How many BTUs per square foot?

A basic rule: 20 BTU per square foot. A 500 sq ft room needs about 10,000 BTU. Adjust for ceiling height above 8 ft, sunny/west-facing rooms (+10%), poor insulation (+20%), high ceilings (+15%), and hot climates (+10%).

What happens if my AC is too big?

An oversized AC short-cycles — it cools quickly but doesn't run long enough to dehumidify properly, leaving your home cool but clammy. It also uses more energy and wears out faster than a properly sized unit.

How many BTUs do I need to cool a 300 square foot room?

At about 20 BTU per square foot, a 300 sq ft room needs roughly 6,000–7,000 BTU as a starting point. Sun exposure, high ceilings, poor insulation and occupancy raise that figure — the calculator adjusts for each to refine the size.

How many BTUs are in a ton of cooling?

One ton of air-conditioning equals 12,000 BTU per hour, so a 3-ton unit delivers 36,000 BTU/hr. Central systems are usually sized in tons; the calculator reports the BTU figure so you can match it to either window units or central tonnage.

What size AC do I need for my whole house?

It depends on square footage, climate, insulation and layout, but a typical 2,000 sq ft home needs roughly 40,000–60,000 BTU (about 3–5 tons). Use the calculator room by room, and have a contractor run a Manual J load calculation before buying.

BTU Calculator — What Size AC Do I Need?

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.

BTU to room size reference (cooling)
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.

Worked examples

12×15 ft room (cooling)

Inputs: 180 sq ft × 20 BTU/sq ft

Result: ≈ 3,600 BTU base (round to a 5,000 BTU unit)

300 sq ft sunny, west-facing room

Inputs: 300 × 20 = 6,000 + 10% sun

Result: ≈ 6,600 BTU (~7,000 BTU unit)

Whole home, 2,000 sq ft

Inputs: 2,000 × 20 = 40,000 BTU ÷ 12,000

Result: ≈ 3.3 tons (3–5 ton central typical)

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.

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BTU / AC Size Calculator

How to use BTU / AC Size Calculator

BTU sizing rules and adjustment factors

SEER rating and running cost

Worked examples

Glossary

Related reading

Frequently Asked Questions

Why use BTU / AC Size Calculator?

Common use cases

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