Engineering Guide

HVAC Engineer's Unit Conversion Cheat Sheet: BTU, kW, Ton, CFM, ACH

Published June 12, 2026 · by Industrial Unit Converter Editorial Team

Why HVAC conversion fluency matters

HVAC design pulls from three different unit systems daily: US imperial (BTU, tons, CFM, FPM), SI metric (kW, m³/h, m/s), and hybrid conventions (ACH, wet bulb, EER, COP). Mixing up these systems during equipment selection, energy modeling, or commissioning is one of the most common — and costly — engineering errors.

Consider a chiller bid: a US manufacturer quotes 50 tons, a European competitor quotes 176 kW. Are they the same equipment? Yes — but only if you know 1 ton = 3.517 kW. A designer who can't make this conversion instantly might spec mismatched cooling towers, pumps, and electrical services.

This cheat sheet covers the everyday conversions organized by the task at hand, with worked examples and the underlying physics where it matters.

Thermal power: BTU/hr ↔ kW ↔ Refrigeration Tons

These three units define every heating and cooling capacity specification. You'll use at least one of these conversions on every project.

From	To	Multiply by	Quick mental shortcut
BTU/hr	kW	0.000293	Divide by 3,412
kW	BTU/hr	3,412	A 10 kW heater = 34,120 BTU/hr
Tons (RT)	kW	3.517	1 ton = 12,000 BTU/hr = 3.517 kW
kW	Tons	0.2843	Divide by 3.517
BTU/hr	Watts	0.2931	A 1,000 BTU/hr baseboard = 293 W
EER	kW/ton	12 ÷ EER	An EER 12 unit = 1.0 kW/ton

Common equipment sizes to memorize:

Capacity	BTU/hr	Tons	kW
Residential window AC	12,000	1	3.52
Small commercial rooftop	60,000	5	17.6
Medium commercial	120,000	10	35.2
Large commercial boiler	500,000	41.7	146.6
Industrial chiller	3,000,000	250	879

Worked example: comparing US and European chiller bids

You receive two proposals for a new chiller plant:

Bid A (US): 150 tons at 0.62 kW/ton
Bid B (EU): 528 kW at EER 19.4

Are they equivalent?

Bid A: 150 tons × 3.517 = 527.6 kW, and 150 × 0.62 = 93.0 kW input Bid B: 528 kW capacity, and 528 ÷ 19.4 = 27.2 kW input

Both provide ~528 kW of cooling, but Bid B uses only 29% of the input power — a dramatically more efficient machine. Without the tons-to-kW conversion, you might have overlooked this entirely.

Use our BTU/hr to kW Calculator for any combination of these units, and Refrigeration Tons to kW for chiller comparisons.

Airflow: CFM ↔ m³/h ↔ ACH ↔ Velocity

Airflow conversions connect duct sizing, ventilation code compliance, and equipment specifications across US and metric systems.

From	To	Multiply by	Typical context
CFM	m³/h	1.699	European AHU catalog comparison
m³/h	CFM	0.5886	US equipment from metric specs
CFM	L/s	0.4719	Laboratory fume hoods
CFM	L/min	28.32	Medical gas, process air
ACH	CFM	(ACH × Room ft³) ÷ 60	Ventilation design
CFM	ACH	(CFM × 60) ÷ Room ft³	Code compliance check

Typical ACH guidelines by space type (commonly referenced in industry practice, derived from ASHRAE 62.1 ventilation rates and other standards):

Space Type	Recommended ACH	Notes
Offices	4-6	Based on occupant density
Classrooms	5-7	Higher for younger children
Laboratories	6-12	Depends on fume hood count
Hospital OR	15-20	Positive pressure, HEPA filtered
Cleanroom ISO 7	60+	Entire volume exchanged per minute

Worked example: sizing ventilation for a classroom

A 900 ft² classroom with 9 ft ceilings (8,100 ft³) requires 6 ACH per ASHRAE 62.1.

Required CFM = (ACH × ft³) ÷ 60 = (6 × 8,100) ÷ 60 = 810 CFM

If the spec called for 15 CFM per person for a classroom of 25 students, that would be 375 CFM — less than half of what the ACH method requires. Always calculate both methods and use the larger value.

Use our ACH to CFM Calculator and Air Changes Per Hour Calculator for direct room-by-room calculations. For duct velocity conversions, see our m/s to FPM Converter.

Sensible and latent heat: the split every load calculation needs

Cooling load is always the sum of two components. Treating them as one number is the most common cause of uncomfortable, clammy spaces.

Total Cooling = Sensible Heat + Latent Heat

Component	Formula	What it handles	Typical share
Sensible (Qs)	1.08 × CFM × ΔT (BTU/hr)	Temperature change (dry bulb)	70% in dry climates
Latent (Ql)	0.68 × CFM × ΔW (BTU/hr)	Moisture removal (condensation)	30% in humid climates
Total (Qt)	4.5 × CFM × Δh (BTU/hr)	Combined sensible + latent	100%
Sensible (SI)	1.23 × L/s × ΔT (Watts)	Temperature change (metric)	—
Latent (SI)	3.0 × L/s × ΔW (Watts)	Moisture removal (metric)	—

Where: ΔT = dry bulb temperature difference (°F or °C), ΔW = humidity ratio difference (grains/lb or g/kg), Δh = enthalpy difference (BTU/lb or kJ/kg).

Worked example: classroom in Miami vs Denver

A 900 ft² classroom needs 810 CFM (from the example above). Design supply air is 55°F, return air is 75°F (ΔT = 20°F).

Sensible load: Qs = 1.08 × 810 × 20 = 17,496 BTU/hr

Now for latent load. In Miami (85 gr/lb outdoor, 65 gr/lb return, ΔW = 20): Ql = 0.68 × 810 × 20 = 11,016 BTU/hr Total = 17,496 + 11,016 = 28,512 BTU/hr (39% latent)

In Denver (30 gr/lb outdoor, 65 gr/lb return, ΔW = -35 — actually adding moisture): Ql = 0.68 × 810 × (-35) = -19,278 BTU/hr (humidification needed) Total = 17,496 + (-19,278) = net different profile entirely

The same classroom in two climates has fundamentally different equipment requirements. A Miami engineer who ignores latent load spec's a coil that hits temperature but leaves the space at 70% RH.

Use our Sensible Heat Calculator and Latent Heat Calculator for automated psychrometric splits.

CFM per ton: the most abused rule of thumb in HVAC

The "400 CFM per ton" rule is useful as a starting point — but dangerous when applied blindly.

Application	Typical CFM/ton	Why
Comfort cooling (standard)	350-400	55°F supply air, 75°F return, ΔT ≈ 20°F
High latent load (humid)	300-350	Deeper coil for more dehumidification
Dry climate (sensible only)	400-450	Less moisture to remove
Data center / process	200-300	High ΔT design, sensible-only load
Hospital OR	15-20 ACH drives	Ventilation-driven, not load-driven
100% outdoor air (DOAS)	Varies widely	Depends on outdoor conditions and energy recovery

The physics behind the rule:

CFM/ton derives from Q = 1.08 × CFM × ΔT, where Q = 12,000 BTU/hr per ton:

CFM/ton = 12,000 ÷ (1.08 × ΔT)

At ΔT = 20°F: CFM/ton = 12,000 ÷ (1.08 × 20) = 556 (not 400!)

The 400 CFM/ton number actually assumes a ΔT closer to 28°F, or it bundles in some latent capacity. At 16°F ΔT (chilled beam or radiant system), it becomes 694 CFM/ton. The rule only works when your system design matches the assumption behind it.

Use our CFM per Ton Calculator for load-specific numbers. For complete load calculation methodology, see our HVAC Load Calculation Guide.

Wet bulb, dry bulb, and psychrometric conversions

Psychrometric calculations often require wet bulb temperature as input, but field instruments frequently measure relative humidity instead.

When the conversion matters:

Cooling tower selection: Tower approach is based on the difference between leaving water temperature and ambient wet bulb, not dry bulb or RH
Evaporative cooling design: Wet bulb depression (dry bulb minus wet bulb) determines the maximum achievable temperature drop
ASHRAE 55 comfort compliance: Uses operative temperature and humidity ratio, both derived from psychrometric state points
Energy recovery: Enthalpy wheels and ERV effectiveness curves reference wet bulb conditions

Condition	Dry Bulb	Wet Bulb	RH	Dew Point
Phoenix summer	110°F	65°F	5%	35°F
Miami summer	92°F	80°F	60%	76°F
Office (design)	75°F	62.5°F	50%	55°F
Data center (supply)	72°F	58°F	45%	50°F

Use our Wet Bulb to Relative Humidity Calculator for quick conversions without a psychrometric chart.

Pressure and duct static conversions

From	To	Multiply by	Use case
in H₂O	psi	0.0361	Duct static to pressure
in Hg	psi	0.4912	Vacuum and refrigerant
Pa	in H₂O	0.00402	European fan curves to US
psi	in H₂O	27.68	Refrigerant pressure to airflow equivalent
kPa	in H₂O	4.02	SI fan curves to US duct design

Fan power is proportional to CFM × static pressure. Overestimating duct static by just 0.5 in H₂O over-specs the fan motor by 20% — a common cause of oversized VFDs and unnecessary energy consumption.

For cleanroom and laboratory pressurization calculations, see our Static Pressure to CFM Calculator. For the full treatment of pressure units across all engineering domains, read our Pressure Units: A Practitioner's Guide.

Frequently asked questions

Q: Why is a refrigeration ton 12,000 BTU/hr?

One ton of refrigeration is the cooling rate required to freeze one short ton (2,000 lb) of water at 32°F into ice at 32°F in 24 hours. The latent heat of fusion for water is 144 BTU/lb, so: 2,000 lb × 144 BTU/lb ÷ 24 hours = 12,000 BTU/hr. This dates from the ice-house era when cooling was measured in tons of ice consumed per day.

Q: When should I use EER vs COP vs kW/ton?

EER (BTU/hr per watt) and COP (kW out per kW in) measure the same thing in different units: COP = EER ÷ 3.412. kW/ton is the inverse — lower is better. Use EER for unitary equipment (rooftops, splits), COP for chillers and heat pumps (especially in SI markets), and kW/ton for chiller plant efficiency comparisons.

Q: How do I convert CFM to kg/s for mass flow calculations?

First convert CFM to m³/s (CFM × 0.0004719), then multiply by air density (typically 1.2 kg/m³ at 20°C sea level). For precise work, correct the density for actual temperature and altitude — our CFM to m³/h Converter and SCFM to ACFM Calculator handle the density corrections.

Q: Can I use the 400 CFM/ton rule for a VRF system?

No. VRF systems typically operate at higher ΔT and variable airflow. A VRF indoor unit at part load might deliver 250-300 CFM/ton while still meeting the load. Always use manufacturer selection software for VRF — the rules of thumb for constant-volume systems don't apply.

Q: What's the difference between ACH and CFM/ft²?

ACH is volumetric — it exchanges a fixed percentage of room volume per hour regardless of ceiling height. CFM/ft² is area-based — it supplies the same airflow regardless of ceiling height. A warehouse with 30 ft ceilings needs dramatically different CFM under each method. Always cross-check both.

The 5 conversions every HVAC engineer should memorize

BTU/hr → kW: ÷ 3,412 (or × 0.000293)
Tons → kW: × 3.517 (1 ton = 12,000 BTU/hr)
CFM → m³/h: × 1.699
ACH → CFM: (ACH × ft³) ÷ 60
m/s → FPM: × 196.85