Interaction to Next Paint (INP)
IntermediateInteraction to Next Paint, or INP, is the delay between a user interaction and the next frame the browser paints in response to that interaction.
Each interaction's latency has three parts: the time other running code blocks the browser from handling the input, the time the event handlers themselves take, and the time the browser needs to paint the updated frame. Only clicks, taps and key presses qualify; scrolling and pointer movement do not. INP replaced First Input Delay as a Core Web Vital in March 2024, because FID measured only the wait before the first handler ran and ignored everything after it. Two common traps: a page nobody interacts with produces no INP at all, so a small sample on read-only pages is normal rather than an error, and averaging hides the single heavy interaction that users experience as a freeze.
How it is calculated
Interaction latency = input delay + event processing time + presentation delay, and INP = the slowest interaction on the page after outliers are discarded
The unit is milliseconds, and a page reports one value at the end of its lifetime rather than one value per click. On heavily used pages one outlier is discarded for every fifty interactions, so a single rare event does not define the number. Assessment is made at the 75th percentile of page visits; per web.dev, under 200 milliseconds is good, 200 to 500 needs improvement and above 500 is poor.