BTU Calculator

🧮 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



sq ft

Range: 50 – 10,000 sq ft



ft

Range: 7 – 20 ft



people

Range: 0 – 50 people

🌡️3. Climate & Location


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)
▼




⚠️ 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

  1. What is a BTU, really?
  2. Why correct sizing matters
  3. Manual J in plain English
  4. Why BTU/sqft rules of thumb fail
  5. Climate multipliers (US, UK, EU & AU)
  6. Field tips for technicians
  7. Superheat & subcooling quick-ref
  8. Common sizing mistakes
  9. 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:

  1. Conduction through walls, roof, floor, glass
  2. Radiation through glass (solar gain)
  3. Infiltration (leakage through doors & windows)
  4. Ventilation (fresh-air intake)
  5. Internal gains from occupants (~600 BTU/hr each, sedentary)
  6. Appliances & lighting
  7. Equipment & machinery
  8. 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

  1. “Customer wants it cold fast, give 2 TR.” Oversizing kills humidity removal and shortens compressor life. Always size by load, not by comfort demand.
  2. Ignoring ceiling height. A 12 ft ceiling needs 40% more cooling than an 8 ft ceiling of the same footprint.
  3. Forgetting the kitchen. Cooking adds ~4,000 BTU/hr (1.2 kW) per burner in most residential kitchens.
  4. Not counting occupants. A home gym with 4 people generates 2,400 BTU/hr just from bodies.
  5. 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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