Quick answer
Window air conditioner capacity comes from a room-area chart, adjusted −10% if heavily shaded, +10% if very sunny, +600 BTU/h for each person beyond two and +4,000 BTU/h for a kitchen. A 120 ft² room with normal sun, two people and no kitchen needs 5,000 BTU/h, about 1.47 kW of cooling.
Enter the room area, sunlight, people and whether it is a kitchen. The calculator returns the recommended BTU/h. Use this window air conditioner size calculator to check your own figures.
Related tools: CFM Calculator: Airflow in cubic feet per minute from duct velocity or air changes. Air Changes per Hour Calculator: Air changes per hour from airflow and room volume.
How to use the Window Air Conditioner Size Calculator
- Enter room area (1 or more).
- Choose area unit: Square feet and Square metres.
- Choose sunlight: Normal, Heavily shaded and Very sunny.
- Enter people regularly in the room (a whole number, between 1 and 30). It starts at 2.
- Choose kitchen: No and Yes.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Room area | A number | ≥ 1 |
| Area unit | Choose one: Square feet; Square metres | Starts at Square feet |
| Sunlight | Choose one: Normal; Heavily shaded; Very sunny | Starts at Normal |
| People regularly in the room | A whole number | ≥ 1, ≤ 30, starts at 2 |
| Kitchen | Choose one: No; Yes | Starts at No |
Formula and method
Formula
base capacity from the area chart; −10% if heavily shaded, +10% if very sunny; +600 BTU/h per person beyond two; +4,000 BTU/h for a kitchen
IEEE-754 double precision; shown to at most 6 decimal places.
Looks up the ENERGY STAR sizing chart (which assumes an 8-foot ceiling) and applies the published adjustments in that order. An area on a boundary uses the smaller capacity (for example exactly 150 sq ft is 5,000 BTU/h). The chart covers rooms from 100 to 1,000 sq ft; outside that range a room air conditioner is the wrong tool and you need a load calculation. Ceiling height, insulation, windows and climate are not modelled.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Room area = 120; Area unit = Square feet; Sunlight = Normal; People regularly in the room = 2; Kitchen = No.
| Result | Value |
|---|---|
| Recommended capacity | 5,000 BTU/h |
| Base capacity from chart | 5,000 BTU/h |
| Room area | 120 sq ft |
| Kilowatts | 1.47 kW |
Example 2
Inputs: Room area = 151; Area unit = Square feet; Sunlight = Very sunny; People regularly in the room = 4; Kitchen = No.
| Result | Value |
|---|---|
| Recommended capacity | 7,800 BTU/h |
| Base capacity from chart | 6,000 BTU/h |
| Room area | 151 sq ft |
| Kilowatts | 2.29 kW |
Example 3
Inputs: Room area = 300; Area unit = Square feet; Sunlight = Heavily shaded; People regularly in the room = 1; Kitchen = No.
| Result | Value |
|---|---|
| Recommended capacity | 6,300 BTU/h |
| Base capacity from chart | 7,000 BTU/h |
| Room area | 300 sq ft |
| Kilowatts | 1.85 kW |
The first line is the headline result (recommended capacity). Below it the window Air Conditioner Size Calculator lists base capacity from chart, room area and kilowatts, which describe the same case from other angles. IEEE-754 double precision; shown to at most 6 decimal places.
Why not bigger
An oversized unit cools the air quickly but shuts off before it removes humidity, leaving a cold, clammy room. An undersized one runs continuously and cannot reach the set temperature. Rooms under 100 or over 1,000 ft² are outside the chart.
Adjustments
The chart assumes an 8-foot ceiling. Add capacity for a sunny, top-floor or poorly insulated room, and for the heat from people and appliances. Check the unit's electrical requirements, since larger units may need a 230 V circuit.
How the result changes with room area
Holding every other input at the first worked example, this table changes only room area. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Room area | Recommended capacity | Base capacity from chart | Room area |
|---|---|---|---|
| 120 | 5,000 BTU/h | 5,000 BTU/h | 120 sq ft |
| 240 | 6,000 BTU/h | 6,000 BTU/h | 240 sq ft |
| 480 | 12,000 BTU/h | 12,000 BTU/h | 480 sq ft |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Room area: left empty shows "Enter room area." beside that field and no result.
Choosing between the Window Air Conditioner Size Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [CFM Calculator](/engineering/cfm/) | use | Airflow |
| [Air Changes per Hour Calculator](/science/air-changes-per-hour-calculator/) | airflow and room volume | Air changes per hour |
| [Wire Size Calculator](/engineering/wire-size/) | current, one-way length and supply voltage | Smallest AWG for the voltage drop |
| [Tire Size Calculator](/automotive/tire-size-calculator/) | section width, aspect ratio and rim diameter | Overall diameter |
| [Sample Size Calculator](/statistics/sample-size-calculator/) | estimate a, confidence level and margin of error | Required sample size |
Background: structural and wiring estimates
Structural and electrical results are only as good as their assumptions. Bending stress is the bending moment divided by the section modulus, and an allowable stress is the yield strength divided by a safety factor that depends on the governing design code and the type of load. For a single-phase or DC run the voltage drop uses the round-trip length: V = 2 × L × R per unit length × I.
Recommended limits, such as keeping branch-circuit voltage drop around 3%, differ between sources and code editions. These pages are estimates: follow the applicable code and a qualified engineer or electrician for real work.
Assumptions and limitations
- Rooms under 100 or over 1,000 sq ft are rejected
- Assumes an 8-foot ceiling
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for room area to confirm this.
Frequently asked questions
What size AC for 300 ft²?
Roughly 7,000 to 8,000 BTU/h on the standard chart; enter the area to see it.
How many kW is 5,000 BTU/h?
About 1.47 kW of cooling.
What is a ton of cooling?
A ton of cooling is 12,000 BTU/h, about 3.517 kW of heat removal.
What does the Window Air Conditioner Size Calculator calculate?
Cooling capacity (BTU/h) for a room, from the ENERGY STAR room air conditioner sizing chart. It returns Recommended capacity, Base capacity from chart, Room area and Kilowatts.
What formula does the Window Air Conditioner Size Calculator use?
base capacity from the area chart; −10% if heavily shaded, +10% if very sunny; +600 BTU/h per person beyond two; +4,000 BTU/h for a kitchen. Looks up the ENERGY STAR sizing chart (which assumes an 8-foot ceiling) and applies the published adjustments in that order.
What do I need to enter in the window Air Conditioner Size Calculator?
You enter Room area, Area unit, Sunlight, People regularly in the room and Kitchen.
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Rooms under 100 or over 1,000 sq ft are rejected.
What are the limits of this calculator?
Rooms under 100 or over 1,000 sq ft are rejected. Assumes an 8-foot ceiling.
What does the “Area unit” option change?
It sets how the window Air Conditioner Size is worked out. The choices are Square feet and Square metres.
What is an example of a window Air Conditioner Size result?
With room area 120, area unit Square feet, sunlight Normal, people regularly in the room 2 and kitchen No, the recommended capacity is 5,000 BTU/h.
Where does the method come from?
The method follows: ENERGY STAR, Room Air Conditioners: sizing chart and special considerations (energystar.gov; US government, public domain); Chart from 450 sq ft up follows the current ENERGY STAR chart (12,000 BTU/h to 550 sq ft, 14,000 BTU/h to 700 sq ft); an older ENERGY STAR sheet printed 500 sq ft for the 14,000 BTU/h band.
Can I rely on this calculator for a decision?
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them. For this calculator in particular: Rooms under 100 or over 1,000 sq ft are rejected.
Related calculators
- CFM Calculator — Airflow in cubic feet per minute from duct velocity or air changes.
- Air Changes per Hour Calculator — Air changes per hour from airflow and room volume.
- Tire Size Calculator — Overall diameter, sidewall height and revolutions per mile from a tire size such as 225/45R17.
- Sample Size Calculator — Sample size for estimating a mean or a proportion.
- Ohm's Law Calculator — Solve voltage, current or resistance.
Sources
- ENERGY STAR, Room Air Conditioners: sizing chart and special considerations (energystar.gov; US government, public domain)
- Chart from 450 sq ft up follows the current ENERGY STAR chart (12,000 BTU/h to 550 sq ft, 14,000 BTU/h to 700 sq ft); an older ENERGY STAR sheet printed 500 sq ft for the 14,000 BTU/h band
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.