Check email subject line length

Empty or whitespace-only subjects are rejected with invalid_input so a blank form cannot become a silent success. Use it free in the browser while writing campaigns, or call the API at $0.002 per successful request when you need reproducible truncation flags in CI, ESP previews, or content pipelines.

What mobile and desktop subject truncation means

The response echoes the subject, reports char_count, repeats the fixed mobile_limit of forty and desktop_limit of seventy, sets truncated_mobile and truncated_desktop booleans, reports how many characters spill past each limit, and returns preview_mobile and preview_desktop as the visible slices. Status is ok when the subject fits both limits, truncated_mobile when it exceeds only the mobile limit, and truncated_desktop when it exceeds the desktop limit as well. The engine either builds an exact length report inside the supported maximum length or rejects the request with invalid_input. Rejected cases include a missing subject field, a null value, an empty string, whitespace-only paste, objects and arrays that are not plain text, and payloads longer than the documented maximum.

How the subject field is read and how limits stay reproducible

The value must be a non-empty string after a meaningful content check; finite numbers and bigints are stringified first so a bare order identifier typed as a number still produces a stable digit subject. Outer spaces that sit around real words still count toward length because inboxes display them; only pure whitespace with no visible content is treated as empty. Character counting uses Array.from on the string so each Unicode code point, including many emoji, counts as one. Desktop previews follow the same rule at seventy. Status never depends on the current date, locale collation tables that reorder letters differently across hosts, or random sampling. The preferred field name is subject, but aliases text, input, title, line, and s are accepted so query strings, spreadsheets, ESP export columns, and typed forms can reuse the same contract without renaming columns. Booleans, plain objects, and arrays are rejected with a clear message so a mistaken JSON body does not become the word true or an unhelpful object dump. Empty string and whitespace-only paste are always invalid_input: the product promise is check a real subject line, not invent a zero success for a blank submit. Length is capped at five hundred characters so a runaway paste cannot monopolize a Worker; longer payloads fail with an explicit limit message rather than a truncated silent cut. Truncation previews slice with the same code-point boundary: when truncated_mobile is true, preview_mobile is the first forty code points; when false, it is the full subject. Pair the free browser run on this page with the priced API when you need the identical report inside an automated pipeline that gates sends before they leave your ESP.

Where subject length checks fit in marketing, support, and software

On this page the work runs entirely in your browser; through the API each successful analysis costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split—free interactive exploration and a priced, identical API—matches the rest of the KIT tier-A catalog and lets you move from a campaign desk demo to an automated gate without changing the numeric result.