Lighthouse emulates a mobile device by default, applying screen dimensions, device scale factor, and a mobile user agent string. This simulates how a typical mobile user experiences your page, independent of the device running Lighthouse.
“Emulation” in Lighthouse refers specifically to screen and user agent emulation. Network and CPU throttling are covered separately in Throttling. Mobile vs desktop
By default, Lighthouse runs in mobile mode. To run with desktop settings and scoring calibration, use the --preset=desktop flag.
# Run with desktop emulation and scoring
lighthouse --preset=desktop https://example.com
--preset=desktop is the recommended way to test desktop performance. It replaces the older --emulated-form-factor=desktop flag, which was removed in Lighthouse v7.
formFactor setting does not control emulation directly. Instead, it tells Lighthouse how to interpret the run — specifically for scoring performance metrics and skipping mobile-only tests when running in desktop mode.
You must always set formFactor when configuring emulation externally. The allowed values are mobile (default) and desktop.
lighthouse --form-factor=desktop https://example.com
Screen emulation
The screenEmulation setting controls the viewport applied to the page. It accepts an object with the following fields:
| Field | Type | Description |
|---|
width | number | Viewport width in pixels. |
height | number | Viewport height in pixels. |
deviceScaleFactor | number | Device pixel ratio. |
mobile | boolean | Enables overlay scrollbars and other mobile-specific rendering behaviors. |
disabled | boolean | When true, Lighthouse skips applying screen emulation entirely. |
lighthouse --screenEmulation.disabled https://example.com
User agent emulation
By default, Lighthouse spoofs a mobile user agent string to ensure pages return their mobile variants. The emulatedUserAgent setting controls this behavior.
| Value | Behavior |
|---|
true (default) | Apply the default Lighthouse user agent string. |
false | Disable user agent spoofing. |
| A string | Apply the provided user agent string. |
emulatedUserAgent: false has no negative effect.
Testing on a real mobile device
When running Lighthouse against Chrome on an actual mobile device, you should disable Lighthouse’s own screen emulation and CPU throttling, since the device provides real conditions.
lighthouse \
--screenEmulation.disabled \
--throttling.cpuSlowdownMultiplier=1 \
https://example.com
--form-factor=mobile is the default and does not need to be set explicitly in this case.
Changes in v7
Lighthouse v7 overhauled the emulation configuration to be more explicit and consistent. Key changes:
| Status | Property |
|---|
| Removed | emulatedFormFactor — replaced by formFactor |
| Removed | TestedAsMobileDevice artifact — replaced by explicit formFactor |
| Removed | internalDisableDeviceScreenEmulation — replaced by screenEmulation.disabled |
| Added | formFactor |
| Added | screenEmulation |
| Added | emulatedUserAgent |
throttling and throttlingMethod settings were not changed in v7. 