Engineering Prefix Calculator
SI (metric) prefixes are shorthand for powers of ten grouped in steps of three: kilo (10³), mega (10⁶), giga (10⁹) going up, and milli (10⁻³), micro (10⁻⁶), nano (10⁻⁹) going down.
Enter Values
How to use this calculator
- Enter the value you want to convert.
- Set 'From prefix' to the power of 10 your value is already in (e.g. 3 = kilo, -6 = micro, leave 0 or blank for no prefix).
- Set 'To prefix' to the power of 10 you want the answer in; read the converted value, the plain (unprefixed) value and the best engineering prefix.
How it works
SI (metric) prefixes are shorthand for powers of ten grouped in steps of three: kilo (10³), mega (10⁶), giga (10⁹) going up, and milli (10⁻³), micro (10⁻⁶), nano (10⁻⁹) going down. Engineering notation always uses these thousand-steps so a quantity reads as a tidy mantissa times a named prefix.
The plain value is value × 10^(from), i.e. the number with no prefix at all. To express it in the target prefix the calculator divides by 10^(to): converted = value × 10^(from − to). The 'best engineering prefix' is found by taking the base-3 floor of log₁₀ of the plain value, so the mantissa lands between 1 and 1000 (0.00047 → 470 µ, 2,500,000 → 2.5 M).
Worked example
Convert 4.7 kilo to milli. Enter Value = 4.7, From prefix = 3 (kilo) and To prefix = -3 (milli). The plain value is 4.7 × 10³ = 4700. Dividing by 10⁻³ gives 4,700,000 m. The tool also reports the best engineering prefix for 4700, which is 4.7 k (kilo).
Common mistakes
- Entering a prefix power that isn't a multiple of 3 — SI prefixes only exist at 10³ steps (3, 6, 9, …), so 3 = kilo and 6 = mega, not 4 or 5.
- Confusing capital M (mega, 10⁶) with lower-case m (milli, 10⁻³); they differ by a factor of a billion.
- Reading the 'From' and 'To' fields backwards — 'From' is the prefix your number already carries, 'To' is the prefix you want the answer in.
Frequently asked questions
Which prefixes are supported?
The full modern SI range in steps of three, from quetta (10³⁰) and ronna (10²⁷) down through kilo, milli and micro to ronto (10⁻²⁷) and quecto (10⁻³⁰). Enter each as its power of ten (kilo = 3, mega = 6, milli = -3, micro = -6, and so on).
What is the 'best engineering prefix' output?
It rewrites the plain value using the largest prefix that keeps the leading number between 1 and 1000 — the convention engineers use. For example 2,500,000 becomes 2.5 M and 0.00047 becomes 470 µ, so the figure is easy to read and speak aloud.
How is this different from scientific notation?
Scientific notation allows any power of ten (e.g. 4.7 × 10⁵). Engineering prefixes restrict the exponent to multiples of three and give each a name (kilo, mega, milli…), which is why cables, resistors and frequencies are quoted as kΩ, MHz or µF rather than raw powers.
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