HVAC Sensible Heat Hs 1.08 CFM Formula Calculator
What This Calculator Does and Why It Matters
The formula Hs = 1.08 × CFM × ΔT is one of the most commonly used equations in mechanical engineering and HVAC design. It calculates the sensible heat capacity of a moving air stream — the heat that changes temperature without changing moisture content.
This free calculator solves the formula in all three directions. You can solve for the sensible heat load (BTU/hr), the required airflow (CFM), or the expected temperature difference (ΔT). Results are automatically converted to tons of cooling and kilowatts for use across different engineering contexts.
For engineers performing full air-side load analysis, this tool pairs naturally with the humidifier steam flow sensible heat gain calculator, which handles the additional heat load introduced by steam humidification in the air stream.
How to Use This Calculator
Step-by-Step Instructions
- Choose what you want to solve for using the three mode buttons at the top: sensible heat (Hs), airflow (CFM), or temperature difference (ΔT).
- Enter the two known values for your selected mode. For example, to find Hs, enter CFM and ΔT.
- Click the Calculate button to see the result.
- Review the full output table showing BTU/hr, tons of cooling, kW, CFM, and ΔT all at once.
- Use the Reset button to clear all fields and start a new calculation.
The Formula Explained
The sensible heat formula used in HVAC is: Hs = 1.08 × CFM × ΔT. Here Hs is the sensible heat in BTU/hr, CFM is the volumetric air flow rate in cubic feet per minute, and ΔT is the temperature difference between the supply and return air in degrees Fahrenheit.
Breaking Down the Formula
The constant 1.08 is not arbitrary — it is a derived value that combines the physical properties of standard air. Specifically, it equals 60 min/hr × 0.075 lb/ft³ (air density) × 0.24 BTU/lb·°F (specific heat of dry air). This gives 60 × 0.075 × 0.24 = 1.08. The formula therefore converts a volumetric flow rate and temperature difference into a true power quantity.
According to Engineering Toolbox’s HVAC air sensible heat reference, this formula is valid at standard sea-level conditions of approximately 70°F and 29.92 inHg. At high altitudes, air density decreases and the constant must be adjusted downward.
Example Calculation with Real Numbers
An air handler supplies 3,500 CFM of air cooled from 75°F to 55°F. The temperature difference is 20°F. Sensible heat: Hs = 1.08 × 3,500 × 20 = 75,600 BTU/hr. Converting: 75,600 ÷ 12,000 = 6.3 tons of cooling. This is the sensible cooling capacity the coil must deliver for that airflow and temperature drop.
When Would You Use This
This formula is used in virtually every HVAC load calculation, equipment selection, and system commissioning task. It appears in heating coil sizing, cooling coil sizing, fan selection, duct design, and energy modeling workflows.
Real Life Use Cases
HVAC engineers use this formula when sizing air handling units, verifying the performance of existing equipment, calculating supply air quantities for zoned systems, and checking that fan capacity matches the cooling or heating load. It is also commonly used during commissioning to verify that measured airflow aligns with design intent.
Specific Example Scenario
A mechanical engineer is designing the HVAC system for a 10,000 square foot open-plan office. The cooling load calculation shows a sensible load of 120,000 BTU/hr. The supply air temperature is designed at 55°F and the space setpoint is 75°F, giving a ΔT of 20°F. Using the rearranged formula: CFM = 120,000 ÷ (1.08 × 20) = 5,556 CFM. This gives the engineer the required supply air quantity to select the proper AHU fan. For complementary duct sizing calculations, the round duct equivalent to rectangular size calculator helps translate this CFM value into a practical duct configuration.
Building energy auditors use this same formula when investigating HVAC systems. If you are working on building energy improvements, the home energy audit savings calculator can help quantify the overall efficiency improvement potential of HVAC upgrades.
Tips for Getting Accurate Results
Adjust for Altitude
The constant 1.08 assumes standard sea-level air density of 0.075 lb/ft³. At elevations above 3,000 feet, air is less dense. For example, at 5,000 feet, air density is approximately 0.0625 lb/ft³ and the effective constant drops to about 0.90. Using 1.08 at high altitudes will overestimate the sensible heat capacity of the system.
Verify You Are Using Dry-Bulb Temperature
The 1.08 formula uses dry-bulb temperature difference only. Do not use wet-bulb temperature or enthalpy difference in this formula. If you need to calculate total heat including moisture changes, you need a different formula using the constant 4.5 and enthalpy difference, which is the total heat version of the HVAC load equation.
Use Consistent Units
CFM must be in cubic feet per minute and ΔT must be in degrees Fahrenheit. Mixing SI and imperial units is a common source of error. If your airflow data is in m³/s or your temperature is in Celsius, convert before entering values into this calculator.
Frequently Asked Questions
What does 1.08 represent in the HVAC formula?
The value 1.08 is a derived constant equal to 60 (min/hr) × 0.075 lb/ft³ (air density) × 0.24 BTU/lb·°F (specific heat of dry air). It allows the formula to work with CFM directly, converting volumetric airflow into a heat rate in BTU/hr.
What is the difference between sensible and total heat in HVAC?
Sensible heat refers to energy that changes the dry-bulb temperature of the air. Total heat (enthalpy) includes both sensible heat and latent heat — the energy needed to change the moisture content of the air. For cooling calculations, both are needed: sensible load uses the 1.08 formula, while total load uses the 4.5 × CFM × Δh formula where Δh is the enthalpy difference.
How do I convert BTU/hr to tons of cooling?
One ton of cooling equals 12,000 BTU/hr. To convert, divide your BTU/hr result by 12,000. For example, 60,000 BTU/hr equals 5 tons of cooling capacity. This is the standard conversion used across the US HVAC industry.
Can this formula be used for heating loads as well?
Yes. The same formula applies to heating systems. When sizing a heating coil or hot water reheat unit, ΔT becomes the temperature rise from entering air to leaving air. The formula calculates how many BTU/hr the coil must add to achieve the desired leaving air temperature at a given CFM.
What CFM value should I use — supply air or return air?
Use the supply air CFM for cooling coil and heating coil calculations. In most systems, supply and return CFM are close or equal. If there is significant outdoor air intake or exhaust, the mixed air CFM at the coil face may differ from supply CFM, and the correct value at the point of heat transfer should be used.
Does this formula work at all temperatures and humidity levels?
The formula is an approximation that assumes standard air properties. It performs well for typical HVAC conditions between 40°F and 100°F. At extreme temperatures or very high humidity levels, the actual properties of moist air deviate from the standard constants, and psychrometric software or the full ideal gas equation gives more precise results.
What is the equivalent formula in SI units?
In metric units, the sensible heat formula becomes: Hs (W) = 1.2 × (m³/s) × ΔT (°C). The constant 1.2 reflects the density of air in kg/m³ multiplied by the specific heat in J/(kg·°C). This is the SI equivalent used in European and international HVAC standards.
Why does altitude affect the 1.08 constant?
At higher altitudes, atmospheric pressure is lower, which reduces air density below the sea-level standard of 0.075 lb/ft³. Since the constant 1.08 includes air density in its derivation, a lower density means less mass flow per CFM. The same CFM carries less heat per degree of temperature difference, so the effective constant is smaller and the system needs more airflow to achieve the same load.
Conclusion
The HVAC Sensible Heat Hs 1.08 CFM Formula Calculator gives engineers, technicians, and students a fast way to work with the most widely used air-side heat formula in mechanical engineering. Whether you need the sensible load, the required CFM, or the expected temperature rise, this tool solves it in one step with results in BTU/hr, tons, and kW.
Understanding and applying this formula correctly is fundamental to designing efficient, well-sized HVAC systems that deliver the right amount of conditioned air to every space.