A preview tool renders your text at roughly the width Google uses. It cannot promise the live result will match, because several things happen between your markup and the rendered listing.
Google rewrites titles and descriptions
A substantial share of displayed titles are not the ones in the <title> tag. Google substitutes text from headings, anchor text or body content when it judges that a better answer to the query. Descriptions are replaced even more often.
Truncation is pixel-based
The limit is rendered width, not character count — around 600 pixels for titles on desktop. Narrow characters fit more; capitals fit fewer. Any character-count rule is a proxy for the real measurement.
Query-dependent rendering
Matched terms render in bold, which changes width. Sitelinks, dates and rich result markup all shift the space available. The same page previews differently against different queries.
What to build against
Treat the preview as a sanity check, keep titles comfortably under the limit rather than exactly at it, and validate uniqueness across the site — that one is deterministic and worth automating.
Try it: Google SERP Snippet Preview on SeoWolf's Notepad.