What Is a Fatberg? How Grease and Wet Wipes Turn to Rock

TL;DR: What is a fatberg? It is a rock-hard mass of congealed grease and wet wipes growing in the sewer, usually out of sight until something backs up. It starts as warm cooking fat poured down a drain, cools and clings to the pipe wall, snags everything that will not break down, then reacts with calcium until it sets as hard as soap.

Key takeaways

• What is a fatberg? A solid mass of fat, oil and grease bound together with wet wipes and other items that never break down.
• It builds in four stages, pour, set, trap and block, and none of them happens quickly.
• What is a fatberg doing while it hardens? Genuine chemistry: fatty acids react with calcium in the sewer to make an insoluble soap.
• GOV.UK puts wet wipes behind 94% of sewer blockages, at a cost of around £200 million to fix.
• The largest recorded in the UK ran over 250 metres and weighed an estimated 130 tonnes.

Last updated: 14 September 2026

What is a fatberg, exactly?

What is a fatberg? A solid, rock-like mass that grows inside a sewer when fat, oil and grease congeal around things that were flushed but never break down: wet wipes, sanitary products, cotton buds. The name is a blend of fat and iceberg, which is fair on both the shape and the hidden bulk.

What is a fatberg, then, if not a record of ordinary habits? Researchers at the University of Manchester’s Sustainable Consumption Institute make the underlying point neatly: fats and oils are liquid at cooking temperature and often at room temperature, which is exactly why the sink feels like the obvious place to put them.

What is a fatberg made of?

What is a fatberg made of? Two ingredients doing very different jobs. The fat, oil and grease supply the glue. The wet wipes, sanitary products and other unflushables supply the reinforcement, giving the mass a skeleton to build around as it thickens.

By source, kitchens do most of the work. Roughly three quarters of the fats, oils and greases reaching UK sewers come from homes rather than commercial premises, so this is not a restaurant problem with a domestic footnote. It is the other way around.

Wet wipes contribute to 94% of sewer blockages, which cost water companies around £200 million to fix. GOV.UK

That is why wipes are being legislated out of the picture. Wet wipes containing plastic are banned in England from 19 May 2027 under the GOV.UK plastic wet wipes ban, partly because flushed wipes break down into microplastics rather than disappearing.

How does a fatberg form, stage by stage?

In four stages, and none of them is fast. Fat goes down the drain warm and liquid. It cools in the sewer and clings to the pipe wall as a hard film. That film snags wet wipes and other items that never rot. Then a reaction with calcium sets the mass solid.

What is a fatberg early on? Barely anything: a film you would not notice. But each stage makes the next one easier. A greasy pipe is sticky, a sticky ledge slows the flow, and slower flow drops more grease. That is why a sewer can look serviceable for a long time and then choke quickly.

Why does a fatberg set as hard as concrete?

Because it turns into soap. What is a fatberg, chemically? A calcium salt of a fatty acid. Free fatty acids released from the grease meet calcium in the sewer, much of it leaching out of concrete, and react into a solid that will not dissolve in water and bonds firmly to the pipe wall.

Aggregation of excess calcium compressing the double layer of free fatty acid micelles and a saponification reaction between aggregated calcium and free fatty acids. He et al., Environmental Science and Technology, 2011

That is the finding of a North Carolina State University study of hardened deposits in sewer lines, which rebuilt them in the laboratory from fatty acids and calcium chloride and matched the result against real sewer samples. Saponification is the same reaction that makes bar soap. In a sewer it runs slowly, in the dark, and welds the deposit to the wall.

How big can a fatberg get?

Bigger than the pipe should allow. The largest recorded in the UK was found during a routine sewer inspection in Whitechapel, east London, in September 2017. It ran to over 250 metres, roughly the length of Tower Bridge, and weighed an estimated 130 tonnes.

Crews broke it up with shovels and high-pressure jets over several weeks. Two sections were cut out, dried and taken into the collection of the London Museum, the first museum in the world to display and conserve one. What is a fatberg of that size made of? Cooking fat, wet wipes and sanitary products, exactly as the museum lists it.

Does this change how you look at a sewer?

A little, yes. What is a fatberg if not a slow event that nobody watched? It leaves no surface sign until the level starts rising, and by then the mass is already hard. That gap between a thing forming and a thing being noticed is the whole reason continuous measurement exists.

It is the part of water we find most interesting. At AQUAIOT we spend our days watching it, through continuous sewer monitoring and non-contact tools like the AQUAIOT Radar for smart water level monitoring. If you would rather read about stopping one, our post on fatberg prevention and FOG blockage monitoring covers that, and our round-up of UK water facts has more oddities like this one.

Frequently asked questions

The questions people ask most once they know what is a fatberg, and the short answers.

What is a fatberg and why does it form?

What is a fatberg? A hardened mass of fat, oil and grease bound together with wet wipes and similar unflushables inside a sewer. It forms because grease poured away warm cools and sticks to the pipe wall, then traps anything fibrous passing by, and finally reacts with calcium to set solid.

How long does a fatberg take to form?

There is no fixed clock. It depends on pipe size, gradient, temperature and how much grease and wipe material the sewer receives. The pattern is consistent even when the timing is not: a thin film first, then a ledge, then a mass. Large examples represent months or years of accumulation.

Are wet wipes really the main cause of sewer blockages?

They are the dominant one. GOV.UK states that wet wipes contribute to 94% of sewer blockages, at a repair cost of around £200 million. Wipes do not disintegrate the way toilet paper does, so they stay intact long enough to catch on a greasy pipe wall and stay there.

What was the biggest fatberg found in the UK?

The one discovered under Whitechapel in east London in September 2017. It measured over 250 metres and weighed an estimated 130 tonnes, and pieces of it were dried and preserved by the London Museum. It remains the largest example recorded anywhere in the UK.

By Gianbattista Porru, Digital and IoT lead at AQUAIOT.