How does an hourglass measure time?

 By using carefully calculated amounts of sand.

The hourglass is sometimes referred to as a sand clock or a sandglass. Like other timepieces, it needs to be carefully calibrated. The hourglass maker must test the instrument and fine tune it to measure the correct length of time.
There are many factors that contribute to the ability of an hourglass to accurately measure time. The type and quality of sand is key. It must have a rate of flow that does not fluctuate. Sand that is too coarse will wear away the glass, eventually making the neck too large. Most important is the ratio of the neck (the hole, or tube) width to the diameter of the sand particles.
Here are the other factors that affect the accuracy of an hourglass:

• The amount or volume of sand used
• The size and angle of the glass bulbs
• The quality of the sand or granular material. It must be fine, dry and consistently formed so it can flow smoothly. (Some substances used in the past were fine grain sand, powdered eggshells, and powdered marble.)
• The width of the neck
• A tight seal so no moisture can get into the chambers. Moisture can add weight to the sand or clog up the neck.
• A flat and level surface on which to rest the hourglass

We still use the hourglass to keep track of time. Just think of its many uses for cooking and for playing games!