A metric unit converter would certainly be a handy addition to any browser. Then you could quickly convert metric units on website pages. You can add just that to the Opera browser with the Metric Length Converter extension.
Open this page  and press the + Add to Opera button to add Metric Length Converter to the browser. That will add a Metric Length Converter button to the toolbar. Press that button to open the tab in the shot below.
There you can convert metric units to a variety of alternatives. For example, click Length and then select Kilometer from the From menu on the left. Input a value in the left text, and select Meter from the To menu on the right. The converter will then convert kilometers to meters as shown below.
There’s much more you can convert with the extension. Click Bytes on the left of the window to convert megabytes, kilobytes, etc. For example, you could select to convert megabytes to kilobytes.
If you need to convert a weather forecast in Fahrenheit to Celsius select Temperature. Then select Fahrenheit and Celsius, and enter a Fahrenheit value in the left text box. That will then convert Fahrenheit to Celsius.
To select further options, press the Opera button at the top left of the window and select Extensions. Select Options under the Metric Length Converter extension. That will open the tab below from which you can select alternative options for opening the extension. For example, you can select to open the extension in a pop-up window, new tab, new window, etc.
So with this extension will give you an alternative to the Windows calculator for metric unit conversions. With it you can convert metric units on website pages directly from the Opera browser.