559.com is unavailable:
10019976.com is available:
But how can I figure out the lowest number available as a ".com" domain name?
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You could find out programatically using a domain search API, such as Dynadot's API, which lets you search for available domains in blocks of 100.
But actually posting the shortest available numerical domain here might only ensure that it wasn't the shortest available domain for much longer.
I think you mean the shortest valid name, because availability shouldn't be your concern since almost all of short names are already taken!
FYI, 559.com is valid and available through GoDaddy auctions.
The shortest valid numerical domain name that I could find by searching on GoDaddy is three digit names, such as 123.com or 111.com, but as you can guess, they're all already taken.
I'm not sure why, but it seems two letter/digit domains are not available by many registrars including GoDaddy.