The short answer
To clear a blocked sink, start by plunging it, then clean out the U-bend trap if plunging fails. Block the overflow with a damp cloth, half-fill the sink, and work a cup plunger firmly over the plughole for 20 seconds to break the blockage with pressure. If that does not work, place a bucket under the U-bend trap beneath the sink, unscrew it, and remove the trapped debris by hand, which is where most sink blockages sit. A mix of baking soda and vinegar followed by hot water can clear light grease, while a fat-laden kitchen drain may need professional jetting.
A blocked sink is one of the most common household drainage problems, and most clear with a plunger or by cleaning the trap, no specialist tools required.
At a glance
- First stepPlunge with the overflow blocked
- Most blockages sitIn the U-bend trap
- Trap fixUnscrew, clear by hand, refit
- Light greaseBaking soda, vinegar, hot water
- AvoidBoiling water on plastic traps
Step 1: Plunge the sink
Plunging uses pressure to dislodge a blockage and is the quickest first step. The key detail people miss is sealing the overflow, otherwise the pressure escapes through it instead of reaching the blockage.
- Stuff a damp cloth firmly into the overflow opening (the small hole near the top of the basin). For a double kitchen sink, block the second plughole too.
- Half-fill the sink with water so the plunger cup is submerged, which helps transmit the pressure.
- Place a cup plunger squarely over the plughole and pump firmly up and down for around 20 seconds, keeping the seal.
- Pull away sharply on the last stroke. Repeat several times. When the water drains away freely, the blockage has cleared.
Step 2: Clean the U-bend trap
If plunging fails, the blockage is most likely in the trap, the U-shaped section of pipe directly below the plughole. This is where food, grease, hair and soap collect, and cleaning it out clears the majority of stubborn sink blockages.
- Place a bucket or bowl under the trap to catch water and debris.
- Unscrew the two collars holding the trap in place (most modern plastic traps undo by hand; older metal ones may need a wrench).
- Remove the trap and tip out the contents, then clear any blockage from inside it and from the connecting pipes with an old brush or your fingers.
- Rinse the trap, check the seals, and screw it back together hand-tight. Run the tap to check for leaks and confirm it drains.
| Sink type | Most common cause | Best approach |
|---|---|---|
| Kitchen sink | Fat, oil and food scraps | Plunge, clean trap, keep FOG out |
| Bathroom basin | Hair and soap scum | Plunge, clean trap, fit a strainer |
| Slow but not fully blocked | Light grease buildup | Baking soda, vinegar, hot water |
| Both sides of double sink | Shared waste pipe blockage | Block one side, plunge the other |
Matching the method to the sink and cause. For guidance only.
Step 3: Home remedies and prevention
For a sluggish rather than fully blocked sink, a gentle home method can clear light grease and odour. Pour a few tablespoons of baking soda (bicarbonate of soda) down the plughole, follow with a similar amount of white vinegar, let the fizzing reaction work for 10 to 15 minutes, then flush with hot, not boiling, water. This is mild and will not harm plastic pipes, though it is no match for a solid blockage.
Avoid pouring boiling water into a basin with a plastic trap, as it can soften or distort the fittings. Be cautious with caustic chemical drain cleaners: follow the safety instructions exactly, wear gloves and eye protection, never mix products, and never use them in a sink you have just plunged, as splash-back of caustic water is dangerous.
To stop sinks blocking again, keep fat, oil and grease out of the kitchen drain by binning them, fit a strainer to catch food scraps and hair, and run hot water through after washing up. If a kitchen drain blocks repeatedly with hardened fat, a drainage engineer's high-pressure jet clears it more thoroughly than home methods.
When the blockage is beyond the trap
If you have plunged the sink and cleaned out the trap and it still will not drain, the blockage is further along the waste pipe than you can easily reach. A handy next step is a hand-operated drain snake or auger, a flexible cable you feed into the pipe past the trap, turning it to grab or break up the obstruction before pulling it back out. Snakes reach blockages that sit beyond the U-bend, where a plunger's pressure does not carry.
A useful diagnostic clue is how many fixtures are affected. If only the one sink is slow, the problem is local to its waste pipe. If the sink, bath and other fixtures are all draining slowly together, or water backs up into the sink when the washing machine drains, the blockage is in the shared waste run or the underground drain that serves several fixtures. In that case, plunging a single sink will not solve it; the drain needs rodding from an inspection chamber, or a drainage engineer.
For a double kitchen sink, remember to block the second plughole and the overflow before plunging, otherwise the pressure escapes through them. If repeated effort does not clear a kitchen sink, the usual cause is hardened fat lining the pipe, which is best removed by professional jetting that scours the walls clean rather than punching a narrow channel through the grease that soon blocks again.
Frequently asked questions
Does baking soda and vinegar really unblock a sink?
It helps with light grease, smells and a sluggish drain, but the fizzing reaction is not powerful enough to clear a solid blockage. For a fully blocked sink, plunging or cleaning the trap is far more reliable.
Why does my kitchen sink keep blocking?
Usually because fat, oil and food scraps are going down it and congealing in the pipe. Keep FOG out of the drain, use a strainer, and the blockages should stop. Repeated blockages can also mean a buildup further along the drain.
Should I use a chemical drain unblocker?
Only with care. They can clear some grease and organic matter but are hazardous, can damage older pipes, and do little against physical blockages like food or hair. Plunging and cleaning the trap is safer and often more effective.
Sources & further reading
Figures on this page are typical UK ranges drawn from published cost guides and are intended as guidance, not a quotation.