HVAC Unit Converter Guide

1. Introduction to HVAC Unit Conversion

HVAC engineering operates across multiple unit systems — Imperial (IP) units are standard in the United States, while SI (metric) units dominate everywhere else. Even within SI, different regions prefer different derived units: China commonly uses W/m² for load indices and Pa for pressure, Europe uses kW and kPa, and Japan uses both kJ/h and W depending on the application context.

Accurate unit conversion is not a clerical afterthought — it is a critical engineering skill. A misapplied conversion factor can lead to undersized ductwork, oversized chillers, or incorrectly selected pumps. For example, confusing CFM with m³/h by a factor of 1.7× can produce a duct that is either too restrictive (causing high fan energy) or too large (adding unnecessary material cost).

This guide provides the conversion factors every HVAC professional needs, organized by physical quantity, along with real-world context for when and why each unit is used.

2. Power Units — Watt (W), Kilowatt (kW), BTU per Hour (BTU/h)

Power units describe the rate of energy transfer — the fundamental metric for heating and cooling capacity, compressor power, fan motor ratings, and pump power consumption. Getting these conversions right is essential for equipment selection, energy audits, and compliance with building energy codes.

From	To	Conversion Factor	Example
BTU/h	kW	÷ 3412	12,000 BTU/h = 3.52 kW
kW	BTU/h	× 3412	1 kW = 3,412 BTU/h
BTU/h	W	× 0.2931	10,000 BTU/h = 2,931 W
W	BTU/h	× 3.412	1,000 W = 3,412 BTU/h
Refrigeration Ton (RT)	BTU/h	× 12,000	1 RT = 12,000 BTU/h
Refrigeration Ton (RT)	kW	× 3.517	1 RT = 3.517 kW
hp (mechanical)	kW	× 0.7457	1 hp = 0.746 kW
kJ/h	kW	÷ 3600	3,600 kJ/h = 1 kW

When to use each unit: BTU/h is the standard capacity rating for residential and light commercial HVAC equipment in North America — a typical window AC is rated at 5,000–12,000 BTU/h. kW is the universal SI standard for equipment capacity and electrical power input worldwide. Refrigeration tons (RT) are used for larger commercial chillers, with 1 RT representing the cooling effect of melting one short ton of ice over 24 hours. Horsepower (hp) is still found on older fan and pump motor nameplates in IP regions.

Practical Note: Refrigeration Ton vs Actual Power

A common point of confusion is that 1 RT equals 3.517 kW of cooling capacity, but the electrical input power to the chiller compressor is much lower — typically 0.8–1.2 kW per RT depending on the chiller efficiency (COP). Do not confuse capacity (output) with power consumption (input).

3. Air Flow Units — m³/h, CFM, L/s

Air volume flow rate is the fundamental quantity for duct sizing, fan selection, ventilation design, and AHU (Air Handling Unit) specification. Three units dominate the industry: CFM (cubic feet per minute) in North America, m³/h in China and most of Asia, and L/s in European ventilation standards.

From	To	Conversion Factor	Example
CFM	m³/h	× 1.699	1,000 CFM = 1,699 m³/h
m³/h	CFM	÷ 1.699	1,000 m³/h = 588.6 CFM
CFM	L/s	× 0.4719	1,000 CFM = 471.9 L/s
L/s	CFM	× 2.119	500 L/s = 1,059.5 CFM
m³/h	L/s	÷ 3.6	3,600 m³/h = 1,000 L/s
L/s	m³/h	× 3.6	100 L/s = 360 m³/h

Duct sizing context: Air velocity in ducts is typically designed between 3–8 m/s (600–1,600 fpm) for main ducts and 2–5 m/s (400–1,000 fpm) for branch ducts. The cross-sectional area of a duct is calculated as Area = Flow Rate / Velocity. Using consistent units throughout this calculation is critical — mixing CFM with m/s will produce incorrect duct dimensions. Our unit converter handles all three air flow units plus velocity and area conversions in a single tool.

4. Pressure Units — Pa, kPa, bar, psi, inH₂O, mmH₂O

Pressure measurements in HVAC span a wide range: from low differential pressures across filters and coils (measured in Pa or inH₂O) to pump discharge pressures and refrigerant system pressures (measured in bar or psi). Each application domain favors specific units, making conversion between them a daily requirement.

From	To	Conversion Factor	Example
Pa	kPa	÷ 1000	100,000 Pa = 100 kPa
Pa	bar	÷ 100,000	100,000 Pa = 1 bar
Pa	psi	÷ 6894.76	6895 Pa ≈ 1 psi
Pa	inH₂O	÷ 249.089	249 Pa ≈ 1 inH₂O
Pa	mmH₂O	÷ 9.80665	98.1 Pa ≈ 10 mmH₂O
psi	bar	÷ 14.504	14.5 psi ≈ 1 bar
inH₂O	Pa	× 249.089	1 inH₂O = 249.09 Pa
mmH₂O	Pa	× 9.80665	10 mmH₂O = 98.07 Pa

Fan and pump applications: Fan total pressure is typically specified in Pa (SI) or inH₂O (IP). A typical rooftop unit fan develops 250–750 Pa (1–3 inH₂O) of static pressure, while large centrifugal fans for central station AHUs may develop up to 2,500 Pa (10 inH₂O). Pump head, when expressed in pressure terms rather than meters of head, appears in kPa or bar. For chilled water pumps, a common design head is 150–300 kPa (1.5–3 bar or 22–43 psi). Conversion errors in pressure units directly affect pump and fan power calculations since fan power is proportional to pressure differential.

5. Temperature Conversion — °C, °F, K

Temperature is perhaps the most frequently converted quantity in HVAC work — design conditions, setpoints, supply air temperatures, and weather data all appear in different units depending on the region and standard. While the conversion formulas are simple, the practical applications require understanding which temperature scale is appropriate for which context.

Conversion	Formula	Example
°C → °F	°F = (°C × 9/5) + 32	25°C = 77°F
°F → °C	°C = (°F − 32) × 5/9	77°F = 25°C
°C → K	K = °C + 273.15	25°C = 298.15 K
K → °C	°C = K − 273.15	300 K = 26.85°C
°F → K	K = (°F + 459.67) × 5/9	77°F = 298.15 K
K → °F	°F = K × 9/5 − 459.67	300 K = 80.33°F

Engineering design conditions: In North American practice, design conditions are typically specified in °F — e.g., summer outdoor design 95°F dry bulb / 78°F wet bulb. In SI regions, the same conditions are 35°C / 25.6°C. Kelvin (K) is used for thermodynamic calculations such as Carnot cycle efficiency, refrigerant property lookups, and psychrometric analysis where absolute temperature ratios are required. A temperature difference of 1°C equals 1 K — this is important because when converting a ΔT (e.g., a 10°C temperature rise), the numerical value is the same in °C and K, but different in °F (18°F). Always distinguish between absolute temperature values and temperature differences in your conversions.

6. Efficiency Units — COP, EER, SEER

Energy efficiency metrics for HVAC equipment appear in three primary forms: COP (Coefficient of Performance), EER (Energy Efficiency Ratio), and SEER (Seasonal Energy Efficiency Ratio). These metrics are used globally but often confused because they measure similar yet distinct performance characteristics.

Metric	Definition	Units	Typical Range
COP	Output (W) / Input (W)	Dimensionless	2.5–6.0 (cooling), 3.0–5.0 (heat pump heating)
EER	Output (BTU/h) / Input (W)	BTU/h·W⁻¹	8–15 (typical), ≥ 11 (efficient)
SEER	Seasonal average of EER	BTU/h·W⁻¹	13–28+ (modern systems)

Conversion relationships: Since 1 W = 3.412 BTU/h, the relationship between COP and EER is straightforward: COP = EER / 3.412, and correspondingly EER = COP × 3.412. For example, a chiller with EER 12 has a COP of 12 / 3.412 ≈ 3.52. SEER is a seasonally weighted average of EER under standardized test conditions defined by AHRI (Air-Conditioning, Heating, and Refrigeration Institute). While SEER and EER are numerically similar for the same equipment, SEER is always higher than EER because it accounts for part-load conditions where efficiency is better.

Chiller and heat pump applications: Chiller efficiency is often specified in kW/RT (kilowatts per refrigeration ton) — this is the inverse of COP. A chiller with COP 6.0 consumes 1 kW/RT ÷ 6.0 × 3.517 = 0.586 kW/RT. Minimum efficiency standards vary by region: US DOE requires chiller COP ≥ 6.1 (full load) for new centrifugal chillers, while the EU Ecodesign directive mandates minimum EER values depending on chiller type and capacity.

7. Common Conversion Mistakes

Even experienced HVAC professionals occasionally slip on unit conversions. Here are the most frequent pitfalls encountered in practice:

1. Confusing refrigeration tons with electrical power. 1 RT = 3.517 kW of cooling capacity, but the compressor electrical input is much lower (typically 0.8–1.2 kW/RT). Always check whether a specification refers to cooling capacity or power input.
2. Using the wrong CFM-to-m³/h direction. The factor 1.699 works in one direction only. A common error is dividing instead of multiplying: 1,000 CFM × 1.699 = 1,699 m³/h (correct), but 1,000 ÷ 1.699 = 588.6 m³/h (too low by factor 2.89).
3. Confusing atmospheric pressure with gauge pressure. Standard atmospheric pressure is 101.325 kPa, 1.01325 bar, or 14.696 psi. Fan static pressure ratings are gauge pressure (relative to atmosphere), while refrigerant pressure specifications are often absolute. Failing to account for this difference can shift pressure readings by 100 kPa.
4. Converting temperature differences using absolute formulas. A ΔT of 10°C equals 18°F — NOT (10 × 9/5 + 32) = 50°F. For temperature differences, use Δ°F = Δ°C × 9/5 only (no +32 offset). The same principle applies converting ΔT to Kelvin: Δ°C = ΔK.
5. Mixing IP and SI units in duct sizing calculations. Using CFM for flow rate with m/s for velocity without converting will produce incorrect duct areas. Always convert all inputs to a consistent unit system before performing calculations.
6. Assuming SEER equals EER. SEER is a seasonal average under part-load test conditions, while EER is a single-point full-load rating. For the same equipment, SEER is typically 10–20% higher than EER. Using SEER values in place of EER for peak-load calculations will underestimate energy consumption.
7. Rounding conversion factors too aggressively. Using 1 inH₂O = 250 Pa (instead of 249.089) creates a 0.37% error that is acceptable for rough estimation. But using 1 psi = 7 kPa (instead of 6.895) introduces a 1.5% error that compounds across multiple conversion steps. Use the full precision factors for final engineering calculations.

8. Frequently Asked Questions

How do I convert BTU/h to kW?

To convert BTU/h to kW, divide the BTU/h value by 3412. For example, 12,000 BTU/h ÷ 3412 = 3.52 kW. This conversion is essential when comparing equipment specifications from US manufacturers (who rate capacity in BTU/h) with international standards that use kW. The reverse conversion is kW × 3412 = BTU/h.

What is the difference between CFM and m³/h?

CFM (cubic feet per minute) and m³/h (cubic meters per hour) both measure air volume flow rate. The conversion factor is 1 CFM = 1.699 m³/h. Alternatively, 1 m³/h = 0.5886 CFM. CFM is the standard air flow unit in North America, while m³/h is used in most other regions including China and Europe. Duct sizing calculations require consistent use of one unit throughout to avoid sizing errors.

How do I convert between inH₂O and Pa?

To convert inches of water gauge (inH₂O) to Pascals (Pa), multiply by 249.089. For example, 1 inH₂O = 249.089 Pa. The reverse is 1 Pa = 0.0040147 inH₂O. For rough estimation, you can use 1 inH₂O ≈ 250 Pa or 1 Pa ≈ 0.004 inH₂O. This conversion is commonly needed when working with fan static pressure ratings (often in inH₂O in US specs) and duct pressure drop calculations (typically in Pa).

What is the formula to convert °F to °C?

To convert Fahrenheit to Celsius: °C = (°F − 32) × 5/9. For example, 77°F is (77−32) × 5/9 = 25°C. To convert Celsius to Fahrenheit: °F = °C × 9/5 + 32. A quick reference: 0°C = 32°F (freezing), 20°C = 68°F (room temperature), 37°C = 98.6°F (body temperature), and 100°C = 212°F (boiling).

What is the relationship between COP, EER, and SEER?

COP (Coefficient of Performance) is the ratio of heating or cooling output to electrical input under specific test conditions. EER (Energy Efficiency Ratio) is BTU/h per watt, and SEER is the seasonal average of EER. The approximate conversions are: COP = EER / 3.412, EER = COP × 3.412, and SEER ≈ EER (though SEER is a seasonal rating while EER is a single-point rating). Higher values indicate greater efficiency for all three metrics.

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