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.
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.
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