Google result preview
Write your title and meta description and see the result preview: the bars tell you whether you fit within the width limits before truncation.
Preview
Why it's measured in pixels
Google doesn't cut the title at a fixed number of characters, but when it exceeds a pixel width (about 580 px on desktop for the title, about 990 px for the description): a «W» takes far more space than an «i». That's why the tool measures your text with the actual results font.
Indicative limits
The limits change over time and between desktop and mobile, and Google may rewrite your title and description when it sees fit: use the bars as guidance (staying under 90% is prudent), not as a guarantee.