Calibrite ColorChecker Gray Balance Mini

The ColorChecker Gray Balance Mini is an 18% gray card scientifically engineered to provide a precise, uniform surface that is spectrally neutral. It works as a reference against changes in perceived white tones, giving you the power to adjust your camera's color sensitivity to exactly match the ambient lighting conditions.
Measuring just 4.3 by 2.5 inches, it's a smaller, more convenient version of the standard grey card, with markings along its edge to serve as a quick measuring tool or to ascertain the scale of the subject it is placed against.