To convert BTU to CFM for cooling, divide the BTU/h by 30 — that's the quick formula. A 12,000 BTU (1-ton) air conditioner needs 400 CFM of airflow, a 36,000 BTU (3-ton) system needs 1,200 CFM, and a 60,000 BTU (5-ton) system needs 2,000 CFM. This relationship between thermal capacity and airflow is the foundation of every duct sizing calculation, and getting it wrong means undersized ducts, uncomfortable rooms, and wasted energy.
The Core Formula: BTU to CFM Conversion
The relationship between BTU and CFM is governed by the sensible heat equation from thermodynamics:
Q = 1.08 x CFM x DeltaT
Where Q is the sensible heat in BTU/h, CFM is the airflow in cubic feet per minute, and DeltaT is the temperature difference between supply and return air in degrees Fahrenheit.
Rearranging to solve for CFM:
CFM = Q / (1.08 x DeltaT)
The constant 1.08 comes from the specific heat of air (0.24 BTU per lb per degree F) multiplied by the density of air at standard conditions (0.075 lb/ft3) multiplied by 60 minutes per hour, which equals 1.08.
Where the "400 CFM Per Ton" Rule Comes From
For standard air conditioning, the typical temperature drop across the evaporator coil (DeltaT) is about 20 degrees F. Plugging that into the formula:
CFM = 12,000 BTU/h / (1.08 x 20) = 12,000 / 21.6 = 556 CFM
Wait — that gives 556 CFM, not 400. What happened?
The answer is that the total cooling capacity (12,000 BTU) includes both sensible cooling (lowering temperature) and latent cooling (removing moisture). Roughly 70–75% of total capacity is sensible in a typical application. So: 12,000 x 0.75 = 9,000 BTU/h sensible, and 9,000 / (1.08 x 20) = 417 CFM — close to the 400 CFM standard.
The 400 CFM per ton rule assumes a sensible heat ratio (SHR) of 0.74 and a 20 degree F DeltaT — conditions that are typical for moderate climates. But these assumptions don't hold everywhere.
The 400 CFM/ton rule is an approximation, not a universal law. Actual airflow requirements vary from 325–450 CFM/ton depending on climate, SHR, coil design, and the specific equipment manufacturer's requirements. Always check the manufacturer's installation manual for the correct airflow specification for your system.
BTU to CFM Conversion Chart
This chart covers the most common HVAC system sizes with airflow requirements for both cooling and heating modes.
Heating Mode: Different CFM Requirements
Heating has different airflow requirements than cooling. For a gas furnace, the DeltaT is much higher (typically 40–70 degrees F rise) but CFM is lower. For a heat pump, the DeltaT is lower (typically 15–30 degrees F) and CFM stays similar to cooling mode.
The critical check for combined systems: If you have an air conditioner or heat pump paired with a gas furnace (dual-fuel system), the duct system must handle the higher of the two CFM requirements. Cooling almost always requires more CFM than gas heating, so duct sizing is typically driven by the cooling load. But always verify both.
Room-by-Room BTU to CFM Calculation
To size ductwork for individual rooms, you need each room's BTU load (from a Manual J calculation) and then convert to CFM.
Step 1: Determine Room BTU Load
A proper Manual J load calculation accounts for room size, orientation, insulation levels, window area and type, internal heat gains, and climate zone. Here are typical ranges for common rooms in a moderately insulated 2,000 sq ft home.
Step 2: Apply the Formula
For each room, use the sensible cooling load (total cooling load multiplied by SHR):
Room CFM = (Room Cooling BTU/h x SHR) / (1.08 x DeltaT)
Example calculation — Master Bedroom:
Room cooling load: 5,000 BTU/h (from Manual J). SHR: 0.75 (typical). DeltaT: 20 degrees F.
CFM = (5,000 x 0.75) / (1.08 x 20) = 3,750 / 21.6 = 174 CFM
Using the ductwork sizing chart, 174 CFM requires a 7-inch rigid duct or 8-inch flex duct. If you split into two registers for better distribution, each branch needs about 87 CFM (6-inch rigid each).
How Climate Zone Affects the BTU-to-CFM Relationship
The 400 CFM/ton standard works well in ASHRAE Climate Zones 3–5 (the middle of the country). But in very humid climates or very dry climates, the optimal airflow changes significantly.
Humid Climates (Southeast, Gulf Coast): Lower CFM per Ton
In humid climates, a larger percentage of the cooling load is latent (moisture removal). Running lower airflow (325–375 CFM/ton) across the evaporator coil allows the coil to get colder, which condenses more moisture from the air. The tradeoff is slightly less sensible cooling capacity, but in a humid climate, dehumidification matters more.
Dry Climates (Southwest, Mountain West): Higher CFM per Ton
In dry climates, almost all the cooling load is sensible (temperature reduction). Running higher airflow (425–450 CFM/ton) maximizes sensible capacity because the coil doesn't need to be as cold — there's minimal moisture to remove. This also improves efficiency.
Many modern variable-speed systems adjust airflow automatically based on conditions. A Carrier Infinity or Trane XV20i, for example, can modulate from 250–500 CFM/ton to optimize both temperature and humidity control. If you have a variable-speed system, the manufacturer's settings take priority over these general guidelines. Your ductwork still needs to handle the maximum airflow the system can produce.
Real-World Examples
Example 1: Sizing Ductwork for a New 3-Ton System
Situation: A 2,000 sq ft home in Nashville (Climate Zone 4A) is getting a new 3-ton (36,000 BTU) heat pump with a variable-speed air handler. The Manual J calculation shows a total cooling load of 34,200 BTU and total heating load of 38,000 BTU.
System CFM calculation: 36,000 BTU / 12,000 = 3 tons x 400 CFM/ton = 1,200 CFM. The manufacturer specs 1,050–1,350 CFM range with 1,200 CFM nominal. The heat pump in heating mode requires the same 1,200 CFM.
Room CFM breakdown (SHR = 0.75, DeltaT = 20°F): Master bedroom: 5,200 BTU = 180 CFM = 8" rigid. Bedroom 2: 3,100 BTU = 107 CFM = 6" rigid. Bedroom 3: 2,800 BTU = 97 CFM = 6" rigid. Living/dining: 8,500 BTU = 295 CFM = 10" rigid. Kitchen: 5,600 BTU = 194 CFM = 8" rigid. Two bathrooms: 1,500 BTU each = 52 CFM each = 5" rigid. Laundry: 1,800 BTU = 62 CFM = 6" rigid. Hallway: 2,200 BTU = 76 CFM = 6" rigid.
Total room CFM: 1,115 CFM. The 85 CFM difference from system capacity accounts for duct losses. The trunk line starts at 18" (1,200 CFM) and reduces as branches peel off.
Example 2: Why Florida and Arizona Need Different Airflow
Scenario: Two identical 2,500 sq ft homes with 4-ton systems — one in Jacksonville, FL (hot-humid) and one in Tucson, AZ (hot-dry).
Jacksonville (SHR = 0.65): Total cooling: 48,000 BTU. Sensible: 31,200 BTU. Optimal CFM: 350 CFM/ton x 4 = 1,400 CFM. The lower airflow drops coil temperature to 38°F for aggressive dehumidification. Supply air: 52°F.
Tucson (SHR = 0.90): Total cooling: 48,000 BTU. Sensible: 43,200 BTU. Optimal CFM: 450 CFM/ton x 4 = 1,800 CFM. Higher airflow keeps the coil at 48°F — no need for deep dehumidification. Supply air: 55°F.
Duct sizing impact: Tucson needs trunk ductwork for 1,800 CFM (20"+ round) while Jacksonville needs only 1,400 CFM (18" round). Same BTU system, very different ductwork requirements.
Example 3: Ducted Mini-Split BTU to CFM Conversion
Situation: A homeowner installs a ducted mini-split for a 600 sq ft addition rated at 18,000 BTU cooling / 21,600 BTU heating.
Cooling CFM: 18,000 / 12,000 = 1.5 tons x 400 = 600 CFM. The manufacturer (Mitsubishi SEZ-KD18) specifies 520–600 CFM on high speed.
Duct sizing: Main supply trunk: 12"–14" round. Three branches at 200 CFM each: 8" rigid. Single return at 600 CFM: 14" round or 10" x 14" rectangular.
Critical caveat: Ducted mini-splits typically have only 0.20–0.40 IWC external static pressure (versus 0.50 for conventional systems). Keep total equivalent duct length under 50 feet, or upsize ducts by one diameter to reduce friction loss.
Example 4: Gas Furnace Temperature Rise Check
Situation: A homeowner wonders if their existing ductwork can handle a new 100,000 BTU (96% AFUE) gas furnace paired with a 4-ton AC.
Cooling CFM: 4 tons x 400 = 1,600 CFM. Ducts must handle 1,600 CFM.
Heating check: Furnace output = 100,000 x 0.96 = 96,000 BTU/h. Data plate temperature rise: 35–65°F.
At 1,600 CFM (cooling airflow): Temperature rise = 96,000 / (1.08 x 1,600) = 55.6°F. This is within the 35–65°F range — the existing ductwork works for both modes.
If the furnace required higher CFM (unlikely with gas, but possible with electric backup), the ducts would need to be sized for that higher number.
The Sensible Heat Ratio: The Missing Variable
Most homeowners overlook the sensible heat ratio (SHR) when converting BTU to CFM. The SHR tells you what percentage of total cooling is temperature reduction (sensible) versus moisture removal (latent).
The range is dramatic: 333 CFM in an extremely humid climate to 528 CFM in a very dry climate — a 59% difference for the same 12,000 BTU system. This is why "400 CFM per ton" is just an approximation.
BTU to CFM Calculator
How to Use the Calculator
Step 1: Enter your system's total cooling capacity in BTU/h (or select the tonnage).
Step 2: Choose your climate zone or manually enter the SHR.
Step 3: Optionally enter the design DeltaT (default is 20°F for cooling).
Step 4: The calculator outputs total system CFM and recommended main trunk duct size. For room-by-room sizing, enter individual room BTU loads to get branch duct sizes.
Altitude Correction Factor
At higher elevations, air is less dense. The 1.08 constant in the sensible heat formula decreases, meaning you need more CFM to move the same amount of heat. This is a critical consideration for mountain and high-plateau regions.
Example: Denver at 5,280 feet
A 3-ton system at sea level needs 1,200 CFM. At Denver's altitude, it needs 1,200 x 1.20 = 1,440 CFM. That means the main trunk that would be 16"–18" at sea level needs to be 18"–20" in Denver. Ignoring altitude correction is one of the most common duct sizing errors in mountain communities.
Advanced: CFM Requirements for Heating
Gas Furnace CFM
Gas furnaces have a specified temperature rise range on the data plate (e.g., 40–70°F). The airflow must produce a temperature rise within this range. Too little airflow means excessive rise, which can crack the heat exchanger. Too much airflow means insufficient rise, producing cool drafts.
Furnace CFM = BTU output / (1.08 x DeltaT)
Example: 80,000 BTU input, 96% AFUE gas furnace
Output: 80,000 x 0.96 = 76,800 BTU/h. Temperature rise range: 40–70°F.
At 40°F rise: CFM = 76,800 / (1.08 x 40) = 1,778 CFM (maximum airflow). At 70°F rise: CFM = 76,800 / (1.08 x 70) = 1,016 CFM (minimum airflow).
The system operates anywhere in the 1,016–1,778 CFM range. If the paired AC requires 1,600 CFM, the furnace temperature rise is: 76,800 / (1.08 x 1,600) = 44.4°F — within the 40–70°F range. Ductwork is sized for 1,600 CFM (the cooling requirement).
Heat Pump CFM in Heating Mode
Heat pumps in heating mode produce much lower temperature rise than gas furnaces — typically 15–30°F depending on outdoor temperature. CFM requirements are similar to cooling mode. Most manufacturers specify the same airflow for both modes.
When auxiliary electric strip heat activates, combined heat output increases and may need more airflow. Check manufacturer specs for auxiliary heat CFM — if it exceeds cooling airflow, size your ducts for the higher number.
Common Mistakes in BTU-to-CFM Conversions
Mistake #1: Using total BTU instead of sensible BTU. The formula uses sensible heat only. Using total BTU overstates CFM by 15–40% depending on SHR, leading to oversized ducts and wasted materials.
Mistake #2: Ignoring heating mode requirements. If your heating system requires more CFM than cooling (possible with large electric furnaces), the ductwork must be sized for the heating number.
Mistake #3: Using nominal system size instead of actual load. A "3-ton" system doesn't necessarily need exactly 1,200 CFM. The Manual J load determines actual BTU, and CFM follows from that.
Mistake #4: Not correcting for altitude. At 5,000 feet, you need roughly 20% more CFM than sea level for the same BTU load. Denver needs bigger ducts than Miami for the same system.
Mistake #5: Forgetting fan heat. The blower motor adds 2,000–5,000 BTU/h to the airstream. This is accounted for in manufacturer ratings but can throw off hand calculations if you're starting from scratch.
Quick Reference: BTU to CFM Shortcut Table
For fast estimates when you don't want to calculate SHR and DeltaT, use these shortcuts.
The divide-by-30 shortcut for cooling is the most useful number to memorize. Room needs 6,000 BTU? Divide by 30 = 200 CFM = 8" rigid duct. System is 48,000 BTU? Divide by 30 = 1,600 CFM = 18"–20" trunk. It's not perfectly accurate for extreme climates, but it gets you in the right ballpark fast.
Key Takeaways
- 400 CFM per ton is the standard rule for cooling — but actual requirements range from 325–450 CFM/ton depending on climate and equipment.
- The quick formula: divide cooling BTU by 30 to get CFM. Divide by 35 for humid climates, 25 for dry climates.
- Sensible heat ratio (SHR) is the key variable most people miss. In humid climates (SHR 0.60–0.70), you need less CFM per ton. In dry climates (SHR 0.85–0.95), you need more.
- Gas furnaces use less CFM than cooling — always size ducts for the higher requirement (usually cooling).
- Heat pumps need similar CFM for heating and cooling — check auxiliary heat requirements separately.
- Altitude matters. At 5,000 feet, add 20% to your CFM requirement. Mountain communities need bigger ducts.
- Always check manufacturer specs — they override general rules. Variable-speed systems adjust airflow automatically but ducts must handle max capacity.
- Manual J + Manual D together give you the correct answer. Manual J determines room BTU loads; the BTU-to-CFM conversion translates those to airflow; Manual D sizes the ducts for that airflow.