Contact
One developer maintains this. Every message is read, and bug reports get answered first.
Reporting a wrong result
This is the most useful thing you can send. Include the tool, the exact values you entered, the result you got and the result you expected. Every calculator states its formula on the page, so if we disagree we can find out which of us is wrong quickly.
Requesting a tool
Tell us the conversion or calculation you need and what you are currently using instead. Requests that name a specific format pair or a specific formula are far easier to act on than general ones.