BTU / AC Size Calculator
Stop guessing on AC size. Enter room dimensions and conditions to get the exact BTU rating needed for efficient, effective cooling without over or under-sizing.
How to use BTU / AC Size Calculator
The BTU calculator tells you the right air conditioner or heater size for any room or whole house. Enter the room dimensions, ceiling height, sun exposure, insulation quality and local climate zone to get the required BTU rating — preventing the waste and poor performance of over- or under-sized HVAC equipment.
- Enter room length, width and ceiling height.
- Select sun exposure (shaded, average or sunny/west-facing).
- Choose insulation quality (poor, average, good).
- Enter the number of people regularly in the room.
- Review BTU needed and the recommended AC or heater size.
Your data never leaves your device — 100% private processing.
BTU sizing rules and adjustment factors
The starting point is 20 BTU per square foot for cooling. Key adjustments: ceiling height above 8 ft adds 15%; sunny or west-facing windows add 10%; poor insulation adds 20%; hot climate (ASHRAE zones 3–4+) adds 10–15%. Add 600 BTU per additional person. For heating, use a Manual J calculation or a rule of 30–60 BTU/sq ft depending on climate severity.
| Room size (sq ft) | BTU needed | Typical AC unit |
|---|---|---|
| 100–250 | 5,000–6,000 | Small window unit |
| 250–350 | 7,000–8,000 | Window or portable AC |
| 350–500 | 9,000–12,000 | Window or mini-split |
| 500–750 | 12,000–18,000 | Mini-split or central |
| 750–1,000 | 18,000–24,000 | Mini-split or 1.5–2-ton central |
| Full home (2,000 sq ft) | ~40,000–60,000 | 3–5 ton central AC |
SEER rating and running cost
SEER (Seasonal Energy Efficiency Ratio) measures cooling output per energy input. Minimum US standard is SEER 14 (15 in southern states). High-efficiency units reach SEER 20–28. A higher SEER unit costs more upfront but saves on electricity — in hot climates, upgrading from SEER 14 to SEER 20 can cut cooling bills 30–40%. The calculator estimates annual running cost using SEER, runtime and your electricity rate.
Glossary
- BTU
- British Thermal Unit — the heat needed to raise 1 lb of water by 1°F; the standard HVAC sizing unit.
- SEER
- Seasonal Energy Efficiency Ratio — cooling BTU per watt-hour; higher = more efficient.
- Ton (HVAC)
- 12,000 BTU/hour; a 3-ton unit provides 36,000 BTU/hr of cooling.
- Manual J
- ACCA standard load calculation method for sizing residential HVAC equipment.
Related reading
Frequently Asked Questions
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Why use BTU / AC Size Calculator?
- Accurate material estimates reduce waste and over-ordering
- Supports both metric and imperial measurements
- Covers common materials: concrete, lumber, flooring, paint
- Results include waste factor buffers for professional accuracy
Common use cases
- Calculate concrete needed for a driveway
- Estimate flooring tiles for a room renovation
- Work out paint quantity for interior walls
- Calculate lumber needs for a deck project
- Estimate roofing material for a pitched roof
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