Normally, it is only necessary and use the tool that converts the special characters apart from a to z (HTML entity converter), but if you want absolutely all characters to be converted to HTML, you can use the tool below which converts all letters and characters. You can also choose decimal or hexadecimal as the conversion form.
Simply paste the text you want to be converted into the first pane, then click "convert" and the text will automatically appear in the pane below. See conversion options below, check and select your preferences from the drop-down.