This is often done through a process called testing.
A set volume of sand can be measured with how long it takes to fall. You can then use math to figure out the exact volume needed for a certain time.
Once this is known, assuming you have a consistent hole, you can mass produce hourglasses with this volume of sand.
Ultimately, it’s all about how much of the particular sand goes through the hole (through the narrow part) in a certain time. Very few people make sandglasses these days to measure hour-long increments very precisely. Most sandglasses these days are used for much shorter periods — a minute or a few minutes. Even when people made bigger sandglasses, such as to time how long someone should stand watch on a ship, longer times often involved having to repeatedly turn the glass back over.
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