Touchscreen test

Drag a finger over the whole screen and see which cells never light up. Includes draw mode, multi-touch and full screen.

Mode Density

Drag your finger across the whole screen. Each cell lights up when touched, so anything still dark at the end is an area that does not respond.

Essential for testing the edges: outside full screen the browser chrome covers exactly the strip where screens fail most.

Coverage 0% 0 / 96 cells

Not responding 96 cells

Simultaneous points 0 Most detected: —

Pointer type —

Multi-touch test 0 fingers at once

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Rest several fingers at once without lifting them. Most phones and touch laptops reach ten; five is normal on entry-level hardware.

Generated right here, in your browser. What you type never leaves your device.

Keep going with Touchpad test Screen test Mouse tester Dead pixel test

This test divides the screen into a grid and marks each cell as you touch it. When you finish, anything still dark is an area that never responded — which is how you locate a dead strip that in daily use only shows up when one keyboard key or one button stops working.

Use it in full screen. Outside of it the browser chrome covers the top strip, and that is exactly where screens fail most, from knocks and from pressure on the frame.

Beyond respond-or-not it measures two things almost nobody shows: how many simultaneous points your screen detects, and the size and pressure of each contact where the device reports them.

Manual

How to use it

1
Go full screen

The step most people skip and the one that matters most. Without it you cannot test the top or bottom edge, which is precisely where faults appear.
2
Cover the whole surface

Drag in a zigzag without lifting, top to bottom and side to side. Work the edges and corners: that is where the digitiser loses sensitivity first.
3
Raise the density and repeat

At 8 cells wide you find large dead areas. At 20 you find narrow strips, about the width of a key, that hide inside a cell that did respond at lower density.
4
Test multi-touch

Rest several fingers at once and watch the maximum counter. Most phones detect five or ten; many touch laptops also stop at ten. If yours stops at two, the digitiser is old or the system is limiting it.
5
Switch to draw mode

Trace slow, straight lines. They should come out continuous. Stepped, jumping or broken lines mean the digitiser is dropping points along the way, which is a different fault from a dead zone.

In depth

Things worth keeping in mind

Projected capacitive, the one in your phone

Under the glass sits a mesh of transparent electrodes, almost always indium tin oxide. That mesh holds an electric field, and your finger — being conductive — disturbs it as it approaches. The controller measures which intersections changed capacitance and works out where you touched.

Because it senses conductivity rather than force, it works with the lightest brush but not through ordinary gloves or with a plastic object. For the same reason water is a problem: a droplet conducts, and the controller reads it as a finger.

Very sensitive, allows genuine ten-point multi-touch or more
Does not work through gloves unless they carry conductive thread
Water and humidity cause phantom touches

Resistive, the one in cash machines and industrial panels

Two conductive layers held apart by microscopic spacers. Pressing makes them meet at that point, and the circuit measures the resulting resistance to compute the position. It detects pressure, not conductivity.

It works with gloves, a stylus or a fingernail, and water does not bother it. In exchange it is less precise, dulls the image because two more layers sit between you and the panel, and rarely handles more than one point at a time. That is why it survives where ruggedness beats refinement.

Works with any object, gloves included
Usually single point: no multi-touch
Wears with use — the top layer eventually gives

Why dead zones appear

The commonest cause is not the glass but the ribbon connector joining the digitiser to the board. A knock or a drop partly loosens it and contact is lost on a few lines of the mesh — and since those lines are whole rows or columns, the result is a straight dead strip, not a blotch.

That geometry is the best diagnosis you have. If what fails is a straight band across the screen, suspect the connector or the digitiser. An irregular blotch is usually physical panel damage. And a fault that comes and goes with temperature or when you squeeze the frame is almost certainly the connector.

Phantom touches

The opposite of a dead spot: the screen registers touches nobody made. The usual causes are a poor charger injecting electrical noise into the panel, moisture under the protector, or a badly applied glass protector with air bubbles.

They are easy to tell apart: if unplugging the charger stops them, it is power. If they always appear in the same area, it is the protector or the panel. If they appear randomly across the screen while the phone is hot, the digitiser may be degrading.

History

Sixty years of touching the screen

The first touchscreen was built by E. A. Johnson at Britain's Royal Radar Establishment in 1965, and it was capacitive, like the one in your phone. He designed it for air traffic control, and British controllers used it into the 1990s.

Resistive came later, in 1970, from G. Samuel Hurst, who invented it while trying to read data from a physics experiment faster. It was far cheaper to make, and so it dominated for decades: cash machines, tills and the early PDAs were all resistive, which is why they came with a stylus.

Multi-touch as we know it began in 1982 at the University of Toronto with Nimish Mehta's system, and was developed through the eighties and nineties in research settings. It took twenty-five years to reach a pocket.

What changed in 2007 with the iPhone was not inventing any of that, but combining a capacitive multi-touch screen with an interface designed from scratch for fingers, with no stylus. Within a few years the whole industry abandoned resistive for consumer devices, and the plastic stylus on a string went with it.

1965: First touchscreen
1970: Resistive touch
2007: Capacitive multi-touch goes mass market
120-240 Hz: Typical touch sampling

How it works

From finger to browser

The touch controller does not wait for something to happen: it scans the electrode mesh between 60 and 240 times a second, measuring capacitance at every intersection. That rhythm is the touch sampling rate, and it is separate from the display refresh rate — a phone can refresh at 120 Hz and sample touch at 240.

When several neighbouring intersections change together, the controller does not pick one: it computes the centroid of the affected area. That is why the position you receive has finer resolution than the mesh itself, and why it can also report contact size — a thumb pad affects more intersections than a little finger.

The operating system groups those points into sequences with identity: this moving point is the same finger you put down half a second ago. That identifier is what makes real multi-touch possible, telling two crossing fingers apart instead of confusing them.

The browser hands all of that over as pointer events, unifying mouse, finger and pen behind one interface with a field saying which it is. This tool uses that API, which is why it behaves the same on a phone, on a touch laptop, and with an active pen.

Pressure and size arrive only if the device reports them. A mouse always returns pressure 0.5 while a button is held, because it has nothing to measure. An active pen returns real pressure. Many capacitive screens report contact size but not force, because they do not measure it: they infer "more area" from a pressed finger, which is not the same thing.

By hand

Finding the cause without opening the device

Before writing a screen off, separate the panel from the software and from the power supply. Four checks, none of them costing anything.

1

A protector with bubbles or a case pressing on the frame causes both dead zones and phantom touches. It is the most frequent cause and the easiest to rule out.
2

Cheap chargers inject noise into the panel. If phantom touches vanish with the cable out, you have your culprit and nothing is broken.
3

A hung process can stop delivering touch events to the interface. If after a restart it behaves for a while and then fails, that is software, not glass.
4

A straight band across the screen: connector or digitiser. An irregular blotch: panel damage. The whole screen intermittent: power or board.
5

Booting into safe mode disables installed apps. If the screen behaves perfectly there, an app is causing it, not the hardware.

Diagnosis

If something is not working

Symptom:A strip of the screen does not respond

Likely cause:: Almost always the digitiser ribbon connector, loosened by a knock. Losing whole lines of the mesh makes the fault come out straight rather than blotchy.
What to do:: Take the case and protector off and retest. Restart. If the strip is still exactly the same, it is hardware: on a phone that means replacing the screen assembly, and it is worth comparing that against the value of the device first.

Symptom:The screen registers touches I am not making

Likely cause:: Electrical noise from the charger, moisture under the protector, or a badly applied protector.
What to do:: Unplug the charger: if they stop, that was it, and a decent charger solves it. If not, remove the protector and clean the glass with a barely damp microfibre cloth. If they persist in the same area, it is the panel.

Symptom:It responds, but I have to press harder than normal

Likely cause:: Usually a protector that is too thick or badly seated, holding your finger away from the glass and weakening the capacitive effect.
What to do:: Try without the protector. If it improves, replace it with a thinner or better-applied one. If it is the same without a protector and the device is old, the digitiser has lost sensitivity and that does not come back.

Symptom:Dragging produces a jumpy line

Likely cause:: The digitiser is dropping points: either it is short of resources, or something is loading the system.
What to do:: Close everything and repeat draw mode. If it comes out clean on a rested device, it was workload. If it jumps in the same place every time, it is the panel.

Symptom:It only detects one or two fingers

Likely cause:: Either the digitiser is old, or the system is limiting points. Some touch laptops reduce multi-touch on low battery.
What to do:: Plug into power and retry. Check power saving is off. If the device is several years old, that may genuinely be its maximum and there is nothing to fix.

Symptom:It only fails at the edges

Likely cause:: The classic pattern of a case pressing on the frame, or a panel slightly separated from the chassis by a knock.
What to do:: Remove the case and test in full screen, which is the only way to reach the true edge. If it behaves without the case, it was mechanical pressure.

Trivia

Things worth knowing

Touchscreen gloves have conductive thread

Nothing special about the fabric: silver or copper thread is stitched into the fingertips so your charge reaches the glass. An ordinary insulating glove leaves the screen blind — and that is why a sausage works: it conducts.

Touch sampling is not display refresh

A phone can show images at 120 Hz and read touch at 240 Hz. Two different circuits, two different rates, and the touch one decides whether dragging feels glued to your finger.

Your screen knows which finger is fatter

The controller computes the affected area, so it can tell a thumb from a little finger. On-screen keyboards use this to correct: if the area suggests you were aiming half a millimetre left, they type the neighbouring letter.

The first one was for directing aircraft

Johnson's 1965 screen was designed for air traffic control, not consumers. It stayed in service in the UK into the nineties — almost thirty years before most people ever touched a screen.

Water behaves like a finger

A droplet conducts and disturbs the field exactly as a fingertip does. That is why a wet screen registers impossible touches, and why waterproof phones still do not work properly under a tap.

Questions

Frequently asked questions

Why do you insist on full screen?

Because outside it the browser chrome occupies the top strip, and that strip is exactly where screens fail most from knocks and frame pressure. Testing without full screen leaves the most problematic area unchecked.

Does it work on a touch laptop?

Yes, the same as on a phone. It also works with an active pen, and in that case you will see real pressure in the point details, which a finger usually does not report.

How many simultaneous points should it detect?

Most modern phones and touch laptops reach ten. Five is normal on entry-level hardware. If yours stops at two, either the digitiser is old or the system is limiting it.

Pressure and size say "not reported" — is that a fault?

No. Many capacitive screens do not measure force, only area, and some do not expose even that to the browser. A mouse always returns pressure 0.5 because it has nothing to measure. Its absence does not indicate a fault.

Can a dead zone be fixed?

It depends on the cause. Protector, case or software: yes, and for free. Ribbon connector or digitiser: the screen assembly needs replacing, and on phones that often costs a significant share of what the device is worth.

Does it work on Android and iPhone, or do I need an app?

Both, with nothing to install. Touch events have been part of the browser for over a decade, so Chrome on Android and Safari on iPhone hand over the same data an app would use: coordinates, finger count and, where the device exposes it, area and pressure. The only thing that changes between models is how many fingers each one reports.

Is there a secret code to test the touchscreen?

On Samsung phones there is: dialling *#0*# on the phone keypad opens a service menu that includes a touch grid test. On other Android phones it depends on the manufacturer and on many it no longer exists, and on iPhone there is none at all, because Apple has never exposed that menu. That is the reason to run the test from the browser: it behaves the same in all three cases and does not depend on your manufacturer having left the door open.

Does this page store what I draw or touch?

No. Touches are processed in your browser to paint the grid and the drawing, and they disappear on reload. Nothing is sent to any server.