How much of the Sahara is covered by sand sheets and dunes?

Sand sheets and dunes cover approximately 25 percent of the Sahara’s surface.1

While these features are iconic, the desert's landscape is diverse and also includes extensive gravel-covered plains known as serirs or regs, rock-strewn plateaus called hammadas, and various mountain massifs. The sand formations themselves range from smaller, crescent-shaped barchans to massive, complex ergs dominated by mountainous sand ridges known as draa.1