Flash Guide Number Calculator
Calculate the correct flash power at any distance, for speedlights and strobes
A flash guide number tells you how powerful it is and how far its light reaches. Enter the guide number and the distance to your subject to get the right aperture or flash power back, for both speedlights and studio strobes.
All apertures at this distance
Flash Guide Number Calculator: how it works
The Flash Guide Number Calculator tells you what flash power you need at a given distance, or how far your flash will reach at a given aperture. Use it when shooting with a manual speedlight or strobe and you want to dial in your exposure before you fire a single test shot. Enter the guide number, distance and aperture, and the result is immediate.
Guide numbers explained for working photographers
The guide number is a straightforward measure of flash output. The formula is: guide number = aperture x distance. A flash with GN 40 (at ISO 100) will correctly expose a subject at 4 metres with f/10, or at 8 metres with f/5. Manufacturers measure this under controlled conditions, so in a room with walls and a low ceiling you will often get more light than the formula predicts.
The calculator works both ways. You can start with the distance and guide number to find the aperture, or enter the aperture and see how far your flash reaches. That second approach is useful for group shots, when you need to know whether everyone in the frame is still within range.
Partial power, strobes and when manual mode matters
Working at full power is rarely the right call. A speedlight at 1/4 power has a lower effective guide number, and the relationship between power fractions and output is not always linear. The calculator accounts for power fractions so you get a reliable estimate even at reduced output.
Strobes follow the same principle, though they are often rated in watt-seconds. As long as you use the manufacturer's guide number for that output level, the calculation holds. For TTL flash this tool is less relevant, but the moment you switch to manual mode it becomes the fastest way to get to a correct starting exposure.
Frequently asked questions
How do I find the right aperture for my flash at a specific distance?
Divide the guide number by the distance. With GN 40 at 3 metres you get f/13.3, which you round to the nearest available stop on your lens.
Why is my flash exposure off even though I used the correct guide number?
Guide numbers apply at ISO 100 in open space with no reflective surfaces. A white-walled room bounces extra light back onto your subject, which throws the calculation off. Some manufacturers also rate their flash generously.
Can I use this calculator when my speedlight is set to half or quarter power?
Yes. Use the guide number that corresponds to that power level, usually listed in the flash manual or on the unit itself. The calculator then runs the same formula with that adjusted value.