Quick Answer: A window air conditioner leaking water inside is almost never a broken unit — it’s a tilt, drain, or airflow problem, and the fix usually takes under an hour. Start by checking that the unit slopes down toward the outside by about 1/4 inch per foot of width (roughly 1/2 inch total); a level or inward-tilting install is the single most common cause. If the tilt is correct, clear the drain hole on the back exterior with a pipe cleaner, then check for a frozen evaporator coil caused by a dirty filter. Industry estimates put the share of AC leaks fixable with simple DIY steps under $50 at around 80% — a cracked drain pan is the rare exception that needs a replacement part or a technician.
Water pooling on the windowsill or dripping onto the floor inside the house is one of the most common window AC complaints, and it almost always traces back to one of five fixable causes. Work through them in order — they’re listed from most to least likely.
1. Check the tilt first
An improperly leveled unit is the single most common cause of indoor leaking. Window units are designed to sit at a slight downward angle toward the outside — about 1/4 inch per foot of width, or roughly 1/2 inch of total slope on a standard 24-inch-wide unit — so the drain pan sheds condensate out the back instead of pooling. If the unit was installed level, or worse, tilting inward, water collects on the room-side edge of the pan and eventually overflows forward instead of draining out.
Set a level across the top of the cabinet to check. Most units rest on adjustable window-frame brackets or accordion side panels that can be shimmed with a wood block or a dedicated support bracket to correct the angle without pulling the whole unit back out of the window. Our window AC installation guide covers getting the tilt right the first time, and our best window AC brackets guide ranks weight-rated support brackets if the factory rails aren’t holding the angle.
Adjustable window AC support bracket
- Corrects a level or inward-tilting install without reinstalling the whole unit.
- Also takes the load off a window sash rated for less weight than the AC itself.
Don’t want to troubleshoot a unit that’s leaking because it’s simply worn out — Prime delivers a replacement bracket or a brand-new window AC to your door in two days, and you can try it free for 30 days.
2. Clear the drain hole
If the tilt checks out, the next place to look is the drain hole itself, a small opening on the back exterior of the cabinet at the bottom of the drain pan. Dust, pollen, and algae accumulate there over a cooling season the same way they do in any outdoor drain, and once it’s blocked, condensate has nowhere to go but back inside.
Unplug the unit, then clear the hole from outside using a straightened pipe cleaner, a toothpick, or a thin zip tie — work gently, since the drain pan underneath is usually thin plastic that punctures easily. Cleaning the hole once a month during cooling season, and more often in dusty or high-pollen areas, keeps this from becoming a repeat problem.
3. Check the filter and coil for ice
A clogged filter restricts airflow over the evaporator coil, and reduced airflow lets the coil run cold enough to freeze — the same failure mode covered in our portable AC not cooling guide, just with a different symptom on a window unit. Instead of just losing cooling power, a window unit’s melting ice sheet dumps more water into the drain pan than it’s built to shed at once, and the overflow runs inside rather than out the back.
Pull the filter and hold it to a light; if you can’t see through it clearly, rinse it under lukewarm water and let it air-dry fully before reinstalling. If ice is already visible on the coil through the front grille, turn the unit off and let it run in fan-only mode for a few hours to melt completely before restarting — running it iced-over just refreezes the same water.
No-rinse AC coil cleaner
- Restricted coil airflow is the same root cause behind both frozen coils and weak cooling.
- No-rinse formulas spray on, foam, and drain without a water hookup.
4. Confirm the window seal isn’t the real source
Occasionally what looks like an AC leak is actually rainwater or condensation finding its way in around a poorly sealed window kit — not the unit’s internal drain pan at all. Check the foam seal and accordion side panels for gaps, especially after a driving rain, before assuming the AC itself is the source. Our window AC installation kit guide ranks foam seal and side-panel kits if the factory seal has gaps or has degraded with age.
5. Rule out a cracked drain pan
If the tilt is correct, the drain hole is clear, the filter’s clean, and the unit still leaks, the drain pan itself may be cracked — usually from age, an impact during installation, or a unit that’s been run through several freeze-thaw cycles with ice on the coil. This is the one cause on this list that isn’t a quick DIY fix; it typically means a replacement pan part from the manufacturer or, on an older unit, that replacement is more economical than repair.
The bottom line
Check the tilt first — a level or inward-sloping install explains the large majority of window AC leaks and takes five minutes to verify with a level. If the angle’s correct, clear the drain hole on the back exterior, then check the filter for the airflow restriction that leads to a frozen coil. A cracked drain pan is the rare cause that actually needs a replacement part rather than a five-minute fix.
Ready to replace instead of repair? Start with our best window air conditioner guide.