Flash Calculator

GN ÷ D = f

Flash Meter

guide number
calculator

Fixed settings

ISO

Guide number (@ ISO 100)

Solve for

Aperture (f-number)

Distance to subject

Flash power 0 stops from full

Required flash power

1/2

1.0 stops down from full

Balance flash with ambient light (optional)

Flash exposure is locked at f/8, ISO 200 — background exposure is set by shutter speed alone.

Ambient light condition

Ambient-only meter reading (no flash), at the aperture & ISO above

1/ s

Your shutter speed

1/ s

Background vs. subject

±0 stops

balanced — background reads as a natural exposure

Sync speed check

Camera sync speed (1/x s)

HSS / FP enabled

Guide number method: aperture = (GN × √power) ÷ distance, where GN scales with √(ISO⁄100) and power is expressed as a fraction of full output. Below sync speed, shutter speed doesn’t change the flash-lit exposure — it only raises or lowers the ambient background relative to it, which is what the balance section models.