Use the free Flash Guide Number Calculator below. Everything runs in your browser, with no sign-up and no upload.
Your flash manual quotes a guide number, but what does that number actually buy you? It buys you the aperture at any distance, or the distance at any aperture. Once you know the relationship, you can set a manual flash in seconds instead of guessing and chimping.
What is Flash Guide Number Calculator?
The guide number, usually quoted at ISO 100 in metres, equals aperture multiplied by distance. A flash with guide number 40 gives f/8 at 5 metres, f/5.6 at about 7 metres, and f/16 at 2.5 metres. The calculator solves whichever value you leave unknown: give it guide number and distance to get aperture, guide number and aperture to get distance, or aperture and distance to find the guide number you need.
How it works
The maths is one multiplication and its two rearrangements. The tool also adjusts the guide number for ISO: doubling ISO multiplies the guide number by the square root of two, about 1.4, because the sensor needs half the light. So a guide number 40 flash at ISO 400 behaves like guide number 80. Distances are in metres; if your flash manual quotes feet, convert first, because mixing units is the most common guide number mistake.
How to use it
- Find your flash’s guide number in its manual, noting whether it is in metres or feet and at which ISO.
- Enter any two of guide number, aperture and distance, leaving the unknown as zero.
- Set your ISO so the tool can adjust the guide number.
- Read the answer and set the flash power or position to match.
Worked example
Your flash has guide number 40 at ISO 100 and your subject stands 5 metres away. Forty divided by five gives f/8. If you want f/4 for a softer background, the same flash now reaches 10 metres, or you can cut its power. Raise the ISO to 400 and the effective guide number doubles to 80, so at f/8 you can light a subject 10 metres away. One number, three decisions, no guessing.
Tips and limits
Guide numbers assume a bare flash in a normal room; a diffuser, a bounce off the ceiling or a softbox all eat light, so treat the result as a starting point and check the first frame. Zooming the flash head changes the guide number, because a narrow beam throws light further. And remember that aperture controls flash exposure while shutter speed controls ambient light, as long as you stay at or below the camera’s flash sync speed. That separation is what makes manual flash so controllable once the arithmetic is handled.
Please note: photography exposure tools are educational calculators. Sunny 16 and Zone System figures are starting-point rules of thumb, and a meter reading governs in real shooting. Time-lapse figures assume the camera can sustain the interval, and buffer or card limits may intervene.
Frequently asked questions
What is the difference between guide numbers in metres and feet?
Only the unit. A guide number of 40 metres equals about 131 feet. Never mix the two in one calculation.
Does the guide number change with flash zoom?
Yes. Most flashes quote the guide number at the longest zoom setting, where it is highest. At wide zoom the number is lower.
Why is my flash photo darker than the calculator says?
Modifiers eat light: diffusers, bounce and softboxes all reduce the effective guide number. Open the aperture a stop or move closer and re-test.
Try the calculator above with your own numbers, and use the result as a planned starting point for your next shoot. A few minutes of planning before you press the shutter usually saves a whole session of corrections afterwards.