Workshop converter
Pressure Converter
Convert workshop, pneumatic, hydraulic, atmospheric, vacuum, and low pressure units, then learn why gauge zero is not a vacuum, why bar and atmosphere disagree, and how liquid columns became pressure units.
Common conversions
Sage’s guided lesson
Pressure is force deciding how much room it gets
Pressure appears in tires, weather maps, compressors, hydraulic cylinders, vacuum systems, gas regulators, water lines, and material stress. The conversion math is straightforward; the difficult part is knowing what kind of pressure was measured, where the reference zero lives, and whether the system was flowing or sitting still. I’ll show you how to convert the units and how to keep a correct number from answering the wrong question.
First, spread out the force
Pressure is force divided by area
Apply the same force over a smaller area and the pressure increases. Spread it over a larger area and the pressure decreases. That is why a sharp edge cuts, a snowshoe helps you stay on top of snow, and a hydraulic piston can turn modest pressure into a large force when its area is large.
The defining relationship
pressure = force ÷ areaP = F ÷ A1 Pa = 1 N/m²This converter uses the pascal as the middle step. It multiplies by the pascals in the starting unit, then divides by the pascals in the target unit.
Pressure is a scalar quantity. In a fluid at rest, it acts in every direction, while the resulting force on a surface acts perpendicular to that surface.
Let’s do one by hand
Converting 100 psi into kilopascals
One pound force per square inch equals about 6.894757293 kilopascals. Multiply by that factor.
Worked example
100 psi × 6.894757293 = 689.4757293 kPa689.4757293 kPa ÷ 100 = 6.894757293 barTo go back to psi, divide kilopascals by 6.894757293.
For a quick estimate, multiply psi by about 7 to get kPa. One hundred psi is therefore close to 700 kPa. Random thought: why did the bicycle fall over? Because it was two tired. Anyway, back to pressure.
Always ask where zero is hiding
Gauge pressure and absolute pressure use different starting lines
Absolute pressure is measured from a perfect vacuum. Gauge pressure is measured relative to the surrounding atmospheric pressure. Differential pressure is the difference between any two pressure points.
A tire gauge that reads 35 psi normally means 35 psi above the local atmosphere, written 35 psig. Its absolute pressure is roughly local atmospheric pressure plus 35 psi. If local atmosphere is about 14.7 psi, the tire is near 49.7 psia.
The reference relationship
absolute pressure = gauge pressure + local atmospheric pressuregauge pressure = absolute pressure − local atmospheric pressureAbsolute pressure cannot be negative. Gauge pressure can be negative when a system is below ambient pressure. A gauge reading of zero does not mean no molecules or no pressure; it means the system matches its reference atmosphere.
Weather borrowed a convenient decimal
One hectopascal is exactly one millibar
One bar is exactly 100,000 pascals. One millibar is one thousandth of a bar, or 100 pascals. One hectopascal is also 100 pascals, so 1 hPa = 1 mbar exactly.
A standard atmosphere is exactly 101,325 pascals, or 1013.25 hPa. That is close to one bar but not equal to it. Weather reports often use hectopascals or millibars because typical sea level atmospheric pressure lands near 1,000 of those units instead of 100,000 pascals.
Atmospheric pressure decreases with altitude because less air remains above you. Weather stations commonly reduce readings to an estimated sea level value so maps can compare weather patterns instead of mostly showing terrain height.
Near standard sea level conditions, atmospheric pressure is about 14.7 pounds force per square inch. We are not crushed because internal body pressure and pressure from every direction largely balance it.
Before electronic gauges, people watched liquids climb
Mercury and water columns turn pressure into height
A manometer compares pressure with the hydrostatic pressure produced by a liquid column. The relationship depends on liquid density, gravity, and height.
Liquid column pressure
pressure = density × gravity × heightP = ρghInches of water are useful for low pressure gas, HVAC, dust collection, and air movement. Inches or millimeters of mercury are common in barometry, vacuum work, and older medical or scientific measurements.
The torr is exactly one 760th of a standard atmosphere. The millimeter of mercury is defined through a conventional mercury density and standard gravity, so torr and mmHg are extremely close but not perfectly identical.
Real liquid density changes with temperature, and local gravity changes slightly with location. Standard conversion factors use defined conventional conditions so that the unit does not wander every time the weather changes.
The empty space above the mercury caused trouble
Torricelli’s barometer helped prove that air has weight
In 1643, Evangelista Torricelli filled a long glass tube with mercury and inverted it into a mercury basin. The mercury column settled with an apparently empty space above it. The height changed with atmospheric pressure, producing the first practical mercury barometer.
Blaise Pascal supported the idea that the atmosphere caused the column height and arranged experiments showing that pressure decreased at higher elevation. His work in fluids also gave us Pascal’s principle: pressure applied to a confined fluid is transmitted throughout the fluid.
The special SI name pascal, symbol Pa, was adopted in 1971 for one newton per square meter. The unit is small, which is why workshop and weather values usually appear as kilopascals or megapascals.
One psi is nearly 6,895 pascals. A typical compressor gauge would need an unreasonable number of plain pascals, so prefixes are doing useful cleanup work.
A static gauge can flatter a tired system
Pressure at rest is not the same as pressure while flowing
A compressor tank may show healthy static pressure, yet a tool can starve when air flows through a long narrow hose, restrictive fittings, a clogged filter, or an undersized regulator. Pressure drop appears because moving fluid loses mechanical energy through restrictions and friction.
Hydraulic force depends on pressure and piston area. A large piston produces large force at the same pressure, but it requires more fluid volume to move the same distance. There is no free force lunch; the trade appears in displacement, speed, and power.
Useful system relationships
force = pressure × piston areapressure drop grows with flow and restrictiongas pressure changes with absolute temperature when volume is nearly fixedNever use a unit conversion as a design approval. Pressure vessels, relief valves, hoses, cylinders, regulators, and piping require rated components, correct codes, manufacturer limits, and competent inspection.
Sage keeps the odd facts under pressure
A few pressure facts worth remembering
Suction does not pull in the way people imagine. A pump lowers pressure on one side, and higher pressure elsewhere pushes the fluid toward the lower pressure region.
Vacuum boiling is real. Lower pressure lowers a liquid’s boiling point. Water can boil below 100 °C at altitude or under vacuum without being “hotter than normal.”
Stress and pressure share units. Mechanical stress is also force per area and can be written in pascals or psi, but stress inside a solid is more complicated than uniform fluid pressure.
Tire gauges usually read gauge pressure. Cold weather often lowers the reading because the gas temperature falls. Driving warms the tire and gas, raising pressure, which is why recommended pressures are normally checked cold.
Deep water becomes serious quickly. Hydrostatic pressure increases with depth because more fluid weight is above the point. Density and gravity determine the exact increase.
Sage’s pressure checklist
Identify gauge, absolute, or differential pressure.Measure at the operating condition that matters.Check temperature, altitude, and instrument range.Use compatible pressure rated components.Do not confuse pressure with flow or force.
The system has stabilized
Class is over. It is officially cheese fries time.
You now know that pressure is force divided by area, that this converter uses pascals in the middle, and that 100 psi equals about 689.4757293 kPa.
You also know why gauge zero is not a vacuum, why bar and atmosphere are close but unequal, why hPa and mbar match exactly, why liquid columns became pressure units, and why a static compressor gauge cannot promise good airflow at the tool.
Quick answers
Pressure conversion questions
How many kilopascals are in one psi?
One psi equals about 6.894757293 kilopascals.
Is one bar the same as one atmosphere?
No. One bar is exactly 100 kilopascals, while one standard atmosphere is exactly 101.325 kilopascals.
What is gauge pressure?
Gauge pressure is measured relative to local atmospheric pressure. A tire gauge reading of zero means the tire is at the same pressure as the surrounding air, not that the tire contains a perfect vacuum.
Can absolute pressure be negative?
No. Absolute pressure is measured from a perfect vacuum and cannot be less than zero. Gauge pressure can be negative when the measured pressure is below the surrounding atmosphere.
Are hectopascals and millibars the same?
Yes. One hectopascal equals exactly one millibar.
Are torr and millimeters of mercury exactly the same?
They are extremely close, but their modern definitions are not perfectly identical. The torr is exactly one 760th of a standard atmosphere.
Why does tire pressure change with temperature?
For gas in a nearly fixed volume, pressure generally rises as absolute temperature rises and falls as it cools. Real tires also change volume slightly and may leak.
Does higher static pressure always mean more airflow?
No. Flow also depends on restrictions, pipe or hose size, length, fittings, fluid properties, and the pressure difference while the system is operating.