AC Size Estimator
Rule-based planning tool for estimating central AC capacity in BTU and tons using square footage, insulation, sun exposure, window count, and occupant count.
This page defines a rough planning model for whole-home central air conditioner sizing. It is intended to help a homeowner, dispatcher, or estimator ask the right questions, estimate a likely cooling load in BTU, convert that load into nominal AC tons, and explain the assumptions used. It is not a Manual J calculation and it should not be treated as a final equipment selection document.
What This Tool Is For
- rough central AC replacement planning
- early-stage tonnage discussion before a site visit
- comparing whether an existing unit may be clearly undersized or oversized
What This Tool Should Not Replace
- a Manual J load calculation
- duct design or airflow verification
- room-by-room zoning design
- final equipment sizing for homes with unusual layouts, large glass areas, or major envelope issues
Intake Questions
| Input | Accepted answers | How the estimator uses it |
|---|---|---|
| Conditioned square footage | numeric square feet | base load starts at 20 BTU per square foot |
| Insulation quality | poor, average, good, excellent | scales the base cooling load up or down |
| Sun exposure | shaded, moderate, high sun | applies a second percentage adjustment |
| Window count | integer | adds 500 BTU per window |
| Occupant count | integer | adds 400 BTU per person |
Calculation Rules
Base Load
- base BTU = square footage × 20
Insulation Adjustment
| Insulation quality | Multiplier | Effect |
|---|---|---|
| Poor | 1.20 | 20% more cooling load |
| Average | 1.00 | no change |
| Good | 0.85 | 15% less cooling load |
| Excellent | 0.60 | 40% less cooling load |
Sun Exposure Adjustment
| Sun exposure | Multiplier | Effect |
|---|---|---|
| Shaded | 0.90 | 10% lower cooling load |
| Moderate | 1.00 | no change |
| High sun | 1.10 | 10% higher cooling load |
Window and Occupant Adders
- windows add 500 BTU each
- occupants add 400 BTU each
Ton Conversion
- raw tons = total BTU ÷ 12,000
- recommended nominal tonnage = round the raw tons up to the next quarter-ton
Formula Sequence
- `base_btu = square_feet × 20`
- `after_insulation = base_btu × insulation_multiplier`
- `after_sun = after_insulation × sun_multiplier`
- `total_btu = after_sun + (windows × 500) + (occupants × 400)`
- `raw_tons = total_btu ÷ 12,000`
- `recommended_tons = round raw_tons up to nearest 0.25`
Worked Example
Input set:
- conditioned area: 2,000 square feet
- insulation: average
- sun exposure: moderate
- windows: 10
- occupants: 4
Calculation:
- base BTU = 2,000 × 20 = 40,000 BTU
- insulation adjustment = 40,000 × 1.00 = 40,000 BTU
- sun adjustment = 40,000 × 1.00 = 40,000 BTU
- windows = 10 × 500 = 5,000 BTU
- occupants = 4 × 400 = 1,600 BTU
- total = 40,000 + 5,000 + 1,600 = 46,600 BTU
- raw tons = 46,600 ÷ 12,000 = 3.88 tons
- recommended nominal size = 4.00 tons
Recommended Output Format
When using this tool, the estimate should return:
- estimated BTU
- raw tons before rounding
- recommended nominal tonnage after rounding
- assumptions used for insulation, sun, windows, and occupants
- confidence level: high, medium, or low
- next step: rough planning only, free estimate, or detailed load calculation
Confidence Rules
| Confidence | When to use it |
|---|---|
| High | square footage is known, insulation is clear, sun exposure is clear, and window and occupant counts are known |
| Medium | square footage is known but one or two adjustment inputs are uncertain |
| Low | square footage is rough or major home-envelope details are unknown |
Stop Conditions
Stop at a planning estimate and recommend a detailed on-site assessment when any of the following are true:
- the home has major additions, vaulted ceilings, or very unusual ceiling height
- there are very large glass walls or extensive west-facing glazing
- the home has major comfort imbalance between floors or rooms
- ductwork condition and airflow are unknown but likely to affect sizing
- the user wants a final installation recommendation rather than a planning estimate