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.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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