How to use the touch test

Go fullscreen: Tap the fullscreen button so the test area is as large as possible.
Cover the whole area: Slowly drag a finger across the entire test area in rows, including the edges and corners. Any gap in the line is a spot that didn't register.
Try multi-touch: Put several fingers down at once. Each touch gets its own color and an ID number, so you can count how many your screen tracks at the same time.
Check the edges: The test area is square, so on a wide screen it covers the middle. Rotate the device to check the remaining edges.

Diagnosing touch screen problems

No touch response: Make sure touch drivers are installed and the touch screen is enabled. A total lack of response can mean a disconnected digitizer cable, a faulty digitizer, or a serious driver or OS problem.
Dead spots: If certain areas don't respond, there's a localized fault in the touch digitizer.
Ghost touches or erratic input: Touches that appear on their own, or land in the wrong place, can be caused by a faulty digitizer, pressure on the screen from a tight case or bezel, electrical interference, a screen protector, or drivers.
Multi-touch failure: If the screen supports multi-touch (pinch-to-zoom) but only one finger shows up here, it could be a driver issue or a fault in the digitizer's multi-touch support.
Working device: Touch registers accurately across the whole screen, and multiple simultaneous touches are tracked correctly.
Malfunctioning device: No response, dead spots, ghost touches, inaccurate touch position, or multi-touch not working.

Restart: Restart the device. Touch controllers occasionally stop responding until a reboot.
Drivers (Windows): In Device Manager, look under 'Human Interface Devices' for 'HID-compliant touch screen'. Make sure it's enabled, and update or reinstall the driver.
Hardware: If nothing helps, the digitizer or its cable may need repair.

Clean the screen: Dirt, grease, and moisture can block or confuse touch input. Wipe the screen with a slightly damp microfiber cloth.
Screen protector and case: Remove any screen protector or tight case and test again. Bubbles and edge pressure cause many touch problems.
Calibration (Windows): Search for 'Calibrate the screen for pen or touch input' in the Start menu and run the calibration.

Charger: Unplug the charger and test again. Some cheap chargers cause electrical noise that triggers phantom touches.
Pressure: Remove the case and check whether any part of the screen is being pressed or flexed.
Hardware: If ghost touches continue with no charger or case, the digitizer is likely faulty.

Use this touch test to separate screen and digitizer problems from settings and accessories before you send a device off for repair.

Does this work on a non-touch screen?

You can draw with a mouse to try it out, but testing multi-touch needs a touch screen.

How many touches should my screen support?

Most phones and tablets track at least five fingers, and many track ten. Some laptops and older devices support fewer.