How the game reads a seed
- A whole number between −9,223,372,036,854,775,808 and 9,223,372,036,854,775,807 is used exactly as typed.
- Anything else, including words, decimals and numbers that are too big, is turned into a number with Java's
String.hashCode. - An empty box gives a random seed.
Text seeds only ever produce numbers between about −2.1 and 2.1 billion, so there are far fewer worlds reachable by text than by number.
Spaces and capitals matter
Glacier, glacier and glacier (with a space) are three different worlds. Spaces at the start and end are trimmed off by the world creation screen, but nowhere else.
Paste the number this tool gives into level-seed in server.properties before the world is first created to get the same world on a server.