What is Interaction to Next Paint?

Interaction to Next Paint (INP) is a Google-developed metric that quantifies a web page's responsiveness to user interactions. It measures the elapsed time from when a user initiates an interaction (such as a click, tap, or keypress) to the moment the browser's next frame is painted after processing the interaction’s event handlers. INP captures the full pipeline: input delay, event processing, and rendering. Unlike its predecessor First Input Delay (FID), which only considered the first interaction’s input delay, INP evaluates all interactions throughout the page’s lifecycle and reports the worst-case (typically the longest) latency.

INP is part of the Core Web Vitals, a set of user experience metrics used by Google for search ranking and site quality evaluation. As of March 2024, INP replaced FID as one of the three official Core Web Vitals. The metric is measured in milliseconds, with Google recommending a target of less than 200 milliseconds for a good user experience. Values between 200 and 500 milliseconds need improvement, and values over 500 milliseconds are considered poor. INP is collected through the Chrome User Experience Report (CrUX) from real users and can also be tested in lab tools like Lighthouse (simulated) and WebPageTest.

INP sits above lower-level performance APIs like the Event Timing API and the PerformanceObserver interface in the browser. Web developers can optimize INP by minimizing long tasks, breaking up heavy JavaScript execution, using requestAnimationFrame for visual updates, deferring non-critical work, and avoiding layout thrashing. Because INP measures every interaction, it provides a more accurate picture of responsiveness than FID, catching issues like unoptimized button clicks or delayed keyboard input that might otherwise go unnoticed.