# Time to First Byte (TTFB)



Time to First Byte (TTFB) is a foundational metric for measuring connection setup time and web server responsiveness in both the lab and the field. It helps identify when a web server is too slow to respond to requests. In the case of navigation requests—that is, requests for an HTML document—it precedes every other meaningful loading performance metric.

## What is TTFB?

TTFB is a metric that measures the time between the request for a resource and when the first byte of a response begins to arrive.

<figure>
  

*A diagram of network request timings. The phases from left to right are Redirect (which overlaps with Prompt for Unload), Cache, DNS, TCP, Request, Response, Processing, and Load. The associated timings are redirectStart and redirectEnd (which overlap with the Prompt for Unload's unloadEventStart and unloadEventEnd), fetchStart, domainLookupStart, domainLookupEnd, connectStart, secureConnectionStart, connectEnd, requestStart, responseStart, responseEnd, domInteractive, domContentLoadedEventStart, domContentLoadedEventEnd, domComplete, loadEventStart, and loadEventEnd.*

  <figcaption>
    A diagram of network request phases and their associated timings. TTFB measures the elapsed time between <code>startTime</code> and <code>responseStart</code>.
  </figcaption>
</figure>

TTFB is the sum of the following request phases:

- Redirect time
- Service worker startup time (if applicable)
- DNS lookup
- Connection and TLS negotiation
- Request, up until the point at which the first byte of the response has arrived

Reducing latency in connection setup time and on the backend will contribute to a lower TTFB.

### What is a good TTFB score?

Because TTFB precedes [user-centric metrics](/user-centric-performance-metrics/) such as [First Contentful Paint (FCP)](/fcp/) and [Largest Contentful Paint (LCP)](/lcp/), it's recommended that your server responds to navigation requests quickly enough so that the **75th percentile** of users experience an [FCP within the "good" threshold](/fcp/#what-is-a-good-fcp-score). As a rough guide, most sites should strive to have Time To First Byte of **0.8 seconds** or less.

<figure>
  <picture>
    <source
      srcset="{{ "image/W3z1f5ZkBJSgL1V1IfloTIctbIF3/ILJ1xKjzVisqOPPyHYVA.svg" | imgix }}"
      media="(min-width: 640px)"
      width="800"
      height="200">
    

*Good TTFB values are 0.8 seconds or less, poor values are greater than 1.8 seconds, and anything in between needs improvement*

  </picture>
</figure>

<p>TTFB is not a [Core Web Vitals](/vitals/) metric, so it's not absolutely necessary that sites meet the "good" TTFB threshold, provided that it doesn't impede their ability to score well on the metrics that matter.<p>
<p>Websites vary in how they deliver content. A low TTFB is crucial for getting markup out to the client as soon as possible. However, if a website delivers the initial markup quickly, but that markup then requires JavaScript to populate it with meaningful content—as is the the case with Single Page Applications (SPAs)—then achieving the lowest possible TTFB is especially important so that the client-rendering of markup can occur sooner.<p>
<p>Conversely, a server-rendered site that does not require as much client-side work could have a higher TTFB, but better FCP and LCP values than an entirely client-rendered experience. This is why the TTFB thresholds are a “rough guide”, and will need to be weighed against how your site delivers its core content.<p>

## How to measure TTFB

TTFB can be measured in [the lab](/user-centric-performance-metrics/#in-the-lab) or in [the field](/user-centric-performance-metrics/#in-the-field) in the following ways.

### Field tools

- [Chrome User Experience Report](https://developer.chrome.com/docs/crux/)
- [`web-vitals` JavaScript library](https://github.com/GoogleChrome/web-vitals)

### Lab tools

- In the [network panel](https://developer.chrome.com/docs/devtools/network/) of Chrome's DevTools
- [WebPageTest](https://www.webpagetest.org/)

### Measure TTFB in JavaScript

You can measure the TTFB of [navigation requests](https://developer.mozilla.org/docs/Web/API/Request/mode) in the browser with the [Navigation Timing API](https://developer.mozilla.org/docs/Web/API/Navigation_timing_API). The following example shows how to create a [`PerformanceObserver`](https://developer.mozilla.org/docs/Web/API/PerformanceObserver) that listens for a `navigation` entry and logs it to the console:

```javascript
new PerformanceObserver((entryList) => {
  const [pageNav] = entryList.getEntriesByType('navigation');

  console.log(`TTFB: ${pageNav.responseStart}`);
}).observe({
  type: 'navigation',
  buffered: true
});
```


Not all browsers support `PerformanceObserver` or its `buffered` flag. To get the largest possible browser support, consider adopting the `web-vitals` package, which is discussed below.

The [`web-vitals` JavaScript library](https://github.com/GoogleChrome/web-vitals) can also measure TTFB in the browser with less complexity:

```javascript
import {onTTFB} from 'web-vitals';

// Measure and log TTFB as soon as it's available.
onTTFB(console.log);
```

### Measuring resource requests

TTFB applies to _all_ requests, not just navigation requests. In particular, resources hosted on cross-origin servers can introduce latency due to the need to set up connections to those servers. To measure TTFB for resources in the field, use the [Resource Timing API](https://developer.mozilla.org/docs/Web/API/Resource_Timing_API/Using_the_Resource_Timing_API) within a `PerformanceObserver`:

```javascript
new PerformanceObserver((entryList) => {
  const entries = entryList.getEntries();

  for (const entry of entries) {
    // Some resources may have a responseStart value of 0, due
    // to the resource being cached, or a cross-origin resource
    // being served without a Timing-Allow-Origin header set.
    if (entry.responseStart > 0) {
      console.log(`TTFB: ${entry.responseStart}`, entry.name);
    }
  }
}).observe({
  type: 'resource',
  buffered: true
});
```

The above code snippet is similar to the one used to measure the TTFB for a navigation request, except instead of querying for `'navigation'` entries, you query for `'resource'` entries instead. It also accounts for the fact that some resources loaded from the primary origin may return a value of `0`, since the connection is already open, or a resource is instantaneously retrieved from a cache.

TTFB for cross-origin requests will not be measurable in the field if cross-origin servers fail to set a [`Timing-Allow-Origin` header](https://developer.mozilla.org/docs/Web/HTTP/Headers/Timing-Allow-Origin).

## How to improve TTFB

An in-depth guide on [optimizing TTFB](/optimize-ttfb/) has been published to give you more guidance on improving your website's TTFB.