Heating and cooling equipment is sized to the house, not to the old box. A replacement quote can reuse the old size, or it can include a load calculation that estimates what the house needs now. The line that says which method was used is worth reading closely, because it sets the scope of the work and shapes how the price is built.
What a load calculation is
A load calculation is a structured estimate of how much heat the house loses in winter and how much it gains in summer. It looks at the building envelope, including walls, attic, windows, doors, and air leakage. It also considers orientation, how the rooms are used, and how the ductwork moves air. The result is a capacity figure that the trade uses to choose equipment.
Why the old size is not a complete guide
The old unit may have been sized for a house that has since changed. Insulation may have been added or removed. Windows may have been replaced. A finished basement or an addition may have changed the load. The old size may also have come from a rule of thumb that does not fit the house. None of this means the old size was wrong. It means the quote should show how the size was reached.
What to look for on the quote
Look for a line that names the sizing method. If it says the new unit matches the old one, ask whether anyone checked the house. If it says a load calculation was done, ask for a copy and read the inputs: window area, insulation, air leakage assumptions, and whether the ductwork was considered. Ask whether the ducts were tested for airflow, because a larger unit on restricted ducts may not deliver what its rating suggests.
How the method moves the price
Sizing work adds time and a separate line. It can also change the equipment choice, since a calculated size may fall between standard sizes or call for a unit with different controls. Matching the old size is often the shorter path on paper, but the house still decides how the system performs. Neither method makes the quote lower or higher by itself. The price follows the equipment, the ductwork, and the access the job needs.
Questions to ask
Ask which method was used and who performed it. Ask what house information went into the calculation and what you can change if a detail is wrong. Ask whether the capacity of the proposed unit falls within the range the calculation supports. Ask whether the sizing covers heating, cooling, or both, because the two loads differ. A clear answer on those points is more useful than any single total.
Where to plan
Check the install year of your current unit and set the replacement window against it. The central air conditioner and heat pump pages on this site set out typical service life and when to start planning. Use the install year as the planning input, then use the load calculation to decide what size you need when the time comes.