Physics Constants List: SI Values, Units, and Where They Are Used

Overview

This hub is designed as a practical reference sheet for students who need a clear physics constants list they can return to during homework, revision, and exam prep. Instead of presenting constants as isolated facts, the goal here is to connect each one to the types of questions where it actually matters.

In introductory physics, many errors come from using the wrong quantity, mixing units, or forgetting whether a value is exact, approximate, or context-dependent. A good constants sheet helps with all three. It also supports faster work under exam pressure because you stop treating constants as trivia and start seeing them as tools.

Below is a compact list of widely used SI constants in physics. Values are presented in standard scientific notation with SI units. In classroom settings, your teacher, exam board, or formula sheet may round some of these differently, so always match the precision expected in your course.

Core physics constants with SI values and units

• Speed of light, c = 2.998 × 10⁸ m/s
  Used in electromagnetic waves, relativity, and photon energy problems.
• Gravitational constant, G = 6.674 × 10^-11 N·m²/kg²
  Used in Newtonian gravitation and orbital mechanics.
• Standard acceleration due to gravity, g ≈ 9.81 m/s²
  Used in kinematics, forces, energy, and projectile motion near Earth’s surface. Many exam questions use 9.8 or 10 m/s² by instruction.
• Planck constant, h = 6.626 × 10^-34 J·s
  Used in quantum physics, photon energy, and wave-particle relations.
• Reduced Planck constant, ħ = 1.055 × 10^-34 J·s
  Used in angular-frequency forms of quantum equations.
• Elementary charge, e = 1.602 × 10^-19 C
  Used in electric charge, electron volt conversions, and atomic physics.
• Electron mass, m_e = 9.109 × 10^-31 kg
  Used in atomic models, particle motion, and quantum topics.
• Proton mass, m_p = 1.673 × 10^-27 kg
  Used in nuclear and atomic problems.
• Neutron mass, m_n = 1.675 × 10^-27 kg
  Used in nuclear physics and mass-energy calculations.
• Vacuum permittivity, ε₀ = 8.854 × 10^-12 F/m
  Used in electrostatics, capacitance, and electric field relations.
• Vacuum permeability, μ₀ ≈ 1.257 × 10^-6 N/A²
  Used in magnetism and electromagnetic wave relations.
• Coulomb constant, k = 8.988 × 10⁹ N·m²/C²
  Used in Coulomb’s law and electric potential formulas.
• Universal gas constant, R = 8.314 J/(mol·K)
  Used in the ideal gas law and thermodynamics.
• Boltzmann constant, k_B = 1.381 × 10^-23 J/K
  Used in statistical physics, thermal energy, and microscopic gas models.
• Avogadro constant, N_A = 6.022 × 10²³ mol^-1
  Used to connect particle counts and moles.
• Faraday constant, F = 9.6485 × 10⁴ C/mol
  Useful in electrochemistry and charge-per-mole relations.
• Stefan–Boltzmann constant, σ = 5.670 × 10^-8 W/m²·K⁴
  Used in blackbody radiation and thermal emission.
• Wien displacement constant, b = 2.898 × 10^-3 m·K
  Used in peak-wavelength radiation problems.

You will not need all of these in every course. A GCSE or introductory mechanics student may mostly use g. An AP Physics or A-Level student may also see c, G, k, R, and h. A college physics student is more likely to use the full range.

Topic map

The easiest way to remember physics constants with units is to group them by topic. This section shows where each constant belongs so you can recognize it quickly in formulas and word problems.

Mechanics and gravitation

The constants most students meet first are g and G, but they are not interchangeable.

• g is the local gravitational field strength near Earth’s surface, used in equations like weight W = mg, free-fall motion, gravitational potential energy, and projectile motion.
• G is the universal gravitational constant, used in Newton’s law of gravitation: F = Gm₁m₂/r².

A common mistake is using G when a problem only needs Earth-surface gravity, or using g in planetary-orbit problems that require the full inverse-square law.

If you need a refresher on force analysis before using constants in mechanics, see Free-Body Diagram Guide: Rules, Examples, and Practice Questions and Work, Energy, and Power Problems with Step-by-Step Answers.

Electricity and magnetism

In electric force and field questions, several constants appear regularly:

• e, the elementary charge, is central when counting charge carriers or converting between microscopic charge and macroscopic current.
• k, the Coulomb constant, appears in Coulomb’s law and electric potential equations.
• ε₀, vacuum permittivity, often appears in more formal electrostatics expressions.
• μ₀, vacuum permeability, appears in magnetic field relations and electromagnetic theory.

In beginner circuit work such as Ohm’s law and series-parallel circuits, you often do not need these constants at all. That is useful to remember: not every electricity problem requires a constants sheet.

Waves, optics, and electromagnetism

The speed of light constant, c, matters in several ways:

• It sets the speed of electromagnetic waves in vacuum.
• It connects wavelength and frequency through c = fλ for light in vacuum.
• It appears in relativity and mass-energy relations.

In optics, students sometimes overuse c. For waves in a medium, use the speed given in the problem or the relevant medium relation, not automatically the vacuum speed of light.

Graph skills also matter here, especially when linking wave quantities and motion data. For that, see Graphing in Physics: How to Read Position-Time, Velocity-Time, and Acceleration-Time Graphs.

Thermodynamics and kinetic theory

Two constants appear often in thermal physics:

• R, the universal gas constant, is used in the ideal gas law PV = nRT.
• k_B, the Boltzmann constant, connects temperature to energy at the particle level.

These two are related conceptually. One works naturally with moles, the other with individual particles. Avogadro’s constant N_A bridges those scales.

Thermal radiation adds two more useful constants:

• σ for total radiated power per unit area.
• b for peak wavelength in Wien’s law.

Modern physics and quantum topics

The Planck constant value is one of the most important entries on any advanced formula sheet. It appears in:

• E = hf for photon energy
• p = h/λ for de Broglie wavelength
• Atomic energy transitions and emission spectra

The reduced Planck constant ħ appears when equations are written in angular form, such as E = ħω.

Mass constants such as m_e, m_p, and m_n become important in atomic and nuclear calculations, especially when converting between particle properties and measurable energies.

Related subtopics

This constants hub works best when paired with the broader study skills and equation resources that students use before and after plugging in numbers. If you are building a personal physics study guide, these are the most useful companion topics.

1. SI units, prefixes, and conversions

Many mistakes that look like “constant mistakes” are really unit mistakes. If your answer is off by a factor of 1000, the problem may be centimeters versus meters, grams versus kilograms, or kilovolts versus volts rather than the constant itself. Review Physics Unit Conversions Guide: SI Units, Prefixes, and Dimensional Analysis before tackling multistep problems.

2. Significant figures and precision

Constants are often given to many decimal places, but your final answer usually should not be. Lab work and measured data call for sensible rounding and uncertainty awareness. See Measurement Uncertainty and Significant Figures in Physics Labs for a practical companion to this reference sheet.

3. Formula sheets by course level

Students preparing for standardized or course exams often need to know not just the constants, but which formulas are supplied and which must be recalled. These guides are useful next steps:

• GCSE Physics Equation Sheet Explained by Topic
• AP Physics 1 Formula Sheet Explained: What Each Equation Means and When to Use It
• A-Level Physics Revision Checklist by Topic and Exam Season
• College Physics Midterm Study Guide: What to Review First

4. Problem-solving method

A constants sheet is only useful if you know when to use each constant. In many physics questions and answers, the hardest step is identifying the correct model before doing any arithmetic. For a repeatable process, read How to Solve Physics Word Problems Step by Step.

5. Equations that often appear with constants

As this hub grows, it can naturally branch into topic pages built around the formulas that most often use these constants. Examples include:

• Projectile motion formulas that use g
• Newtonian gravitation formulas that use G
• Coulomb’s law and electric field formulas that use k or ε₀
• Photon energy and de Broglie relations that use h
• Ideal gas and kinetic theory formulas that use R and k_B

That is why a constants page works well as a long-term hub: each constant can lead to a family of solved examples, derivations, calculators, and revision notes.

How to use this hub

This section is the practical core of the page. If you want this resource to improve your grades rather than just sit in your bookmarks, use it actively.

Build a two-step lookup habit

Whenever you start a problem, do these steps in order:

1. Identify the model first. Ask what topic this belongs to: kinematics, gravitation, circuits, gases, waves, or modern physics.
2. Check whether a constant is actually needed. Many problems are solved with given values only. Do not force a constant into a problem that does not require one.

This habit prevents a common error in step by step physics solutions: students notice a familiar symbol and jump to a formula too early.

Keep units attached at every step

Never write only the number for a constant in your working. Write the number and the unit together. For example, write c = 3.00 × 10⁸ m/s, not just 3.00 × 10⁸. The unit often tells you whether the constant fits the equation.

If units do not combine into the expected answer, stop and reassess before finishing the calculation.

Match the precision to the course

Your class may use rounded values such as:

• g = 9.8 m/s² or 10 m/s²
• c = 3.0 × 10⁸ m/s
• h = 6.63 × 10^-34 J·s

That is normal. For exam prep, use the exact style your course expects. For deeper study, it helps to know the more precise standard values as well.

Create your own tiered constants sheet

One of the best revision techniques is to separate constants into three levels:

• Level 1: Must know cold — values you use frequently, such as g, c, and possibly G, R, or e.
• Level 2: Recognize quickly — values you may be given but should identify immediately, such as ε₀, μ₀, and k_B.
• Level 3: Look up as needed — specialized constants for less frequent topics.

This reduces cognitive overload and turns a long list into a workable exam tool.

Use constants to check whether an answer is reasonable

Constants can also act as a sense-check. For example:

• If a calculated speed exceeds c in an ordinary mechanics problem, something is wrong.
• If a near-Earth free-fall answer implies an acceleration wildly different from g, recheck units or signs.
• If thermal energy per particle comes out many orders of magnitude above typical k_BT scales for the situation, re-evaluate the setup.

This kind of estimation is especially valuable during timed tests.

When to revisit

Return to this hub whenever your course moves into a new unit, whenever you start using a new formula sheet, or whenever you notice that unit errors are costing you marks. A constants reference is most useful at transition points: before an exam, at the start of electricity or modern physics, or when moving from high school to college-level work.

This page is also worth revisiting when new related subtopics are added. Over time, a strong constants hub can expand to include:

• mini-guides for each constant
• worked examples sorted by topic
• downloadable equation sheets
• calculator links for gravitation, projectile motion, circuits, and gas laws
• notes on common symbol confusion, such as g versus grams or k versus spring constants

For now, the most effective next step is simple: bookmark this page, pair it with your course formula sheet, and build the habit of checking symbol, value, unit, and topic before calculating. If you do that consistently, a constants list stops being a passive reference and becomes part of your everyday physics help workflow.