlight | GN 40–50 | Good for most events |
| Pro speedlight | GN 55–65 | Full-room coverage |
| Studio strobe (100Ws) | GN 30–40 | Studio use |
| Studio strobe (500Ws) | GN 65–85 | Large space coverage |
Worked example: Your flash has GN = 56 meters (ISO 100). Subject is 7 meters away. What aperture do you need?
Aperture = GN ÷ Distance = 56 ÷ 7 = f/8
At ISO 400 instead of ISO 100: Effective GN = 56 × √(400 ÷ 100) = 56 × 2 = GN 112 Aperture = 112 ÷ 7 = f/16 — OR keep f/8 and shoot from 14 meters (double the distance)
Practical application: Use the GN formula to quickly set manual flash power without a light meter when shooting off-camera flash in predictable conditions.