Flash Guide
Use the guide-number relationship for a transparent, local estimate. Enter a guide number at ISO 100, distance, aperture, ISO, and flash power as needed.
Use the guide-number relationship for a transparent, local estimate. Enter a guide number at ISO 100, distance, aperture, ISO, and flash power as needed.
Enter positive values to calculate.
Results
:
Effective GN at ISO: · GN after power:
Reference distances
| f-stop | Max distance |
|---|
A QUICK WALKTHROUGH
How to use this tool
- Choose whether to solve for aperture, distance, or guide number.
- Enter the known values and choose matching metres or feet.
- Review the calculated value and reference distances at common f-stops.
Transparent relationship
GN = distance × f-stop. ISO changes the guide number by √(ISO ÷ 100), and flash power changes it by √(power fraction). The displayed result shows those intermediate values.
Unit handling
Guide number and distance can be entered in metres or feet. The calculator converts distance before applying the relationship and reports the result in the guide-number unit.
Planning estimate only
This is a simplified calculation for learning and planning manual flash settings. It does not measure a flash, account for modifiers or ambient light, or establish a camera or equipment compatibility result.
GOOD TO KNOW
Common questions
What does GN mean?
Guide number is the product of distance and f-stop for a stated ISO and flash power.
Why does ISO change the result?
The calculator adjusts a GN stated at ISO 100 by √(ISO ÷ 100), following the simplified relationship used here.
Does this measure my flash?
No. It only calculates from the values you enter; actual exposure depends on the scene, equipment, and technique.