Charging is work too
Filling a battery isn't free. Power from the adapter is converted and pushed into the battery, and some of that energy always turns into heat. The battery itself also warms as it charges, most of all when it's low and charging fast.
So a MacBook on the charger has two sources of heat: the charging, and whatever you're doing. Light browsing while it charges is usually only a little warm. A video call, an export or a game while it charges from nearly empty is where it gets properly hot.
What's normal, and what isn't
Warm, even quite warm, near the hinge and underneath while charging is normal. It should ease off as the battery fills and once heavy work stops.
It's worth a closer look if the MacBook stays hot while charging even when you're doing very little. That points to something busy in the background, and the main guide shows how to find it in Activity Monitor.
Ways to keep it cooler
- Use a hard, flat surface. Apple says to use a Mac laptop on a stable surface that allows good ventilation, and never on a bed, a pillow or under bedding. That matters most while charging.
- Use an Apple-authorized adapter. Apple says to use only Apple-authorized power adapters. Cheap, unbranded adapters and cables are a common source of trouble.
- Charge before heavy work, not during it, when you can. Topping up while the Mac is idle spreads the heat out.
- Mind the room. Apple's guidance is a room between 10° and 35° C (50° and 95° F). A charger on a sunny windowsill fights a losing battle.
- Try Low Power Mode on the adapter too. In System Settings › Battery, the Energy Mode menus let you choose Low Power Mode separately for On battery and On power adapter on supported Macs.
Why it sometimes stops at 80%
If your MacBook sits at 80% on the charger, that's usually on purpose. With Optimized Battery Charging on, macOS can hold the battery around 80% for a while to slow battery aging, and finish charging when it expects you'll need it. You can choose Charge to Full Now from the battery menu in the menu bar when you need the full charge.
Apple also notes that if Optimized Battery Charging is off and the Mac still won't charge past 80%, the battery may be too warm. Move it somewhere cooler and better ventilated, and it should carry on.
Signs to stop and get help
Unplug and contact Apple if the adapter, cable or port gets too hot to touch, if you see discoloration or melting, or smell burning. The same goes if the MacBook's case starts to bulge or no longer sits flat: that can mean a swollen battery.
Some owners of Intel MacBook Pros from around 2016 to 2019 report their Mac runs cooler when charged from the ports on one side rather than the other. Apple hasn't documented this, so treat it as something to try, not a fix.