I am looking to add a couple of new circuits to my panel, including a 2000 watt baseboard heater. My panel (100 amp) is close to being full, so with some advice I got a multimeter w/ amp clamp, and turned on everything in the house I could. Then I used the meter and got readings of 53 amps and 48 amps. Does this mean that I am already maxing my panel out and is it time for an upgrade?

The readings were taken where?

These readings don't surprise me one bit. This is about 10,000 watts of use. Maybe you have 500 watts of lighting. Once you get past the toaster (1500 watts) the hair dryer (1,500 watts) self defrost frig (900 watts) all of which are short term loads, unless you have electric stove, water heater, and other heat producing loads the only large load is a/c. Figure a/c is about 1,200 watts per ton (12,000 btu) So 5 tons is 6,000 watts. Makes you wonder about all those 200 amp services.

If you measured each leg, then the neutral would have the difference or about 5A. This is good.

Your using about 1/2 capacity, but you really don't want to load it more than 80% of 100A.

Turning everything on is not how a load calc is done. Nothing wrong with it, interesting data, however, I am sure you will agree that all you had on is never all on at once.

Confirming Kiss's assumption and question, knowing where these reading were taken would be great.