TL;DR
A cooler smells because it was closed while still damp, and mildew grew in the gasket, the drain plug, and the hinge seams. Drain and scrub it with warm soapy water, treat the smell with a baking soda paste or a diluted bleach soak, rinse thoroughly, and dry it with the lid propped open. Never store a cooler shut.
Why coolers go bad
Everything in a cooler ends up in the bottom of it: melted ice, marinade, condensation off drink cans, and whatever leaked out of a bag of food.
That liquid pools around the drain plug and soaks into the gasket. Then the cooler gets shut and put in a garage, warm and dark and wet, and mildew does exactly what mildew does.
The parts that hold the smell are the ones nobody scrubs: the gasket groove under the lid, the drain plug threads, the hinge seam at the back, and on a soft cooler, the seams of the liner.
How do you clean a cooler?
Empty it, pull the drain plug, and rinse it out. Scrub every interior surface with warm water and dish soap, including the lid, the gasket, and the drain channel. Rinse thoroughly, then dry it completely with the lid propped open before storing.
The step-by-step for a hard cooler
1. Empty and drain it. Take the drain plug all the way out and rinse it separately. The threads collect the worst of it.
2. Rinse the interior. A hose or a bathtub sprayer. Get the loose debris out before soap.
3. Scrub with warm soapy water. A soft brush or a sponge, covering the walls, the floor, the underside of the lid, and the exterior. Use a narrow brush or an old toothbrush in the gasket groove and the hinge seam.
4. Clean the drain channel. The slope leading to the plug traps residue. Brush it out.
5. Rinse thoroughly. Soap left behind will smell and will taint the next batch of food.
6. Treat the odor if it is still there. Options are below. One method only, never combined.
7. Dry with the lid open. In the sun if you can, or somewhere with airflow. Fully dry, all the way into the gasket.
Odor treatments, one at a time
Baking soda. A paste of baking soda and water scrubbed over the interior, left twenty minutes, then rinsed. Also works dry: a shallow open container of baking soda in a closed cooler for a day or two absorbs residual smell.
Diluted bleach. About one tablespoon of household bleach per gallon of water, wiped or swabbed over the interior, left a few minutes, then rinsed very thoroughly. Good for visible mildew. Ventilate the space.
White vinegar. A one-to-one vinegar and water solution, sprayed on, left ten minutes, then rinsed. Milder, and it handles light musty smell well.
A soak. Fill the cooler with warm water and your chosen solution, leave it a few hours, drain and rinse.
Do not combine any of these. Never mix bleach with vinegar or any acid, which releases chlorine gas. Never mix bleach with ammonia-based cleaners, which releases chloramine gas. Never mix hydrogen peroxide and vinegar in one container. Rinse fully between methods if you try a second one.
Why does my cooler smell even after washing?
Because the smell is not on the surfaces you washed. Mildew lives in the gasket groove, the drain plug threads, the hinge seam, and inside foam insulation that got wet through a crack. Scrub those specific spots, and if the smell survives, moisture has reached the insulation and the cooler may not recover.
Soft coolers and cooler bags
Soft coolers are harder, because they cannot dry the way a hard shell can.
Check the manufacturer's tag first. Most soft coolers are hand-wash only, and machine washing can delaminate the waterproof liner.
Wipe the liner with warm soapy water and a cloth, working into the seams where the liner meets the walls. Rinse with a clean damp cloth rather than flooding it, since water trapped between the liner and the insulation is the thing that causes the smell in the first place.
Clean the zipper. Grit in the teeth ruins zippers, and a waterproof zipper needs its track wiped and, on some models, lubricated per the manufacturer.
Dry it open, propped so air reaches the inside, for as long as it takes. A soft cooler can take a full day.
Removable liners and the ice-chest gasket
Some coolers have a removable gasket. Pull it, wash it separately, dry it, and refit it. That single step solves a lot of stubborn smells.
If the gasket is cracked or compressed flat, replacement gaskets are available for many models and cost far less than a new cooler. A failed gasket also means the cooler holds ice poorly, so replacing it fixes two problems.
Storage is the whole game
A cooler stored shut will smell no matter how well you cleaned it.
Store it with the lid propped open, or with a wedge, a rolled towel, or the drain plug removed and a block under the lid. Airflow is the requirement.
Keep it off a damp concrete floor and out of direct sun, since ultraviolet light makes plastic brittle over years.
For long storage, an open container of baking soda inside keeps the interior neutral. The same open-storage rule applies to a tent, covered in how to clean a tent and rainfly, and to almost everything in our guide to cleaning outdoor gear and equipment.
The food safety part
Cleaning a cooler removes soil. It does not make a cooler safe to hold food at an unsafe temperature.
Keep perishable food at 40 degrees F or below, and use an appliance thermometer if you are not sure. Raw meat goes in a sealed container or a separate cooler from ready-to-eat food, and any leaking meat juice means the cooler needs a full clean before the next use.
Melted ice water is not clean water. Drain it rather than letting food sit in it.
Source: United States Department of Agriculture Food Safety and Inspection Service cooler guidance
After a fishing or camping trip specifically
Fish coolers need washing immediately, not next weekend. Fish oil soaks into plastic and gets much harder to remove once it dries in.
Camping coolers come home with soil, food residue, and sometimes ash. Rinse the exterior too, and clean the stove and the rest of the kit at the same time as described in how to clean a camping stove and burner.
Doing it while the truck is still unpacked takes ten minutes. Doing it in the spring takes an hour and sometimes does not work.
FAQ
How do you get the smell out of a cooler?
Scrub it with warm soapy water first, paying attention to the gasket groove, drain plug threads, and hinge seam. Then use one odor treatment: a baking soda paste, a diluted bleach wipe, or a vinegar solution. Rinse thoroughly and dry with the lid propped open. Never combine the treatments.
Can you use bleach to clean a cooler?
Yes, diluted to roughly one tablespoon per gallon of water, wiped on and rinsed very thoroughly. Ventilate the area. Never mix bleach with vinegar or any acid, which produces chlorine gas, or with ammonia-based cleaners, which produces chloramine gas. Use one product at a time.
How do you clean a soft-sided cooler?
Hand wash it unless the manufacturer's tag says otherwise. Wipe the liner with warm soapy water, work into the seams, then wipe clean with a damp cloth rather than flooding it. Clean the zipper track, and dry the cooler propped fully open, which can take a day.
Why does my cooler still smell after cleaning?
The smell is usually in a spot that did not get scrubbed: the gasket groove, the drain plug threads, or the hinge seam. If those are clean and the smell persists, water has likely reached the foam insulation through a crack, and that is generally not fixable.
How should you store a cooler so it does not get musty?
Completely dry, with the lid propped open so air circulates, off a damp concrete floor and out of direct sunlight. An open container of baking soda inside helps during long storage. A cooler put away shut and even slightly damp will smell by the next season.
Is it safe to put a cooler in the dishwasher?
No. Coolers are too large, and dishwasher heat can warp the plastic shell and damage the insulation and gasket. Removable parts like a drain plug or a small food tray may be dishwasher safe if the manufacturer says so, but the body is always washed by hand.
Gear that comes home from a trip has a way of leaving the mess behind in the house. If that part is what you would rather hand off, get a quote or read more about house cleaning services.