With LED bulbs, flickering is almost always a compatibility problem, not a defect. Unlike an incandescent bulb, LED electronics react sensitively to their surroundings: to the dimmer, to the number of bulbs on the circuit, to any loose contact. The good news: in most cases the cause can be narrowed down in 10 minutes, without a meter.
Check dimmer compatibility
The most common cause of all. Old dimmers are built for incandescent bulbs (leading/trailing edge, often rotary dimmers with no manufacturer details) and don't regulate LED bulbs cleanly – the result is flicker, especially at the lower end of the dimming range, or a faint hum from the bulb itself. The fix: only use bulbs that are explicitly "dimmable", together with an LED-compatible dimmer (stated on the device or in the data sheet, usually trailing edge with LED marking). If in doubt: non-dimmable bulbs on a normal switch, dimmable ones only on a dimmer approved for LED. You'll find a selection of suitable bulbs under dimmable bulbs.
Minimum load not reached
Many dimmers need a minimum load of 20 to 40 watts to switch cleanly – a single LED bulb at 4 to 7 watts is well below that. The dimmer's electronics then oscillate and the light flickers irregularly, often worse at low brightness. The fix: run several bulbs on the same dimmer (e.g. a multi-light fitting or a spider with three to five lamp holders from our multi-outlet ceiling roses), or retrofit a dimmer with a low minimum load (from 3 to 5 watts, designed for LED). An electrician usually swaps the dimmer in the flush box in under an hour.
Loose contact at the lamp holder or terminal
If the light flickers when there's a jolt, when the light gets knocked or when it turns slightly in a draught, it's almost always mechanical: a strand is sitting next to the terminal rather than in it, the strain relief in the ceiling rose isn't tightened properly, or a wire in the lamp holder has worked loose over time. The fix: switch off the power, open the ceiling rose, retighten every terminal and check that no single fine strand is sticking out to the side. With fine-stranded fabric cables, ferrules (crimper from 15 euros) help the terminal keep a firm contact for good. How to do this in principle is covered in Fitting a ceiling rose and Lamp holders explained.
Power supply or transformer as the cause
With low-voltage bulbs or LED strips with an external driver, the driver itself can also be the source: undersized, too old (electrolytic capacitors age and lose capacity) or unsuitable for the connected load (electronic transformers often can't handle a very low load, much like dimmers). The fix: match the driver's output to the total of the connected bulbs (wattage is stated on both); if in doubt, use a new driver approved for LED. With normal 230 V lamp holders without a transformer, this point doesn't apply – it only affects special cases such as LED strips or low-voltage systems.
Switch with glow lamp (indicator light)
Some light switches have a small indicator lamp that glows when switched off so you can find the switch in the dark. For that, a tiny residual current flows through the circuit – unnoticeable with an incandescent bulb, but enough to make a highly sensitive LED flicker or glow faintly in the "off" state. The fix: use a switch without an indicator lamp, or connect a small capacitor (a residual current compensator, a few euros from an electrical wholesaler) in parallel with the light. Matching switches and adapter plugs are under Switches & plugs.
Safety note
All work on the fixed ceiling wiring (swapping a dimmer, opening terminals in the flush box) is a job for a qualified electrician. What you can safely check yourself is everything from the light's ceiling rose onwards – switch off the power at the fuse box and confirm with a two-pole voltage tester that there's no voltage left. If you'd rather avoid wiring altogether, Plug-in light without a ceiling connection shows an alternative with no ceiling mounting.
Narrow it down systematically instead of guessing
Before you replace anything, a quick observation helps: does it flicker only when dimmed or at full brightness too? Just one bulb or all of them on the same switch? Only when you touch the light, or all the time? These three questions almost always narrow the five causes down to one – only when dimmed points to the dimmer or minimum load, only when touched to a loose contact, constantly and on several bulbs at once more likely to the switch or driver. That saves you opening the ceiling rose when the dimmer is actually the cause.
If you're not sure which bulb and which dimmer go together, put your light together in the configurator – there you can see straight away for each bulb whether it's dimmable, and plan a second bulb for a multi-outlet ceiling rose if needed.