Convert Atto to Milli
Atto (a) to Milli (m) SI prefix conversion — enter any value below to get an instant result, or use the table for common values.
Results from this calculator are estimates provided for general informational purposes only, based on formulas, rates, and standards commonly accepted as of 2026. Figures may differ slightly from other calculators or professional sources due to rounding methods, differing assumptions, or regional regulations, and rules may change over time. Always consult a qualified professional — such as a financial advisor, healthcare provider, or other relevant specialist — before making decisions based on these results.
Result
1 a = 1E-15 m
1 Atto = 1E-15 Milli
1 Milli = 1E+15 Atto
1 a in every supported prefix
Conversion chart: Atto to Milli
Conversion table
| Atto (a) | Milli (m) |
|---|---|
| 0.01 a | 1E-17 m |
| 0.1 a | 1E-16 m |
| 1 a | 1E-15 m |
| 2 a | 2E-15 m |
| 3 a | 3E-15 m |
| 5 a | 5E-15 m |
| 10 a | 1E-14 m |
| 20 a | 2E-14 m |
| 50 a | 5E-14 m |
| 100 a | 1E-13 m |
| 1000 a | 1E-12 m |
Atto (a)
Definition: An SI prefix denoting a factor of 10⁻¹⁸ — one quintillionth of the base unit. From the Danish/Norwegian 'atten' (eighteen).
History: Adopted by the CGPM in 1964 alongside femto-, extending the SI down to the sub-atomic measurement scale.
Current use: Used in high-precision physics measurements, including attosecond-scale laser pulses used to study electron motion within atoms.
Milli (m)
Definition: An SI prefix denoting a factor of 10⁻³ — one thousandth of the base unit.
History: One of the original six metric prefixes adopted in France in 1795, from the Latin 'mille' (thousand).
Current use: Ubiquitous in science, medicine, and engineering — milligrams for dosages, milliliters for small volumes, and millimeters for precise length measurement.
Supported Units
| Prefix | Symbol | Multiplier (× base) |
|---|---|---|
| Yotta | Y | 1E+24 |
| Zetta | Z | 1E+21 |
| Exa | E | 1E+18 |
| Peta | P | 1E+15 |
| Tera | T | 1E+12 |
| Giga | G | 1E+09 |
| Mega | M | 1000000 |
| Kilo | k | 1000 |
| Hecto | h | 100 |
| Deka | da | 10 |
| None (base unit) | — | 1 |
| Deci | d | 0.1 |
| Centi | c | 0.01 |
| Milli | m | 0.001 |
| Micro | µ | 1E-06 |
| Nano | n | 1E-09 |
| Pico | p | 1E-12 |
| Femto | f | 1E-15 |
| Atto | a | 1E-18 |
| Zepto | z | 1E-21 |
| Yocto | y | 1E-24 |
About These Parameters
- Value
- The number of "From" prefix units you want to re-express in the "To" prefix. Since prefixes are just multiplier scales, this value can represent anything from a physical quantity's leading digits to a plain abstract number.
- From Unit
- The SI prefix your value is currently scaled by — for example, the "kilo" in "kilogram" or the "nano" in "nanosecond," entered here without the base unit attached.
- To Unit
- The SI prefix you want the value re-expressed in. Use the swap button to flip From and To instantly, handy when converting a datasheet figure from one prefix scale to another.
How Prefix Conversion Works
The Formula
Every SI prefix is defined by a fixed power-of-ten multiplier relative to the unprefixed base value. To convert a value from one prefix scale to another:
result = value × (multiplier of "From" prefix ÷ multiplier of "To" prefix)
For Atto → Milli: multiply by 1E-15. For example, 1 a × 1E-15 = 1E-15 m.
A History of Powers of Ten
The original six metric prefixes — kilo, hecto, deka, deci, centi, milli — were adopted by the French Academy of Sciences in 1795 alongside the metric system itself. As science and industry needed to describe ever-larger and ever-smaller quantities, the CGPM (Conférence Générale des Poids et Mesures) extended the table repeatedly: mega and micro in 1960 when the modern SI was formalized, giga and nano the same year, tera and pico following soon after, then peta/femto (1975), exa/atto (1975), and finally zetta/zepto and yotta/yocto in 1991 — the classical range covered on this page. In 2022 the CGPM went further still, adding ronna/ronto and quetta/quecto to keep pace with data-storage and astrophysical quantities that had begun to outgrow even yotta.
Why a Prefix-Only Converter Is Useful
Every other unit converter on this site attaches these same prefixes to one specific base unit — grams, watts, bytes, farads — because that's how the prefix is actually used in practice. But the underlying math of "how much bigger is giga than mega" never changes no matter which unit it's modifying, so it's often faster to reason about the prefix scale on its own: quickly checking that a "giga" figure is a billion and not a million, or converting a "nano" measurement from one datasheet into the "micro" scale used by a different one, without needing to know or care what physical quantity either figure describes.
Example
1 a equals 1E-15 m. Whatever base unit these prefixes are actually attached to — grams, bytes, watts, seconds — the same multiplier ratio applies: 1 kilo- is always 1,000 times 1 (unprefixed), and 1 mega- is always 1,000 times 1 kilo-, regardless of the quantity being measured.
Frequently Asked Questions
How many Milli are in 1 Atto?
1 Atto (a) equals exactly 1E-15 Milli (m).
Is giga a billion or a million?
Giga is always a billion (10⁹) times the base unit — a common point of confusion is with "mega," which is a million (10⁶). The two are frequently mixed up in casual conversation about data storage and computing speed, which is exactly the kind of mistake this converter is designed to catch.
Does this converter work for any unit, like grams or bytes?
Yes — this converter deliberately ignores the underlying unit and works purely with the multiplier scale. "1 kilo- to mega-" gives the same ratio (0.001) whether you mentally attach it to grams, watts, bytes, or any other quantity, because the prefix's multiplier is independent of what it's modifying.
What does "None (base unit)" mean in the unit list?
"None (base unit)" represents a value with no prefix attached at all — a plain gram, meter, or watt rather than a kilogram, kilometer, or milliwatt. It's included so you can convert directly between an unprefixed value and any prefixed scale, such as figuring out how many kilo- units equal one plain (unprefixed) unit.
Are ronna, ronto, quetta, and quecto included here?
Not yet — this page covers the classical SI prefix range from yotta (10²⁴) down to yocto (10⁻²⁴), which was complete from 1991 until 2022. The newer 2022 additions (ronna/ronna-, quetta/quecto) sit beyond even yotta and yocto and aren't included in this table.