Is this a substitute for Manual J?

No. It is a sizing sanity check. Equipment selection, permits and warranty compliance need a full room-by-room Manual J and Manual S.

What happens if the system is oversized?

An oversized unit short-cycles, removes little humidity, wears components faster and gives uneven room temperatures.

Plan on roughly 600 BTU/h total per person at rest, plus the nameplate heat of major appliances and lighting.

Formula

BTU/hr ≈ sqft × base(zone, insulation) + window + people, × ceiling factor

Order-of-magnitude only. Use ACCA Manual J software for permit / equipment selection.

Conditioned floor area

sq ft

Climate zone

Envelope / insulation

Window area

% of floor

Occupants

Ceiling height

Recommended AC size

4.5tons

54150 BTU/hr cooling

Heating design load

52.3kBTU/hr

52290 BTU/hr at design day

Round DOWN, never up. Oversized AC short-cycles and won't dehumidify.

Produce a quick load estimate from area, insulation, glazing, occupancy and climate.

Enter conditioned floor area and ceiling height.
Select insulation level, window area and orientation.
Enter occupancy, appliance load and design outdoor temperature.
Read estimated cooling tons and heating BTU/h, then confirm with a full Manual J before selecting equipment.

Is this a substitute for Manual J?: No. It is a sizing sanity check. Equipment selection, permits and warranty compliance need a full room-by-room Manual J and Manual S.
What happens if the system is oversized?: An oversized unit short-cycles, removes little humidity, wears components faster and gives uneven room temperatures.
How much load do occupants and appliances add?: Plan on roughly 600 BTU/h total per person at rest, plus the nameplate heat of major appliances and lighting.