SI prefixes are multipliers used with SI units to express very large or very small quantities without writing long strings of zeros. Every engineer works with them daily — kN, MPa, mm, μm, GHz — but the full table from yotta to atto is rarely memorized. This guide covers all 20 SI prefixes with engineering examples.
Complete SI prefix table
| Prefix | Symbol | Factor | Engineering example |
| yotta | Y | 1024 | – |
| zetta | Z | 1021 | – |
| exa | E | 1018 | – |
| peta | P | 1015 | – |
| tera | T | 1012 | 1 TN = 10¹² N |
| giga | G | 109 | 1 GN = 10⁹ N |
| mega | M | 106 | 1 MPa = 10⁶ Pa |
| kilo | k | 103 | 1 kN = 10³ N |
| hecto | h | 102 | 1 hPa = 100 Pa |
| deka | da | 101 | – |
| (base) | – | 100 | m, Pa, N, g |
| deci | d | 10-1 | – |
| centi | c | 10-2 | 1 cm = 0.01 m |
| milli | m | 10-3 | 1 mm = 0.001 m |
| micro | μ | 10-6 | 1 μm = 10⁻⁶ m |
| nano | n | 10-9 | 1 nm = 10⁻⁹ m |
| pico | p | 10-12 | 1 pF = 10⁻¹² F |
| femto | f | 10-15 | – |
| atto | a | 10-18 | – |
| zepto | z | 10-21 | – |
| yocto | y | 10-24 | – |
Most common SI prefixes in structural engineering
In day-to-day structural and civil engineering, six prefixes cover the vast majority of calculations:
Mega (M, 10⁶) — stress and pressure: MPa is the standard unit for concrete strength, steel yield stress, and modulus of elasticity.
Kilo (k, 10³) — force and length: kN is the standard unit for structural loads. km for distances in road design.
Milli (m, 10⁻³) — length: mm is the standard unit for structural dimensions, reinforcement bar diameters, and deflections.
Micro (μ, 10⁻⁶) — strain: microstrain (με) is used to express structural deformation measurements.
Giga (G, 10⁹) — stiffness: GPa is used for modulus of elasticity of steel (210 GPa) and concrete (20–40 GPa).
Kilo + base — mass: tonnes (t) and kilograms (kg) for material quantities.
How to convert between SI prefixes
To convert a value from one prefix to another, find the difference in exponents and apply it to the value.
Example: convert 350 kN to MN
kilo = 10³, mega = 10⁶
Difference = 10³ ÷ 10⁶ = 10⁻³
350 kN = 350 × 10⁻³ MN = 0.35 MN
Example: convert 0.025 m to mm
base = 10⁰, milli = 10⁻³
Difference = 10⁰ ÷ 10⁻³ = 10³
0.025 m = 0.025 × 10³ mm = 25 mm
SI prefixes with cubic units
When a unit is raised to a power (squared or cubed), the prefix factor must be raised to the same power. This is a common source of errors.
mm² to m²: (10⁻³)² = 10⁻⁶ — multiply by 10⁻⁶
mm³ to m³: (10⁻³)³ = 10⁻⁹ — multiply by 10⁻⁹
cm² to m²: (10⁻²)² = 10⁻⁴ — multiply by 10⁻⁴
A value of 1500 mm² = 1500 × 10⁻⁶ m² = 0.0015 m²
Use our SI prefix calculator
Need to convert between any SI prefixes instantly? The Buildref SI Prefix Scaler covers all 19 prefixes from yotta to atto, with a step-by-step breakdown of each conversion.
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