WordPress

1. In Settings → General, set both WordPress Address (URL) and Site Address (URL) to https://example.com. Log in again afterwards.
2. WordPress itself does not redirect http:// to https://. Add the redirect at the hosting level (cPanel, Plesk or .htaccess tab) or with the free Really Simple SSL plugin (now called Really Simple Security): after activating it, turn on SSL and enable its redirect. Where it offers a .htaccess 301 redirect, prefer that over the PHP redirect: it is faster and also covers images and files.
3. Clear the page cache (WP Rocket, LiteSpeed Cache, your host's cache).

If the two URL fields are greyed out, they are set in wp-config.php (WP_HOME and WP_SITEURL); change them there.

cPanel

1. Make sure the domain has a certificate: Security → SSL/TLS Status should show it as covered (AutoSSL issues one for free on most hosts).
2. Open Domains → Domains.
3. Turn on Force HTTPS Redirect next to the domain.

If the toggle is missing or greyed out, add the rule from the Apache tab to the .htaccess in the site's root (File Manager, with Show Hidden Files on).

Plesk

1. Under Websites & Domains → example.com → SSL/TLS Certificates, make sure a valid certificate is installed (Let's Encrypt is free).
2. Open Websites & Domains → example.com → Hosting & DNS → Hosting (Hosting Settings in older Plesk versions).
3. Under Security, check that SSL/TLS support is on and the certificate is selected, then turn on Permanent SEO-safe 301 redirect from HTTP to HTTPS.
4. Click OK (or Apply).

DirectAdmin

1. Make sure the domain has a certificate: Account Manager → SSL Certificates (Let's Encrypt is free).
2. Open Account Manager → Domain Setup and click the domain.
3. Turn on Force SSL with https redirect and save.

nginx

Give port 80 its own server block that does nothing but redirect. It lives in your site's file: /etc/nginx/sites-available/example.com on Debian and Ubuntu, /etc/nginx/conf.d/example.com.conf on Rocky, Alma and CentOS.

server {
    listen 80;
    listen [::]:80;
    server_name example.com www.example.com;

    return 301 https://example.com$request_uri;
}

Write your main address in the return line (with or without www), so both variants reach it in one hop. Remove any root, location or PHP settings from this block: on port 80 it should not serve anything.

sudo nginx -t && sudo systemctl reload nginx

If you use Certbot, sudo certbot --nginx --redirect -d example.com -d www.example.com adds an equivalent block for you.

Apache and LiteSpeed

With server access (Apache). In the port 80 virtual host (/etc/apache2/sites-available/example.com.conf on Debian and Ubuntu, a file in /etc/httpd/conf.d/ on Rocky, Alma and CentOS):

<VirtualHost *:80>
    ServerName example.com
    ServerAlias www.example.com
    Redirect permanent / https://example.com/
</VirtualHost>

sudo apachectl configtest && sudo systemctl reload apache2

(httpd instead of apache2 on Rocky, Alma and CentOS.)

Without server access (shared hosting, LiteSpeed). Put this at the top of the .htaccess in the site's root, above the # BEGIN WordPress block if there is one:

RewriteEngine On
RewriteCond %{HTTPS} off
RewriteCond %{HTTP:X-Forwarded-Proto} !https
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

The X-Forwarded-Proto line stops a loop when a proxy or CDN in front of the server already speaks HTTPS to the visitor and plain HTTP to your server.

Cloudflare

1. Open the domain and go to SSL/TLS → Overview. Set the encryption mode to Full (strict) (your server needs a valid certificate; a free Cloudflare Origin Certificate works too).
2. Go to SSL/TLS → Edge Certificates and turn on Always Use HTTPS. Cloudflare answers every http:// request with a 301 to https://.

Warning: Do not combine the Flexible mode with a redirect on your server. In Flexible mode Cloudflare talks to your server over plain HTTP, your server redirects to HTTPS, Cloudflare asks again over HTTP, and the browser stops with too many redirects. Use Full (strict), or leave the redirect to Cloudflare alone.

IIS and ASP.NET

Install the URL Rewrite module for IIS, then add this rule to the site's web.config:

<configuration>
  <system.webServer>
    <rewrite>
      <rules>
        <rule name="HTTP to HTTPS" stopProcessing="true">
          <match url="(.*)" />
          <conditions>
            <add input="{HTTPS}" pattern="^OFF$" />
          </conditions>
          <action type="Redirect" url="https://{HTTP_HOST}/{R:1}" redirectType="Permanent" />
        </rule>
      </rules>
    </rewrite>
  </system.webServer>
</configuration>

redirectType="Permanent" sends a 301. The site also needs an http binding on port 80, or the request never reaches the rule.

In ASP.NET Core, app.UseHttpsRedirection() redirects with a temporary 307 by default. Make it permanent:

builder.Services.AddHttpsRedirection(options =>
{
    options.RedirectStatusCode = StatusCodes.Status308PermanentRedirect;
});