Online Tool Store
Flash Guide Number Calculator
Calculate the aperture, distance, or power a flash needs from its guide number, adjusted for ISO and fractional power settings.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Image
Flash Guide Number Calculator
Frontend preview — no upload or external service.
Result
GN 58 at ISO 100 scaled to ISO 400 becomes GN 116. At 4.5 m that gives f/25 at full power — so around 1/8 power for f/8.
How the Flash Guide Number Calculator works
- Find the flash's guide number and the ISO and units it is quoted for.
- Scale the guide number for your ISO — doubling ISO twice doubles the effective guide number.
- Divide by distance for the aperture, then reduce power to reach the aperture you actually want.
The method
Guide number ties aperture and distance together: their product is constant for a given flash power and ISO.
aperture = GN ÷ distance; GN scales with the square root of the ISO ratio
GN 58 at ISO 100 becomes 58 × √4 = 116 at ISO 400. At 4.5 m that is f/25 — far more light than most shots need, which is what fractional power is for.
FAQ
Why does guide number scale with the square root of ISO?
Because aperture is a square-root scale too — each stop doubles light while the f-number changes by √2. Four times the ISO is two stops, which doubles the guide number.
Is a manufacturer's guide number reliable?
Treat it as optimistic. Guide numbers are often quoted at the flash's narrowest zoom setting, which concentrates light and inflates the figure. Test your own flash to find its real number.
Does guide number apply with modifiers?
Not directly. A softbox or umbrella absorbs and spreads light, typically costing one to two stops. Guide number describes the bare flash, so measure the loss for each modifier you use.
How we compare
| Feature | Online Tool Store | A desktop editor | An upload-based service |
|---|---|---|---|
| ISO scaling handled | ✓ | Manual | Yes |
| Solves in any direction | ✓ | ✗ | ✓ |
| No meter needed | ✓ | ✗ | ✗ |
| Accounts for modifiers | ✗ | Testing | ✗ |
Flash Guide Number Calculator handles the ISO scaling correctly and is clear that quoted guide numbers assume a bare flash at its narrowest zoom.