What size window air conditioner for 900 sq ft?18,000 BTU/h
Use the ENERGY STAR room chart, then adjust for sun, people and kitchen heat.
Your starting point
18,000 BTU/h
Planning range
16,000–20,000 BTU/h
A screening estimate under the assumptions below. The range is a shopping guide, not a guarantee.
Planning estimate. A licensed contractor must confirm the final size, installation and local code requirements. Electrical work needs a licensed electrician.
Adjust your home
The calculation, step by step.
ENERGY STAR chart per equal-area zone, added and adjusted for sun and ceiling height; 600 BTU/h per person beyond two per zone and 4,000 BTU/h for a kitchen.
| Input or step | Calculated amount |
|---|---|
| Assumed equal-area zones | 1 × 900.0 sq ft |
| Base cooling | 18,000 BTU/h |
| Envelope / sun / height | 18,000 BTU/h |
| Extra occupants | 0 BTU/h |
| Kitchen | 0 BTU/h |
| Estimated load | 18,000 BTU/h |
What moves this number?
Sun exposure
Heavy shade reduces room-chart capacity 10%; strong sun increases it 10%.
Served zones
Different room partitions can change the total. Equal-area zones and shared conditions are only a planning assumption.
People and cooking
Each person beyond two per served zone adds 600 BTU/h; a kitchen adds 4,000 BTU/h.
Your location and assumptions.
Location does not change this model. The selected appliance loads, room conditions or peak usage determine the result.
- ENERGY STAR room table; upper boundary includes the displayed area.
- Height scaling and +/-10% range are SizeMyUnit assumptions.
- Insulation and climate are not additional multipliers in the published room chart.
- 8 ft ceilings, moderate sun, 2 occupants
What might it cost to run?
Illustrative full-load cooling cost: about $0.27/hour at an assumed EER of 10 BTU/Wh (1.80 kW). Actual efficiency, cycling and fan power differ. Uses the published 2024 EIA Texas residential average of $0.1494/kWh, not your current tariff.
Before you choose equipment.
Oversizing can reduce humidity control and comfort. Manual J determines loads; Manual S selects equipment; Manual D sizes ducts.
Bring this calculation to your installer. They will check the measured load, manufacturer performance, distribution or connections, installation conditions and local requirements.
A few more sizing questions.
What size room cooling unit for 900 square feet?
The default screen gives 18,000 BTU/h, with a planning range of 16,000–20,000 BTU/h. Change the inputs above to reflect your actual home.
How many BTU are in one ton?
One ton of cooling equals 12,000 BTU per hour. The estimated load here is 18,000 BTU/h; an equipment nameplate is a capacity rating, not electricity consumption.
Should I choose a larger unit just in case?
Excess capacity can cause short cycling and poor humidity removal. Use the range to discuss uncertainty, then select using a professional load calculation and manufacturer data.
How do sun and ceiling height affect the answer?
Shade uses a 0.9 multiplier and strong sun 1.1. This screen scales ceiling height relative to 8 feet. The sun adjustments come from ENERGY STAR room guidance; height scaling is our assumption.
Can one room unit cool the whole house?
The served area needs air circulation and a suitable layout. Closed bedrooms, multiple stories and separate zones cannot be sized as a single open room. This case assumes 1 equal-area zones; a total is combined capacity, not one unit. Equal-area zoning is our assumption and real rooms need separate calculations. Mini-split heating needs a separate winter capacity check.
Is this a final equipment size?
No. A licensed contractor must measure your building or actual loads, check manufacturer performance and local code, and confirm the final selection.