You need a load calculation that accounts for climate, insulation, windows, orientation, ceiling height, and infiltration — typically ACCA Manual J in U.S. residential practice. Rules of thumb used on manufacturer and retailer pages (often about 20–25 Btu/h per square foot for cooling, sometimes more if the unit will also heat) are only a starting point. Oversizing a mini-split can hurt humidity control and cycling; undersizing leaves a room uncomfortable at design temperature.
This is a high-intent long-tail query, which is exactly why it should not be answered with a pretend calculator. If you see a 400-square-foot room × 25 Btu/ft² = 10,000 Btu/h, treat that as an illustration of the rule of thumb, not as a specification. A sun-facing room with poor insulation in a hot climate can need more; a well-shaded, tight room in a mild climate can need less.
Have an HVAC contractor (or a designer who will stamp a Manual J) size the system, and have an electrician confirm the circuit and the panel. If the question is actually about operating cost or about whether the panel can take the circuit, use the operating-cost calculator and the panel calculator.
This is not a permit calculation, a utility approval, or a substitute for a licensed electrician, HVAC contractor, or the authority having jurisdiction. Panel Headroom is an independent educational site. It is not a utility, a certification body, or a contractor. Electrical and generator work that is done incorrectly can cause fire, shock, or carbon monoxide poisoning. Use the adopted electrical code and manufacturer instructions; pull permits where required.