Try the following:
RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^.*$ https://%{SERVER_NAME}%{REQUEST_URI} [R,L]
Also, you can also redirect based on port number, for example:
RewriteCond %{SERVER_PORT} ^80$
RewriteRule ^.*$ https://%{SERVER_NAME}%{REQUEST_URI} [R=301,L]
This will redirect all requests received on port 80 to HTTPS.