The goal of an air density calculator is to measure molecules in one object. You get the air density (ρ)’s estimate by establishing pressure conditions and temperature. Such information can help you perform other measurements. You can work out aerodynamic drag forces or even how a wind turbine works. Air density is reliant on many factors. It can also change fast with pressure, temperature, and relative humidity. Even air pressure and altitude play a part in understanding air density. The higher you stand, the less air, and the lower the pressure. Understand air density better than you ever have by reading on.
You would define the density of air as the measurement of air mass per unit volume. The Greek letter ρ denotes it, and unit volumes follow that, such as g/m3. Dry air is mostly composed of Nitrogen and Oxygen, with 78 percent and 21 percent respectively. The one percent remaining is a mixture of different gases such as helium and carbon dioxide. When water vapors appear, the air is no longer dry air. With the air containing many different things, it doesn’t have a constant density. Its value depends on air composition. Most components won’t influence the air density, but water vapor will. With more water vapor in the air, the lower its density. Have you ever wondered what the density at sea level is? Let’s say the parameters are 59°F and 14.7 PSI. Air density at sea level would be 0.0765 lb/cu ft. If altitude, temperature, or humidity changes, air density does too.
If you want to calculate air density, visit a weather station website. Make the calculations with a few parameters. Air pressure: You measure barometric pressure in hPa. Air temperature: The outside temperature is in °C. Relative Humidity: You will need the dew point or relative humidity to work out one or the other. The dew point is when water vapor begins to condensate. You measure this parameter in °C. The air density calculation from this point is straightforward. Divide the pressure of the air into dry air and water vapor. Combine the values to get your parameter. 1. At dew point (T), calculate your saturation vapor pressure. The formula is: p₁ = 6.1078 x 10^ [7.5T / (T + 237.3)] The saturation vapor pressure determines 100% relative humidity. 2. Identify the vapor pressure. Multiply the saturation vapor pressure by relative humidity. pv = p₁ x RH 3. Take away the vapor pressure data from the total air pressure. The result will be the pressure of dry air. pd = p – pv 4. Put the values into the formula below: ρ = (pd / (Rd x T)) + (pv / (Rv x T)) pd = dry air pressure (in hPa) pv = water vapor pressure (in hPa) T = air temperature (in Kelvins) Rd = specific gas constant for dry air (equal to 287.058 J/(kg.K) Rv = specific gas content for water vapor (equal to 461.495 J/(kg.K) Air density is like regular density. Both tell us how much air weighs in an area. You express it using the following formula: ρ = mass of air / volume It may look like air is a constant value describing a gas property, but it’s not. The density of gas, liquid, and solid depends on many things. You also have to factor the substance’s chemical composition and external conditions. With these factors and the various gases on Earth, air density is ever-changing. It’s also a curious subject that leads you to ask whether moist air is heavier or lighter than dry air. The more water vapor you add to air, the less dense it becomes. That might seem strange, but it’s quite logical. Avogadro’s law states gas volumes have the same molecules, pressure and temperature. Let’s say you put dry air into a container with a set pressure and temperature. One percent is argon molecules with an atomic weight of 18u. Twenty-one percent is oxygen, with two O atoms weighing 32u. The largest part is nitrogen. There is 78 percent nitrogen with two atoms at 28u. Every molecule in the container is equal to or heavier than 18u. Add water vapor of 18u to the gas, with two hydrogen atoms of 1u and one of oxygen at 16u. When you obey Avogadro’s law, all the molecules remain the same. Water vapor will then take over other gases due to the weight difference. The total mass decreases, decreasing the density with it.
You can define dry air as air that doesn’t contain any water vapors. You would also say it’s air with low relative humidity – or with a low dew point. You use the dew point and relative humidity with logarithmic functions. When the logarithm reaches zero, the value goes to minus infinity. As a result, for zero-relative humidity, a dew point doesn’t exist. This air density calculator lets you calculate dry air by using ‘dry air’ in the ‘air type’ field. Dry air contains oxygen and nitrogen molecules that move at lightning fast speeds. To put it into perspective, let’s say you have a nitrogen molecule with 14 u mass. The room temperature is 670 m/s. That molecule will move at twice the speed of sound. When you increase the temperature, the gas molecules move quicker. They push harder at the borders of their surroundings, expanding the gas volume. When volume increases, density decreases in response. You can now deduce that when you heat air, the air density decreases. The opposite happens with pressure. If you have a gas cylinder with the same volume, higher pressure equates to higher air density. Temperature and pressure both affect air density, but so does altitude. The higher your elevation, the more pressure there is and the lower the temperature. There is also less air at higher altitudes. You won’t see too many people climbing Mount Everest without an oxygen mask! In a plane, the cabin has pressure to mimic the air pressure on the ground. If that doesn’t convince you about changing pressures, then check out the table below. It shows you dry air density at different pressures, altitudes, and temperatures.
| Altitude [ft (m)] |
Temperature [°F (°C)] |
Pressure [psi (hPa)] |
Air density [lb / cu ft (kg / m³)] |
|---|---|---|---|
| sea level | 59 (15) | 14.7 (1013.25) | 0.077 (1.23) |
| 2 000 (610) | 51.9 (11.1) | 13.7 (941.7) | 0.072 (1.16) |
| 4 000 (1219) | 44.7 (7.1) | 12.7 (873.3) | 0.068 (1.09) |
| 6 000 (1829) | 37.6 (3.1) | 11.7 (808.2) | 0.064 (1.02) |
| 8 000 (2438) | 30.5 (-0.8) | 10.8 (746.2) | 0.06 (0.95) |
| 10 000 (3048) | 23.3 (-4.8) | 10 (687.3) | 0.056 (0.9) |
| 12 000 (3658) | 16.2 (-8.8) | 9.2 (631.6) | 0.052 (0.84) |
| 14 000 (4267) | 9.1 (-12.8) | 8.4 (579) | 0.048 (0.77) |
| 16 000 (4877) | 1.9 (-16.7) | 7.7 (530.9) | 0.045 (0.72) |
The standard measurement type for air density is kilograms per cubic meter (Kg/m3). Many people also use the following:
g/cm³ = gram per cubic centimeter
1 g/cm³ = 0.001 kg / m³
kg/L = kilogram per liter
1kg/L = 1,000 kg / m³
g/mL = gram per milliliter
1g/mL = 1,000 kg / m³
What you use for measuring can depend on your situation. You might need it in liters one day, but cubic meters the next. With the air density calculator, you can switch between many different measurement values.
You can also use imperial units for measuring air density, such as these below:
• lb/ cu ft = pound per cubic feet
• lb / cu yd = pound per cubic yard
• oz / cu in = ounce per cubic inch
• lb / US gal = pound per gallon
You can use a density converter for converting any mass density units into gas, liquid, or solid form.
Temperature and air pressure will change no matter where you go. What makes it challenging is when the “standard” is also not the same, depending on the industry. Always check the author’s definition of “standard” when using scientific calculations. It might not be what you think. The standards often change, with some having more than one definition as well. Read about different standard references using pressures (p₀) and temperatures (T₀) below. The International Union of Pure and Applied Chemistry (IUPAC) Standard Temperature and Pressure: p₀ = 10⁵ Pa, T = 0 °C The Institute of Standards and Technology (NIST) ISO 10780: p₀ = 1 atm, T = 0 °C The International Civil Aviation Organization (ICAO) International Standard Atmosphere (ISA): p₀ = 1 atm, T = 15 °C The US Environmental Protection Agency (EPA) Normal Temperature and Pressure: p₀ = 1 atm, T = 20 °C The International Union of Pure and Applied Chemistry Standard Ambient Temperature and Pressure (SATP): p₀ = 10⁵ Pa, T = 25 °C To define the standard air density, you need first to set standard conditions. If you assume relative humidity is small, you won’t have trouble with this air density calculator. Standard air density for STP is: ρ₀ = 1.2754 kg / m³ NTP = ρ₀ = 1.2923 kg / m³ SATP = ρ₀ = 1.1684 kg / m³.
Air pressure is a measure of how much strength gas has in particular surroundings. Let’s say you trap air in a container. If you use the kinetic theory of gas, you can assume the gas molecules are moving around a lot. Their movement depends on thermal energy. When the particles in the container collide, there is an extra force. The molecules will reach around 10²³, making the effect measurable. That force is now pressure in the vessel.
Relative humidity (RH) is the ratio of partial water vapor pressure and equilibrium vapor water pressure. The pressure is on one component with a consistent volume and temperature. When you add the partial pressures of all the gas in the air, you get the total pressure. It looks like this: p_total = p_N₂ + p_O₂ + p_Ar + p_H₂O + ... The water’s equilibrium vapor pressure is the result of vapor in thermodynamic equilibrium with its liquid at a set temperature. It’s the process of measuring atoms and molecules that come from a liquid as it becomes a gas. When the temperature increases, so do the equilibrium vapor pressures. The relative humidity range is between 0 and 100 percent. At zero, it’s dry air. At 100, it contains water vapor mixed in with it. At this point, the cooling air produces condensation.
Dew point directly links to air humidity. It’s when the water vapor’s temperature reaches a saturation state. If you keep cooling the air, the water vapor becomes dew. There are many ways to find out when this point is. The air density calculator uses the following formulas to work it out: DP = 243.12 x α / (17.62 - α) α = depends on temperature and relative humidity parameters and: α = ln (RH/100) + 17.62 * T / (243.12 + T). When displaying relative humidity, you do so in percentages. You use degrees Celsius for temperatures. All these formulas and new languages can be confusing. So, here’s a real-life example of the process. Let’s say you’re sweating on a hot day. Your body evaporates sweat to cool you down. The perspiration evaporation speed depends on how much moisture is in the air. If there is 100 percent humidity, your sweat will not evaporate. If you move into a light breeze, that sweat will disappear faster. You may experience discomfort when the dew point is low. The dry air causes your skin to crack and irritate. The table below covers the relative humidity and dew points at a temperature of 68°F. The dew point is not higher than the air temperature because moisture does not go above 100 percent. If the relative humidity is 0 percent, the air is dry, and there is no dew point.
| Dew point [°F (°C)] |
Relative humidity at 61.6°F (20°C) [%] |
|---|---|
| over 60 (16.4) | over 80 |
| 57.8 (14.3) | 70 |
| 53.6 (12) | 60 |
| 48.7 (9.3) | 50 |
| 42.8 (6) | 40 |
| 35.4 (1.9) | 30 |
| below 25.4 (-3.7) | below than 20 |
