How to convert .torrent file into magnet link

μTorrent is compatible with Magnet links, so you can use them.

Short version:

Instead of downloading the .torrent file from a webserver, you download it directly from a seed/leecher. The biggest advantage is that you might be able to download the content of the torrent, even if the tracker is down or closed for registration.

Long version:

Traditionally, .torrent files are downloaded from torrent sites. A torrent client then calculates a torrent hash (a kind of fingerprint) based on the files it relates to, and seeks the addresses of peers from a tracker (or the DHT network) before connecting to those peers and downloading the desired content.

Sites can save on bandwidth by calculating torrent hashes themselves and allowing them to be downloaded instead of .torrent files. Given the torrent hash – passed as a parameter within a Magnet link – clients immediately seek the addresses of peers and connect to them to download first the torrent file, and then the desired content.

It is worth noting that BitTorrent can not ditch the .torrent format entirely and rely solely on Magnet links. The .torrent files hold crucial information that is needed to start the downloading process, and this information has to be available in the swarm.

In order to do this, you would pretty much have to do it the roundabout way. A magnet link downloads the .torrent file by torrenting it from the peers, as you've said, so you would basically need a torrenting app to get this done. This could be avoided if there is a site that runs the magnet, downloads the torrent and then hosts it for downloading, but I don't know of any such site.