Posted by Kevin Vollquartz
A washing machine smells because of biofilm: a soft, slimy layer of bacteria bound together with detergent residue, body oils and lint. It builds up in the places that stay wet and never properly dry, mainly the door seal, the detergent drawer, the filter and the sump beneath the drum. Getting rid of it takes about half an hour with a cloth and one hot empty cycle, and a monthly maintenance wash keeps it from coming back.
Why does my washing machine smell when it is full of detergent every week?
Because the detergent is doing a job on your clothes, not on the machine. A wash cycle sends water and surfactants through the drum, but the folds of the rubber seal, the underside of the drawer and the filter housing sit outside that flow. Residue collects there, stays damp, and becomes food for bacteria.
A 2024 review in Antibiotics reports research identifying as many as 15 separate points inside a domestic washing machine where biofilm can establish itself, including the drawer, the rubber door seal, the pump, the filter and the hoses. Once a biofilm is settled it is physically protected, which is why a normal wash cycle does not shift it and why the smell comes back within a day or two of an ordinary hot wash.
Where exactly is the smell coming from?
Four places in the machine account for almost every case, and a fifth pattern points at the laundry instead. The smell itself narrows it down. Researchers sampling 13 domestic machines swabbed the drawer, the door seal, the sump and fibres from the wash water, and found the bacterial community was strongly site dependent, with the highest diversity inside the detergent drawer. The same study found Moraxella osloensis concentrated at the door seal more than anywhere else in the machine, a bacterium known for producing the compound behind that particular musty, damp-flannel odour.
| What it smells like | Most likely source | Check this first |
|---|---|---|
| Musty, damp, like a wet towel | Door seal folds and the drum left shut while wet | Peel back the seal and wipe the trapped water out |
| Sour or sickly sweet | Detergent drawer and its housing | Pull the drawer right out and look at the roof of the cavity |
| Drains, eggs, sewage | Filter and pump, clogged with lint, hair and coins | Open the filter flap at the bottom front |
| Stale water, strongest in the first minutes of a cycle | The sump, the low point under the drum where water sits between washes | Drain it through the filter housing, then run a hot empty cycle |
| Fine on the machine, bad on the clothes | Residue left in fabric, usually from overdosing | Reduce the dose and rewash one load with no detergent |
Does washing at 30 degrees cause it?
It contributes, though giving up cold washes is not the answer. Modern detergents work at low temperatures, but low temperatures also mean less microbial kill inside the machine, and most liquid detergents contain no bleaching agent to make up the difference. The same Antibiotics review is direct about this: at 30 degrees with a bleach-free liquid, inactivation of microorganisms drops and survivors go on to cross-contaminate.
An earlier study by Callewaert and colleagues in Frontiers in Microbiology ran five household machines at 30 degrees with non-bleach detergent and found biofilm-forming genera enriched in the water leaving the machine, along with skin bacteria transferring onto clean cotton washed alongside worn clothing. So a cold wash is not sterilising anything. It moves organisms around the load and leaves a few of them in the machine.
Keep washing cold for everyday loads anyway. Pushing every cycle up to 60 degrees costs energy and is hard on fabrics, and it is not the efficient way to deal with a machine that already smells. Clear the residue directly instead.
How do I clean a smelly washing machine, step by step?
Work from the outside in, then finish with a hot empty cycle. Total time is around 30 minutes, most of it hands-off.
- Start with the door seal. Peel the rubber lip back all the way round. There will be water sitting in the bottom of the fold, usually with lint, hair and a grey film in it. Wipe it out, clear the drainage holes if your seal has them, and dry the rubber off.
- Take the drawer right out (most release with a catch in the middle) and wash it in the sink. Then look up into the empty cavity, which is where the worst of it hides, and scrub the roof of it with an old brush.
- Open the filter behind the small flap at the bottom front. Put a towel and a shallow tray down first, because a litre or so of stagnant water comes out with it. That is the sump draining, and the housing is as close to it as you get. Rinse the filter, clear the housing, then screw it back in firmly.
- Wipe the drum and the underside of the door glass, where residue dries into a ring you stop noticing after a while.
- Finish with a hot empty cycle: the longest, hottest programme you have, nothing in the drum, and a washing machine cleaner tablet in with it. Ours is a plant and mineral-based drop-in aimed at the drum, pumps, filters and seals, which are the parts you cannot reach with a cloth.
The sump is the part no cloth reaches, so that hot cycle is what deals with it. If the machine has been smelling for months, plan on repeating the hot cycle a week later. Biofilm that has had time to settle usually needs a second pass before the smell goes for good.
How often should I run a maintenance wash?
Once a month, for a household washing several loads a week. That is frequent enough to stop biofilm getting a foothold again without burning energy on empty hot cycles you do not need. Our older post on keeping a washing machine fresh covers the small between-times habits.
If you own a dishwasher, it has the same problem and the same fix, which is why we sell both cleaners together as a machine cleaner set. Doing them on the same day of the month is the easiest way to remember.
Does hard water make it worse?
Yes, in an indirect way. Hard water is dissolved calcium and magnesium, and the Drinking Water Inspectorate notes that the resulting scale on heating elements shortens their life and makes appliances less efficient. Texture probably matters too: the Antibiotics review reports that surface topography is thought to be one of the main factors in how much bacteria a surface picks up, and scale is rougher than clean metal. Hard water also pushes people towards overdosing, because lather is poorer and the instinct is to add more, which leaves more residue behind for bacteria to feed on.
If your kettle furs up quickly, you live in a hard water area and the monthly tablet is doing double duty on limescale and grime. We go through the whole picture in our guide to hard water and limescale at home.
How do I stop it coming back?
Four habits, none of which cost anything.
- Leave the door and the drawer ajar between washes. A drum that dries out properly does not grow much.
- Unload the moment the cycle ends. Damp laundry sitting in a closed drum overnight is the fastest way we know to make a machine smell.
- Dose correctly, which usually means less than people think. Pre-measured formats take the judgement out of it, which is part of why we make laundry detergent tablets as a fixed 12 g tablet instead of a liquid you pour by eye. The rest of what we make for the wash sits in our laundry collection.
- Run something hot now and again. Towels and bedding at 60 degrees, with a detergent containing oxygen bleach or a booster, gives the machine a break from permanently cold running.
One caveat on that last point. The same 13-machine study found the monthly number of cycles at 60 degrees or above was the only user habit significantly linked to the bacterial community inside, though not in the direction you would expect: machines run hot more often had a more diverse bacterial community, not less. The researchers say themselves that what that means for hygiene is not settled. Heat alone does not clear the residue. The wiping and the monthly tablet are what do that.
What if the smell is on the clothes, not in the machine?
Then the problem is usually drying, not washing. Fabric that takes a day and a half to dry on an airer in a cold room grows the same organisms all over again, which is why clean laundry can come out fine and smell musty by the time it is folded. Spin speed and airflow matter more here than anything you put in the drawer, and we go through the practicalities in drying laundry indoors without the damp smell.
If the machine is clean and the drying is sorted and clothes still smell, look at dosing next. In our experience the culprit is nearly always too much detergent rather than too little.
