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The angle of repose is an engineering property of granular materials. When bulk granular materials are poured onto a horizontal surface, a conical pile will form. The angle between the surface of the pile and the horizontal surface is known as the angle of repose and is related to the density, surface area, and coefficient of friction of the material. Material with a low angle of repose forms flatter piles than material with a high angle of repose. Another way to say it is that the angle of repose is the angle a pile, usually made of sediments, form with the ground.

This property is sometimes used in the design of equipment for the processing of particulate solids. For example, it may be used to design an appropriate hopper or silo to store the material. It can also be used to size a conveyor belt for transporting the material. It can also be used in determining whether or not a slope will likely collapse; the talus slope is derived from angle of repose and represents the steepest slope a pile of granular material will take. This angle of repose is also crucial in determining the correct calculus of stability in vessels.

There are numerous methods for measuring angle of repose and each produces slightly different results. Results are also sensitive to the exact methodology of the experimenter. As a result, data from different labs is not always comparable.

An alternative measurement, useful for many of the same purposes, is testing with a shear cell.

An antlion larva traps small insects such as ants by digging a conical pit in loose sand, such that the slope of the walls is very close to the angle of repose for the sand^[1]. Thus when a small insect blunders into the pit, its weight causes the sand to collapse below it, drawing the ant toward the center where the antlion larva lies in wait. The antlion larva assists this process by vigorously flicking sand out from the center of the pit, when it detects a disturbance, undermining the pit walls and causing them to collapse toward the center, bringing the prey with them.

1. ^ Effects of slope and particle size on ant locomotion: Implications for choice of substrate by antlions, Botz, Jason T.; Loudon, Catherine; Barger, J. Bradley; Olafsen, Jeffrey S.; Steeples, Don W.; (J. Kans. Entomol. Soc.), ISSN 0022-8567, 2003, vol. 76, no3, pp. 426-435 abstract