The kitchen sink is blocked — how do I clear it?
Before you buy a bottle of caustic drain cleaner: it sometimes dissolves grease, but it also attacks your pipes — and it becomes a genuine hazard for whoever opens the trap afterwards while it's still sitting in there. There are better methods, and they're cheaper.
The problem
Water drains very slowly or not at all. You may hear gurgling, smell drains, or see water back up into the second sink bowl when you run the first.
Likely causes
Grease build-up (the number one kitchen cause)
Oil goes down the drain as a liquid, cools, and hardens inside the pipe — and food debris sticks to it. Over months it becomes a thick layer that narrows the pipe until it closes. This isn't one object stuck in a pipe; it's an accumulation.
Food scraps and peelings
Vegetable peel, coffee grounds, rice and pasta (both swell in water) all catch on the grease. Coffee grounds in particular set into a concrete-like paste at the bottom of the trap.
A full trap (the U-bend)
The bend under the sink is designed to hold a little water so sewer smells can't come back into the house. It's also the first place everything collects. The good news: it unscrews by hand, empties, and cleans.
Hair and soap (bathrooms, not kitchens)
A bathroom drain is a different animal: hair wrapped around the stopper, bound together with soap. The fix there is a cheap plastic hair-grabber, not a chemical.
A deeper blockage in the main line
If every drain in the house is slow at once, or water comes up in another drain when the washing machine empties, the problem isn't your sink — it's the main line. That's a plumber.
Fix it — step by step
- Try boiling water first (with one caveat)Boil two litres and pour it down the drain slowly, in stages, pausing thirty seconds between pours. The heat melts the grease layer.⚠️ If your pipes are PVC (white plastic), use very hot but not boiling water — boiling can soften PVC joints.
- Bicarbonate of soda and vinegarPour half a cup of bicarbonate of soda down the drain, then a cup of white vinegar. It will foam — plug the drain with a cloth for 20 minutes, then flush with very hot water.This won't shift a hard blockage, but it breaks down light grease and kills the smell — and it's completely safe for the pipes.
- Use the plunger — properlyFill the sink with enough water to cover the plunger head (a plunger works on water, not air). If there's a second bowl, block its drain hard with a wet cloth — this is the step almost everyone skips.Without blocking the second drain, your pressure just escapes through it and never reaches the blockage.
- Plunge hard, 15-20 timesSeat the plunger over the hole, push down to expel the air, then pump firmly up and down 15-20 times without breaking the seal. Then pull it off sharply.The upstroke matters more than the downstroke — it's the suction that breaks the blockage up.
- Remove and clean the trapPut a basin under the trap (the U-bend). Unscrew the two plastic nuts by hand (or with a wrench if they're tight). Take the trap off and empty it into the basin.⚠️ Skip this step entirely if you've poured chemical cleaner down — caustic liquid is trapped in there and will pour out onto you.
- Scrub the trap and pipe from the insideScrub inside the trap with a pipe brush or an old toothbrush. You'll find a grey layer of grease. Clean as far into the wall pipe as you can reach.Photograph the assembly before you take it apart — and watch the rubber washers, they must go back exactly where they came from.
- If it's deeper, use a drain snakeWith the trap off, feed a drain snake into the pipe going into the wall. Turn it as you push until you feel resistance, then keep turning and pull back.A cheap barbed plastic strip (a 'zip-it') handles most cases and costs less than a bottle of drain cleaner.
- Reassemble and flush hardRefit the trap (hand-tight first, then a quarter-turn with the wrench — don't crank it, plastic cracks). Run hot water for 3-5 minutes straight and check underneath with a dry cloth.Dry under the sink completely, then come back half an hour later — a slow weep only shows up with time.
Tools & parts you'll need
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When to call a professional
Call a plumber if: every drain in the house is slow at the same time (the main line is blocked); water backs up out of a drain when the washing machine or toilet empties (sewage backflow — potentially a health hazard); the pipes under the sink are metal, rusted and crumbling; you've poured chemical cleaner and it didn't work (opening the trap is now hazardous — tell the plumber there's caustic in there); or the blockage returns within weeks of being cleared (that suggests a fall/gradient problem, not just grease).
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Try photo diagnosisPeople also ask
Why is chemical drain cleaner a bad idea?
Three practical reasons: (1) it generates heat and can deform PVC and, with repeated use, corrode metal pipes; (2) if it doesn't clear the blockage it just sits on top of it — and the first person to open the trap gets caustic liquid on their hands; (3) it rarely shifts a serious grease blockage anyway, so you've spent money and still have the problem. A plunger and a snake are cheaper, more effective, and safer.
Can I pour cooking oil down the sink with hot water?
No. The hot water carries the oil a little further along, where it cools and sets — so you've just moved the problem somewhere harder to reach. Let used oil cool, pour it into a container, and bin it.
My bathroom drain is blocked — same method?
No. In a bathroom the cause is almost always hair wound around the stopper. The fix is a thin barbed plastic strip: push it down, pull it out. The result is disgusting but instant. Chemical cleaner is an even worse choice here.
There's a drain smell coming from the sink
Either the water in the trap has evaporated (common in a sink that isn't used) or there's rotting grease build-up. Pour a litre of water down it — if the smell goes, that was it. If not, clean the trap.
Water comes back up in the other sink bowl
That means the blockage is past the point where the two bowls join. The plunger will only work if you seal the other drain properly. If it still doesn't shift, the blockage is deeper — trap or wall pipe.
How do I stop it happening again?
A strainer over the drain (costs pennies), oil in the bin rather than the sink, and coffee grounds in the bin. Then once a month: a litre of very hot water with dish soap down the drain. Those three habits eliminate the problem almost entirely.