The flash guide number (GN) lets you calculate manual flash exposure without test shots. The formula: GN = f-stop × distance. Rearranged: f-stop = GN ÷ distance or distance = GN ÷ f-stop. Guide numbers are published at ISO 100; adjust for other ISOs using the effective GN multiplier below.
Flash Settings
Check your flash manual. Typical speedlight: 30–60
Result
How to Use Flash Guide Number for Manual Exposure
The guide number formula (GN = f-stop × distance) gives you manual flash control without trial-and-error test shots. If your flash has GN 40 (meters, ISO 100) and your subject is 4 meters away, divide: 40 ÷ 4 = f/10. Set your aperture to f/10 and your exposure should be correct. This calculation assumes direct, on-axis flash without modifiers.
ISO and Flash Power Adjustments
Guide numbers double with each 4× increase in ISO (two stops). At ISO 400, a GN 40 flash becomes GN 80. This means you can shoot with smaller apertures or work at greater distances when you increase ISO. At ISO 1600, effective GN quadruples to 160, giving you f/20 at 8 meters — useful when you need deep depth of field in a large room.
Flash Power Fractions and Guide Number
At half power (1/2), flash GN decreases by √2 (×0.71). At quarter power (1/4), GN decreases by half. A GN 60 flash at 1/4 power is GN 30. This is why guide numbers published in flash manuals are at full power — always account for your flash power setting in the calculation, especially at telephoto distances where you may be running the flash at full power.
FAQ
Is this calculator free?
Yes, completely free with no signup required.
What is a flash guide number?
A flash guide number (GN) is a standardized measurement of a flash unit's power output at a specific ISO (typically ISO 100). The formula is: GN = f-stop × distance. If your flash has GN 40 (meters, ISO 100), at 4 meters you can use f/10. At 2 meters you can use f/20. Higher GN means more powerful flash.
How does ISO affect the guide number?
The published guide number is specified at ISO 100. When you change ISO, effective guide number changes proportionally to the square root of the ISO ratio. At ISO 400 (4× ISO 100), effective GN = published GN × √4 = published GN × 2. So a GN 40 flash at ISO 100 has effective GN 80 at ISO 400.
What is a typical guide number for a speedlight?
Entry-level speedlights: GN 24–32 (meters, ISO 100). Mid-range speedlights (Canon 470EX, Nikon SB-700): GN 32–42. Professional speedlights (Canon 600EX, Nikon SB-910): GN 42–60. Studio monolights at full power: GN 55–80+. The guide number drops significantly at partial power — at 1/4 power, GN is halved.
What does GN 40 mean in practice?
GN 40 (at ISO 100, in meters) means: at 4 meters distance, use f/10. At 2 meters distance, use f/20. At 8 meters, use f/5. The guide number is simply the product of f-stop and distance. Use it to quickly calculate manual flash exposure without test shots.
Does bounce flash change the guide number calculation?
Yes, significantly. Bouncing flash off a ceiling or wall spreads light in a larger area and adds distance — typically losing 2–3 stops of light compared to direct flash. The guide number calculation applies to direct flash. For bounce flash, increase your ISO or open up your aperture 1.5–2 stops from what the GN calculation suggests, then refine by test shot.