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A Hegman gauge, sometimes referred to as a grind gauge, grind gage, or grindometer, is an instrument which indicates the fineness of grind or the presence of coarse particles and agglomeration in a dispersion.^[1] It is commonly used to determine how finely ground the particles of pigment (or other solid) dispersed in a sample of paint (or other liquid) are. This is important because many types of solid materials must be ground into finer particles in order to be dispersed in liquids.^[2] The resulting properties of the dispersion vary based on the size of individual particles and the degree which they are dispersed.

The Hegman gauge usually consists of a stainless steel block with a series of very small parallel grooves machined into it. The grooves decrease in depth from one end of the block to the other, according to a scale stamped next to them. A typical Hegman gauge is 170mm by 65mm by 15mm, with a channel of grooves running lengthwise, 12.5mm across and narrowing uniformly in depth from 100 m to zero and used to determine

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A Hegman gauge is used by placing a sample of paint at the deep end of the gauge and drawing the paint down with a flat edge along the grooves. Grind gages are sold with machined flat 'drawdown bars' specifically for this purpose. The paint fills the grooves, and the location where a regular, significant "pepperyness" in the appearance of the coating appears, marks the coarsest-ground dispersed particles.^[3] This is the point where oversized particles start to appear in high density and determines the rating for that material.^[4] The reading is taken from the scale marked next to the grooves, in dimensionless "Hegman units" (or National Standard units; NS) and/or mils or micrometres.^[5] Hegman units are defined in terms of an inverted size scale as shown below:^[6]

A lesser-used scale, North (or PCU), is also occasionally employed in the paint industry. Like the Hegman scale, this is also inverted compared to the value in microns:

Determining the fineness of a paint's grind is important, because too coarse a grind may reduce the paint's color uniformity, gloss, and opacity.^[7] The Hegman gauge is widely used for this purpose because it requires minimal skill and only a few seconds' work.^[3]