Pressure formula

Here you will learn about the pressure formula, including how to use the formula. Students will first learn about the pressure formula as part of ratio and proportionality in 7 th grade.

What is the pressure formula?

The pressure formula is a formula connecting pressure, area, and force.

Pressure is a measure of force per unit area. Weight is calculated by multiplying mass by the acceleration of gravity so heavier objects exert more pressure.

To calculate the pressure, area or force applied to an object, you need to use their intrinsic relationship. Pressure is calculated by dividing the amount of force by the area.

The pressure formula (or pressure equation) is therefore

\text{Pressure}=\cfrac{\text{Force}}{\text{Area}} or P=\cfrac{F}{A}

Pressure is a compound measure made from force and area. Pressure equals the force per unit area. Pressure is measured using pressure gauges.

Force is the energy attributed to a movement or a physical action. The SI unit for force is Newtons (N).

Area is the amount of a surface on which the force is applied to. The standard unit for area is square meters (m^{2}).

The pressure formula can be used in a formula triangle where it can be adapted to calculate force and area.

To find pressure \hspace{1.3cm} To find force \hspace{1.5cm} To find area

The units of force are Newtons (N) and the derived SI units of area are square meters (m^{2}). So the units of pressure are Newtons per square meter (N/m^{2}).

However, the standard unit of pressure is Pascals (Pa). \, 1 Pascal is equal to 1 \, N/m^{2} and 100,000 \, Pa=1 Bar (another unit of pressure).

Air pressure (or atmospheric pressure) can change depending on where you are on planet Earth. If you are on a mountain, the air pressure is less than when you are at sea level. The air pressure at sea level is approximately 14.7 pounds per square inch or 1.01325 Bar.

What is the pressure formula?

Common Core State Standards

How does this relate to 7 th grade math?

• Grade 7 – Ratio and Proportionality (7.RP.A.2)
  Recognize and represent proportional relationships between quantities.

How to calculate the pressure, force or area using the pressure formula

In order to calculate the pressure, force or area using the pressure formula:

1. Write the formula with the correct subject.
2. Substitute known values into the formula and do the calculation.
3. Write down the solution including the units.

Pressure formula examples

Example 1: calculating pressure (N/m²)

A force of 600 \, N acts on an area of 20 \, m^{2}. Calculate the pressure.

1. Write the formula with the correct subject.

As P=\cfrac{F}{A} and you want to calculate the pressure, you do not need to rearrange the formula, so you have,

2Substitute known values into the formula and do the calculation.

3Write down the solution including the units.

Pressure is 30 \, N/m^{2}.

The solution can also be written in Pascals, 30 \, Pa.

Example 2: calculating pressure (N/m²)

A force of 980 \, N acts on an area of 120 \, m^{2}. Calculate the pressure. Give your answer correct to the nearest hundredth.

Example 3: calculating area (m²)

A force of 560 \, N exerts a pressure of 70 \, N/m^{2}. Calculate the area of the surface that the force is applied to.

Write the formula with the correct subject.

As P=\cfrac{F}{A} and you want to calculate the area, you need to rearrange the formula to make A the subject. You do this by multiplying both sides by A, and then dividing both sides by P, so you have,

\begin{aligned}P&=\cfrac{F}{A} \\\\ P\times{A}&=\cfrac{F}{A}\times{A} \\\\ P\times{A}&=F \\\\ P\times{A}\div{P}&=F\div{P} \\\\ A&=\cfrac{F}{P} \end{aligned}

Alternatively you could use the pressure formula triangle,

Example 4: calculating area (m²)

A force of 460 \, N exerts a pressure of 18 \, N/m^{2} onto a flat surface. Calculate the area of the surface that the pressure is being exerted on. Give the answer correct to the nearest tenth.

Write the formula with the correct subject.

As P=\cfrac{F}{A} and you want to calculate the area, you need to rearrange the formula to make A the subject. You do this by multiplying both sides by A, and then dividing both sides by P, so you have,

\begin{aligned}P&=\cfrac{F}{A} \\\\ P\times{A}&=\cfrac{F}{A}\times{A} \\\\ P\times{A}&=F \\\\ P\times{A}\div{P}&=F\div{P} \\\\ A&=\cfrac{F}{P} \end{aligned}

Alternatively you could use the pressure formula triangle,

\text{Area}=\cfrac{\text{Force}}{\text{Pressure}}

Example 5: calculating force (N)

A pressure of 40 \, N/m^{2} is exerted on an area of 6 \, m^{2}. Calculate the force.

Write the formula with the correct subject.

As P=\cfrac{F}{A} and you want to calculate the force, you need to rearrange the formula to make F the subject. You do this by multiplying both sides by A, so you have,

\begin{aligned}P&=\cfrac{F}{A} \\\\ P\times{A}&=\cfrac{F}{A}\times{A} \\\\ P\times{A}&=F \\\\ F&=P\times{A} \end{aligned}

Alternatively you could use the pressure formula triangle,

\text{Force}=\text{Pressure}\times \text{Area}

Example 6: calculating the force (N)

A pressure of 83 \, N/m^{2} is exerted on an area of 53.2 \, m^{2}. Calculate the force.

Write the formula with the correct subject.

As P=\cfrac{F}{A} and you want to calculate the force, you need to rearrange the formula to make F the subject. You do this by multiplying both sides by A, so you have,

\begin{aligned}P&=\cfrac{F}{A} \\\\ P\times{A}&=\cfrac{F}{A}\times{A} \\\\ P\times{A}&=F \\\\ F&=P\times{A} \end{aligned}

Alternatively you could use the pressure formula triangle,

\text{Force}=\text{Pressure}\times \text{Area}

Example 7: converting units of area

A pressure of 29 \, N/m^{2} is exerted on an area of 20000 \, cm^{2}. Calculate the force applied to the area.

Write the formula with the correct subject.

As P=\cfrac{F}{A} and you want to calculate the force, you need to rearrange the formula to make F the subject. You do this by multiplying both sides by A, so you have,

\begin{aligned}P&=\cfrac{F}{A} \\\\ P\times{A}&=\cfrac{F}{A}\times{A} \\\\ P\times{A}&=F \\\\ F&=P\times{A} \end{aligned}

Alternatively you could use the pressure formula triangle,

\text{Force}=\text{Pressure}\times \text{Area}

Teaching tips for pressure formula

• Provide practice problems where students calculate pressure given force and area, and vice versa. Include units to reinforce the importance of proper unit usage.

• Use visual aids and experiments, such as standing on one foot versus both feet, to demonstrate how distributing force over a larger surface area affects pressure.

• Introduce the Ideal Gas Law PV=nRT to explain gas pressure. Discuss how gas pressure increases with temperature and decreases with volume.
  • In thermodynamics, the ideal gas law states that the gas constant R is equal to the pressure times the volume of a gas, divided by the number of moles and temperature of the gas.
    R=8.3145J/mol/K (where J= Joules, mol= Moles, K= Kelvin)

• Introduce the concept of fluid pressure and explore topics such as Pascal’s principle, buoyancy, and Bernoulli’s principle. Bernoulli’s principle states that in a fluid flow, an increase in velocity results in a decrease in pressure. This principle is crucial in explaining phenomena like the lift on airplane wings and the operation of Venturi meters.
  

Easy mistakes to make

• Using incorrect units
  As pressure is a measure of force per unit area (N/m^{2}), the units for force will be given (N), as well as the units for area (m^{2}). Use these to determine the units of your answer. If you are given the pressure in Pascals, 1 \, Pa = 1N/m^{2} and so you will need to remember this to determine the units of your solution.

• Not fully reading the question
  Check that you give the answer in the right form. You may have an answer which needs to be rounded to a specific decimal place.

• Not converting units of area correctly
  To convert centimeters to meters, you divide by 100 as 1 \,m=100 \, cm. To convert square centimeters to square meters, you divide by 100^{2}=10,000 as 1 \, m^{2} = 10,000 \, cm^{2}.

• Confusing scalar and vector
  Area is a scalar quantity as it has size. Force is a vector quantity as it has size and magnitude. Pressure is a scalar quantity too.

Related compound measures lessons

• Speed distance time
• Speed formula
• Average speed formula
• Density formula
• Density mass volume triangle
• Mass from density and volume
• Pressure force area
• Flow rate
• Population density formula

The next lessons are

• Math Equations
• Math Formulas

Still stuck?

At Third Space Learning, we specialize in helping teachers and school leaders to provide personalized math support for more of their students through high-quality, online one-on-one math tutoring delivered by subject experts.

Each week, our tutors support thousands of students who are at risk of not meeting their grade-level expectations, and help accelerate their progress and boost their confidence.

Find out how we can help your students achieve success with our math tutoring programs.