SI Base Units Quick Reference
The Seven SI Base Units:
- Length: meter (m)
- Mass: kilogram (kg)
- Time: second (s)
- Electric Current: ampere (A)
- Temperature: kelvin (K)
- Amount of Substance: mole (mol)
- Luminous Intensity: candela (cd)
SI Base Units (International System)
| Quantity | Unit | Symbol | Definition |
|---|---|---|---|
| Length | meter | m | Distance traveled by light in 1/299,792,458 of a second |
| Mass | kilogram | kg | Defined by the Planck constant (6.62607015×10⁻³⁴ J⋅s) |
| Time | second | s | Duration of 9,192,631,770 periods of Cs-133 radiation |
| Electric Current | ampere | A | Defined by elementary charge (1.602176634×10⁻¹⁹ C) |
| Temperature | kelvin | K | Defined by Boltzmann constant (1.380649×10⁻²³ J/K) |
| Amount of Substance | mole | mol | Exactly 6.02214076×10²³ elementary entities |
| Luminous Intensity | candela | cd | Defined by luminous efficacy of 540 THz radiation |
Metric Prefixes
| Prefix | Symbol | Factor | Decimal |
|---|---|---|---|
| yotta | Y | 10²⁴ | 1,000,000,000,000,000,000,000,000 |
| zetta | Z | 10²¹ | 1,000,000,000,000,000,000,000 |
| exa | E | 10¹⁸ | 1,000,000,000,000,000,000 |
| peta | P | 10¹⁵ | 1,000,000,000,000,000 |
| tera | T | 10¹² | 1,000,000,000,000 |
| giga | G | 10⁹ | 1,000,000,000 |
| mega | M | 10⁶ | 1,000,000 |
| kilo | k | 10³ | 1,000 |
| hecto | h | 10² | 100 |
| deca | da | 10¹ | 10 |
| (base) | - | 10⁰ | 1 |
| deci | d | 10⁻¹ | 0.1 |
| centi | c | 10⁻² | 0.01 |
| milli | m | 10⁻³ | 0.001 |
| micro | μ | 10⁻⁶ | 0.000001 |
| nano | n | 10⁻⁹ | 0.000000001 |
| pico | p | 10⁻¹² | 0.000000000001 |
| femto | f | 10⁻¹⁵ | 0.000000000000001 |
Example: 1 kilometer (km) = 1000 meters (10³ m), 1 nanometer (nm) = 0.000000001 meters (10⁻⁹ m)
Common Scientific Conversions
Temperature
- Celsius ↔ Kelvin
K = °C + 273.15
- Fahrenheit ↔ Celsius
°C = (°F - 32) × 5/9
- Kelvin ↔ Fahrenheit
°F = (K - 273.15) × 9/5 + 32
Energy
- Joules ↔ Calories
1 J = 0.239 cal
- eV ↔ Joules
1 eV = 1.602×10⁻¹⁹ J
- kWh ↔ Joules
1 kWh = 3.6×10⁶ J
Pressure
- Pascal ↔ Atmosphere
1 atm = 101,325 Pa
- Bar ↔ Pascal
1 bar = 100,000 Pa
- mmHg ↔ Pascal
1 mmHg = 133.322 Pa
Energy Unit Conversions
| Unit | Value | Calories | Electron Volts | BTU |
|---|---|---|---|---|
| Joule (J) | 1 J | 0.239 cal | 6.242×10¹⁸ eV | 9.478×10⁻⁴ BTU |
| Calorie (cal) | 1 cal | 1 cal | 2.611×10¹⁹ eV | 3.968×10⁻³ BTU |
| Kilocalorie (kcal) | 1 kcal | 1000 cal | 2.611×10²² eV | 3.968 BTU |
| Electron Volt (eV) | 1 eV | 3.827×10⁻²⁰ cal | 1 eV | 1.519×10⁻²² BTU |
| Kilowatt-hour (kWh) | 1 kWh | 860,421 cal | 2.247×10²⁵ eV | 3,412 BTU |
Pressure Unit Conversions
| Unit | Value | Atmosphere | Bar | mmHg | PSI |
|---|---|---|---|---|---|
| Pascal (Pa) | 1 Pa | 9.869×10⁻⁶ atm | 10⁻⁵ bar | 7.501×10⁻³ mmHg | 1.450×10⁻⁴ psi |
| Atmosphere (atm) | 1 atm | 1 atm | 1.013 bar | 760 mmHg | 14.696 psi |
| Bar | 1 bar | 0.987 atm | 1 bar | 750.06 mmHg | 14.504 psi |
| mmHg (Torr) | 1 mmHg | 1.316×10⁻³ atm | 1.333×10⁻³ bar | 1 mmHg | 0.0193 psi |
| PSI | 1 psi | 0.068 atm | 0.069 bar | 51.715 mmHg | 1 psi |
Fundamental Physical Constants
| Constant | Value | Use |
|---|---|---|
| Speed of Light (c) | 2.998×10⁸ m/s | Universal constant in relativity |
| Planck Constant (h) | 6.626×10⁻³⁴ J⋅s | Quantum mechanics, energy quantization |
| Gravitational Constant (G) | 6.674×10⁻¹¹ N⋅m²/kg² | Universal gravitation |
| Avogadro Number (Nₐ) | 6.022×10²³ mol⁻¹ | Mole definition, chemistry |
| Boltzmann Constant (kᵦ) | 1.381×10⁻²³ J/K | Statistical mechanics, thermodynamics |
| Gas Constant (R) | 8.314 J/(mol⋅K) | Ideal gas law |
| Elementary Charge (e) | 1.602×10⁻¹⁹ C | Charge of electron/proton |
| Electron Mass (mₑ) | 9.109×10⁻³¹ kg | Atomic and particle physics |
Laboratory Best Practices
Always Use SI Units in Publications
Scientific journals require SI (International System of Units). Convert all measurements to SI base or derived units before submission.
Significant Figures Matter
Maintain appropriate significant figures through conversions. Your final answer should have no more precision than your least precise measurement.
Know Your Metric Prefixes
Memorize common prefixes (nano, micro, milli, kilo, mega, giga) for quick mental conversions. Essential for lab work and data analysis.
Use Dimensional Analysis
Set up conversion factors so units cancel properly. This catches errors and ensures correct conversions in complex calculations.
Temperature Scale Awareness
Kelvin has no degrees symbol (just K, not °K). Absolute zero is 0 K = -273.15°C = -459.67°F. Always use Kelvin in thermodynamic equations.
Frequently Asked Questions
What is the difference between Celsius and Kelvin?
Celsius uses the freezing point of water (0°C) as its zero point, while Kelvin uses absolute zero (0 K = -273.15°C) as its zero point. The size of one degree is the same in both scales. Formula: K = °C + 273.15. Kelvin is the SI base unit for temperature.
How do I convert electron volts (eV) to joules?
Multiply eV by 1.602176634×10⁻¹⁹ to get joules. This is the elementary charge constant. Example: 1 eV = 1.602×10⁻¹⁹ J. Use our eV to J converter for precise calculations.
Why use SI units in scientific work?
The International System of Units (SI) provides a universal standard, eliminating ambiguity and errors in global scientific communication. All SI units are rigorously defined by fundamental physical constants, ensuring accuracy and reproducibility across all research.