Inverter vs fixed-speed AC: does sizing change?

The BTU load is the same. With an inverter AC you can right-size because it modulates down. With a fixed-speed AC, always round up to the next standard size.

What size air conditioner do I need for a 12x12 room?

A 12 by 12 ft room is 144 sq ft (about 13 m2) and usually needs about 5,000 to 6,000 BTU/hr (roughly 0.5 ton or 1.5 kW), a standard small window or portable unit. Add 10 to 20 percent for a top-floor room, west-facing sun, or an adjacent kitchen.

What SEER or SEER2 rating should I choose in the US?

SEER2 is efficiency, not size, so it does not change the BTU load, but it changes running cost. US minimums are SEER2 13.4 (North) and 14.3 (South) as of 2023. Most homeowners pick a 15 to 18 SEER2 inverter or variable-speed unit for the best balance of price and energy savings.

BTU Load Calculator — HVAC Cooling Load Tool (Manual J)

Q: What size AC do I need for a 200 sq ft room?

A 200 sq ft bedroom typically needs 6,000 to 9,000 BTU/hr (1.5 to 2.5 tons) in a hot-humid US climate, and 4,500 to 6,000 BTU/hr (1.2 to 1.6 tons) in a mild marine climate (Pacific NW, UK, northern Europe). Use the calculator for your specific climate, sun exposure, insulation, and ceiling height.

Q: How many BTU per square foot do I need?

In hot climates (US Sun Belt, southern Europe) 25 to 30 BTU/hr per sq ft (about 250 to 300 W per m2) is typical. Mild climates (Pacific NW, UK, northern EU, Mountain West) need 18 to 22 BTU/hr per sq ft (about 180 to 220 W per m2). Server rooms, gyms and commercial kitchens need 35 to 45 BTU/hr per sq ft. The calculator applies ceiling height, sun, windows and occupants on top of these baselines.

Q: How many square feet does 1 ton of AC cool?

Roughly 100 to 150 sq ft (about 9 to 14 m2) per ton in average US Sun Belt conditions. The right answer varies up to 3x by climate region, ceiling height and sun exposure, so calculate it with the tool rather than relying on a single number.

Q: Is this Manual J compliant?

This is a simplified Manual-J-inspired field calculator. It applies Manual J heat-gain categories (envelope, occupants, appliances, equipment, ventilation) with condensed factors suitable for residential field estimates. For commercial, hospital or clean-room projects, a full ACCA Manual J from a licensed engineer is required.

Q: Can I use this for a VRF / VRV system?

Yes for indoor unit sizing. For VRF total capacity and refrigerant pipe sizing, use the manufacturer selection software (Daikin, Mitsubishi, LG, etc.) with a full Manual J load summary per indoor unit.

Q: What size air conditioner do I need for a 1,500 sq ft house?

A 1,500 sq ft (about 140 m2) home typically needs 30,000 to 45,000 BTU/hr (2.5 to 3.5 tons or 9 to 13 kW). Hot US Sun Belt and southern-Europe homes are at the top of that range, mild marine climates at the bottom. Ceiling height, insulation, sun and window area shift it about 30 percent either way.

Q: How many kW of cooling do I need for 50 m2?

For a 50 m2 (about 540 sq ft) space, budget roughly 5 kW in a mild UK or northern-EU climate and up to 6.5 kW in a hot southern-EU or Australian climate (about 100 to 130 W per m2). Switch the calculator to Metric to size it in m2 and kW directly.

🧮 BTU Load Calculator — Manual J Cooling Load & AC Sizing

Instant air conditioner sizing for the US, UK, Europe, Canada & Australia. ACCA Manual-J inspired, field-tested, in BTU/hr, tons and kW — works in any climate.

🌍 Prefer metric? Switch units here

🏠1. Space Type

📐2. Floor Area & Ceiling

Floor Area 200 sq ft

or type exact value: sq ft

Range: 50 – 10,000 sq ft

Ceiling Height

or type: ft

Range: 7 – 20 ft

Occupants

or type: people

Range: 0 – 50 people

🌡️3. Climate & Location

Country / Region

Tip: Phoenix, Vegas or Riyadh pick Very Hot. London, Seattle or Berlin pick Mild. Miami, Houston or Singapore pick Hot & Humid.

🧱4. Insulation Quality

☀️5. Sun Exposure

🪟6. Windows & Glazing

⚙️ Advanced Loads (kitchen, appliances, equipment) ▼

Kitchen in the cooled area (adds heat) Ducts run through unconditioned attic or basement Adds +15% load — common US Sun Belt / Gulf install penalty (ACCA Manual J 14.4)

Major Appliances fridge / oven / washer

Heat Equipment servers, gym gear, medical

Exhaust Fans kitchen / bathroom

⚠️ Field estimate only. This calculator uses simplified Manual-J factors for fast on-site sizing. For code-compliant installations, hospital / clean-room / data-centre projects, or any load above 5 TR, request a full ACCA Manual J calculation from a licensed HVAC engineer. Refcon HVAC is not liable for installations sized solely from this tool.

The Complete Guide to AC Sizing & BTU Load Calculation

If you've ever sized an AC unit by guessing "1 ton per 100 sqft" and watched the customer complain about high bills, uneven cooling, or a unit that short-cycles every three minutes, this guide is for you. Below we walk through the math, the field rules of thumb, and the practical shortcuts that separate a profitable install from a callback.

On this page

What is a BTU, really?
Why correct sizing matters
Manual J in plain English
Why BTU/sqft rules of thumb fail
Climate multipliers (US, UK, EU & AU)
Field tips for technicians
Superheat & subcooling quick-ref
Common sizing mistakes
FAQ

What is a BTU, really?

A BTU (British Thermal Unit) is the energy needed to raise one pound of water by 1°F. In HVAC we use it as a rate — BTU/hr — to express how much heat a cooling system can remove per hour. One ton of refrigeration = 12,000 BTU/hr, defined as the heat absorbed by melting one ton of ice in 24 hours. So a 1.5 TR AC = 18,000 BTU/hr.

Why correct sizing matters

An undersized AC runs continuously, never reaches setpoint on peak days, and burns out compressors early. An oversized AC short-cycles, leaves humidity high, and never dehumidifies properly — exactly what causes the "cold but clammy" complaint. Correct sizing costs you 20 extra minutes of paperwork and saves you a service call six months later.

Manual J in plain English

ACCA's Manual J is the residential load-calculation standard used across the US and Canada; the UK and Europe use the equivalent BS EN 12831 / EN 16798 heat-load method (and Part L for efficiency), while Australia follows AS/NZS guidance. They all compute the same thing — sensible + latent cooling load — from eight heat sources:

Conduction through walls, roof, floor, glass
Radiation through glass (solar gain)
Infiltration (leakage through doors & windows)
Ventilation (fresh-air intake)
Internal gains from occupants (~600 BTU/hr each, sedentary)
Appliances & lighting
Equipment & machinery
Duct losses (5–15% depending on routing)

For a residential field estimate, we condense this into the area × factor × climate × envelope approach used by the calculator above.

Why Rules of Thumb (BTU/sqft) Fail for Load Calculation

You'll see "1 ton = 100 sqft" or "1 ton = 120 sqft" all over the internet. Those numbers come from one specific condition: 8 ft ceiling, a baseline US/EU moderate climate (≈30 °C / 86 °F), average insulation, double-glazed windows, 2 occupants. Change any of those and the right answer moves 30–80%.

Examples from real Refcon surveys:

Master bedroom, 200 sqft, Orlando humid climate, top floor, west-facing sun: needs 1.5 tons — the "1 ton = 200 sqft" rule undersizes by 50%.
Living room, 400 sqft, Seattle mild marine climate, north-facing, well insulated: needs 1.5 tons — the "1 ton = 100 sqft" rule oversizes by 25%.
Server room, 80 sqft: needs at least 1 TR regardless of area. Airflow and redundancy dominate.

The calculator above applies climate, insulation, sun, windows, ceiling, occupants, and appliance loads automatically — it's the "1 TR = X sqft" rule but with all the corrections built in.

Quick BTU & Tonnage Reference Table

Baseline only — assumes 8 ft ceilings, moderate climate, average insulation. Always verify with the calculator above for your specific conditions.

Floor Area	Typical Cooling Load	Recommended AC Size
100–200 sq ft (9–19 m²)	5,000–6,000 BTU/hr	½ Ton (≈ 6,000 BTU)
200–300 sq ft (19–28 m²)	6,000–8,000 BTU/hr	¾ Ton (≈ 9,000 BTU)
300–500 sq ft (28–46 m²)	8,000–12,000 BTU/hr	1 Ton (12,000 BTU)
500–800 sq ft (46–74 m²)	12,000–18,000 BTU/hr	1.5 Ton (18,000 BTU)
800–1,200 sq ft (74–111 m²)	18,000–24,000 BTU/hr	2 Ton (24,000 BTU)
1,200–1,800 sq ft (111–167 m²)	24,000–36,000 BTU/hr	3 Ton (36,000 BTU)

Round UP to the next standard unit size. Hot climates, west-facing glass, kitchens and top floors will push you one size higher — re-run the calculator above for accurate results.

Climate multipliers (US, UK, Europe, Canada & Australia)

The base factor assumes a 30 °C / 86 °F design day. Adjust as follows:

Coastal-moderate (San Francisco, Lisbon, Sydney): × 0.90
Hot dry (Sun Belt, S. Europe, inland Gulf): × 1.10
Hot humid (Miami, Houston, Singapore, Dubai coastal): × 1.20 (latent load dominates)
Very hot desert (Phoenix, Riyadh, inland Australia): × 1.30
Cool-mountain (Denver, Munich, Stockholm): × 0.70

Field tips for technicians

Always round UP to the next standard size. A 1.4 TR load still gets a 1.5 TR unit, never 1 TR.
Top floor? Add 15% — heat gain through roof is brutal.
West-facing glass? Add 25% if no external shading.
Kitchen adjacent? Negative-pressure the kitchen (exhaust fan ≥ 300 CFM) and add 4 BTU/hr per sqft of cooled area.
Commercial vs residential? Commercial spaces need Manual N (not Manual J). Use this calculator for residential offices only.
Data centre, hospital, lab? Don't use field calculators. Engage a design engineer.

Superheat & subcooling quick-ref

After sizing correctly, you still have to charge correctly. Here's the field cheat sheet:

System	Superheat (SH)	Subcooling (SC)
Fixed-orifice (piston / cap-tube)	10–14°F	—
TXV / EEV	8–12°F	8–12°F
R-410A, R-32 split AC	8–12°F	6–10°F
R-22 legacy	12–18°F	10–15°F
Chilled water AHU	—	8–12°F

5 common sizing mistakes

"Customer wants it cold fast, give 2 TR." Oversizing kills humidity removal and shortens compressor life. Always size by load, not by comfort demand.
Ignoring ceiling height. A 12 ft ceiling needs 40% more cooling than an 8 ft ceiling of the same footprint.
Forgetting the kitchen. Cooking adds ~4,000 BTU/hr (1.2 kW) per burner in most residential kitchens.
Not counting occupants. A home gym with 4 people generates 2,400 BTU/hr just from bodies.
Ignoring duct losses. A poorly insulated duct in an attic can lose 15% of cooling capacity.

Frequently asked questions

What size AC do I need for a 200 sqft room?

A 200 sqft bedroom typically needs 6,000 to 9,000 BTU/hr (1.5 to 2.5 tons) in a US Gulf climate or similar hot-humid region, and 4,500 to 6,000 BTU/hr (1.2 to 1.6 tons) in a mild marine climate (Pacific NW, UK, N. Europe). Use the calculator above for your specific climate, sun exposure, insulation, and ceiling height.

How many BTU per square foot do I need?

In hot climates (Sun Belt, Gulf, S. Europe), 25–30 BTU/hr per sqft (~250–300 W/m²) is typical. Mild climates (Pacific NW, UK, Northern EU, Mountain West) need 18–22 BTU/hr per sqft (~180–220 W/m²). Server rooms, gyms, and commercial kitchens need 35–45 BTU/hr per sqft (~350–450 W/m²). The calculator above applies ceiling height, sun, windows, and occupants on top of these baselines.

How many square feet does 1 ton of AC cool?

Roughly 100–150 sqft (~9–14 m²) per ton in average US Sun Belt conditions. The right answer varies 3× by climate region, ceiling height, and sun exposure. Don't rely on a single number — calculate it with the tool above.

Is this Manual J compliant?

This is a simplified Manual-J-inspired field calculator. It applies Manual J's heat-gain categories (envelope, occupants, appliances, equipment, ventilation) with condensed factors suitable for residential field estimates. For commercial, hospital, or clean-room projects, a full ACCA Manual J from a licensed engineer is required.

What about inverter vs fixed-speed AC?

The BTU load you calculate is the same — but with an inverter AC you can right-size (e.g. a 1.5 TR inverter on a 1.2 TR load), because the inverter modulates down. With a fixed-speed AC, always round UP to the next standard size.

Do I need to consider altitude?

Yes, but the air-density effect is small below 5,000 ft (1,500 m). Above that, AC sensible capacity drops roughly 3–4% per 1,000 ft of additional altitude. Our cool-mountain factor (0.70) captures most of it.

Can I use this for a VRF / VRV system?

Yes for the indoor unit sizing. For VRF total capacity and refrigerant pipe sizing, use the manufacturer's selection software (Daikin, Mitsubishi, LG, etc.) with a full Manual J load summary per indoor unit.

What size air conditioner do I need for a 1,500 sq ft house?

A 1,500 sq ft (≈140 m²) home typically needs 30,000–45,000 BTU/hr (2.5–3.5 tons / 9–13 kW). Hot US Sun Belt and southern-Europe homes land at the top of that range; mild marine climates (Pacific NW, UK, northern Europe) at the bottom. Ceiling height, insulation, sun exposure and window area shift it 30% either way — enter your details in the calculator above for an exact figure.

What size air conditioner do I need for a 12×12 room?

A 12×12 ft room is 144 sq ft (≈13 m²) and usually needs about 5,000–6,000 BTU/hr (roughly 0.5 ton / 1.5 kW) — a standard small window or portable unit. Add ~10–20% for a top-floor room, west-facing sun, or a kitchen next door.

How many kW of cooling do I need for 50 m²?

For a 50 m² (≈540 sq ft) space, budget roughly 5 kW in a mild UK/northern-EU climate and up to 6.5 kW in a hot southern-EU or Australian climate (about 100–130 W/m²). Switch the calculator above to Metric to size it in m² and kW directly.

What SEER or SEER2 rating should I choose (US)?

SEER2 is efficiency, not size — it doesn't change the BTU load you calculate here, but it changes running cost. US minimums are SEER2 13.4 (North) / 14.3 (South) as of 2023. For the best balance of price and energy savings, most homeowners pick a 15–18 SEER2 inverter/variable-speed unit; ENERGY STAR models start around SEER2 15.2.

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Last updated: 2026. Maintained by the Refcon HVAC engineering team. Found an error or have a suggestion? Tell us.

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