Which Of The Following Are Not Units Of Pressure

The Question That Trips Up Students Every Semester

Here's a question that shows up on exams, homework, and late-night study sessions across physics and chemistry departments: "Which of the following are not units of pressure?" It sounds straightforward until you realize how many different measurement systems exist, and how easy it is to confuse them.

I've seen students who can solve complex fluid dynamics problems stumble on this one. Why? Because pressure units are everywhere, and some look deceptively similar. Let's clear this up once and for all.

What Pressure Actually Is

Pressure is force applied per unit area. On the flip side, the formula is simple: P = F/A. You push on something with a certain amount of force, spread over a certain area, and that gives you pressure.

The key insight? Pressure has units that combine force and area in some way. That's your litmus test. If a unit doesn't involve force divided by area (or something equivalent), it's not a pressure unit.

The Real Units of Pressure

Let's start with what actually counts as a pressure unit.

Pascals and Newtons Per Square Meter

The SI unit for pressure is the pascal (Pa), which equals one newton per square meter. One pascal is a tiny amount of pressure — about the weight of a dollar bill spread over a square meter. Most pressures we encounter are measured in kilopascals (kPa) or hectopascals (hPa).

Atmospheres and Millimeters of Mercury

Atmospheric pressure at sea level is approximately 101,325 pascals. Scientists defined this as one atmosphere (atm). Another common unit is millimeters of mercury (mmHg), based on the height of a mercury column that balances atmospheric pressure. One atmosphere equals roughly 760 mmHg.

Pounds Per Square Inch

In the imperial system, pressure is often measured in pounds per square inch (psi). That's why one atmosphere equals about 14. Practically speaking, tire pressure gauges, scuba tanks, and many engineering applications use psi. 7 psi.

Bars and Millibars

Meteorologists frequently use bars and millibars. One bar equals 100,000 pascals, which is close to one atmosphere. Weather reports often express pressure in millibars (mb), where standard atmospheric pressure is about 1013 mb.

The Trap: Units That Look Like Pressure But Aren't

Now here's where it gets interesting. Many units sound like they could measure pressure, but they don't.

Newtons Alone Are Not Pressure

A newton is a unit of force, not pressure. Force divided by area gives pressure. Just force by itself? That's not pressure. You need that area component.

Joules Are Not Pressure

A joule measures energy, which is force times distance. Pressure is force divided by area. These are fundamentally different quantities, even though both involve force.

Watts Don't Measure Pressure Either

A watt measures power, or energy per time. Again, completely different from force per area.

Seconds, Meters, and Kilograms

Basic SI units like seconds (time), meters (length), and kilograms (mass) are not pressure units. They're components that might appear in pressure equations, but they're not pressure units themselves.

Why This Matters More Than You Think

Understanding which units are and aren't pressure units isn't just academic. It's a gateway to thinking correctly about physical quantities.

When you mix up units, equations break. Dimensional analysis fails. Which means real-world calculations go wrong. Engineers designing pressure vessels, meteorologists predicting storms, and chemists working with gases all depend on using the right units.

I've watched students lose points on entire exam problems because they used the wrong unit conversion. They knew the physics perfectly — they just couldn't keep their units straight.

How to Identify Pressure Units Quickly

Here's a practical approach that works every time.

Check the Dimensions

Every pressure unit has dimensions of force divided by area. Because of that, in SI base units, that's kg·m⁻¹·s⁻². If a unit doesn't reduce to these dimensions, it's not pressure.

Look for the Area Component

Real pressure units always include something about area — square meters, square inches, or even the implicit area in millimeters of mercury. If there's no area involved, it's not pressure.

For more on this topic, read our article on what is the x intercept of the function graphed below or check out a simcell with a water-permeable membrane that contains 20 hemoglobin.

Remember the Force Component

Pressure units always involve force — newtons, pounds, or the force exerted by a column of fluid. Without force, you don't have pressure.

Common Mistakes Students Make

Let me share what I've observed over years of tutoring and teaching.

Confusing Force with Pressure

Students see newtons and think pressure. Pressure needs force per area. But a newton is just force. This is the single most common mistake.

Mixing Up Energy and Pressure Units

Joules, calories, and kilowatt-hours all measure energy. Plus, they're not pressure units. Students sometimes group them together because they all sound "scientific.

Getting Tripped Up by Compound Units

Units like newton-meters (torque or energy) or joules per mole (energy per amount of substance) are not pressure units. The compound nature makes them look more complex, but they're still not force per area.

Assuming All Imperial Units Are Pressure

Not every imperial unit measures pressure. On top of that, feet, pounds, and gallons are length, force, and volume respectively. Only combinations like pounds per square inch qualify as pressure.

Practical Tips for Getting It Right

Here's what actually works when you're solving problems or taking tests.

Write Out the Dimensions

When in doubt, write out what the unit represents. This leads to does it have force divided by area? So naturally, if not, it's not pressure. This simple check catches most errors.

Use Unit Conversion Chains

Set up conversion problems carefully. If you're converting from one pressure unit to another, make sure both units have the same dimensions. If they don't match, you've made an error.

Memorize the Common Pressure Units

Know the standard ones: pascals, atmospheres, mmHg, psi, bars, and torr. These are your pressure units. Everything else needs justification.

Practice Dimensional Analysis

Regular practice with dimensional analysis builds intuition. You'll start to recognize which units go together and which don't.

Frequently Asked Questions

Is torr a unit of pressure? Yes. The torr is based on millimeters of mercury and equals approximately 133.322 pascals. It's named after Evangelista Torricelli, who invented the barometer.

Are pascals and newtons the same thing? No. Pascals measure pressure (force per area), while newtons measure force. One pascal equals one newton per square meter.

Can you convert joules to pascals? Not directly. Joules measure energy, and pascals measure pressure. You'd need additional information about area to relate them.

Is pounds per square foot a pressure unit? Yes. It's another way to express force per area, just using different units than psi.

Why do some pressure units use fluid height? Units like millimeters of mercury or feet of water work because the weight of a fluid column creates pressure proportional to its height. The density and gravity are built into the unit definition.

The Bottom Line

The question "which of the following are not units of pressure" tests more than memorization. It tests whether you understand what pressure actually is.

Pressure is force per unit area. Worth adding: always. Any unit that doesn't represent this relationship isn't a pressure unit, no matter how scientific it sounds.

When you encounter unfamiliar units, break them down. Do they involve force and area? Now, what are they measuring? If not, they're not pressure units.

This kind of thinking will serve you well beyond homework problems. It's the foundation for understanding how physical quantities relate to each other, and it's what separates students who memorize formulas from those who truly understand the material.

So the next time you see that question on a test, don't panic. Just ask yourself: does this unit represent force divided by area? If not, it's not pressure.