Convert Kilometer to Attometer
Convert kilometers to attometers instantly. 1 kilometer = 1e+21 attometer — use the live calculator, the exact formula, a conversion table and worked examples. Also check the Attometer to Kilometer converter for the reverse conversion.
Units explained
Kilometer
A kilometer is a metric unit of length equal to one thousand meters. It is the standard unit for measuring road distances, geographic distances, and other large-scale measurements in metric countries.
The kilometer was defined alongside the meter in 1795 using the standard SI prefix kilo- (from Greek chilioi, "thousand"), denoting one thousand units.
Kilometers are used worldwide (except the United States and a few others) for road signage, geographic distance, athletic events, and scientific distances at planetary scale. Speed limits in most countries are given in km/h.
Adopted 1795 in France as part of the original metric system; the kilometer became the global standard for road and geographic distance through the 19th and 20th century metric adoption.
Attometer
An attometer is a metric unit of length equal to one quintillionth of a meter (1×10⁻¹⁸ m). It is among the smallest length units in the SI system and is used almost exclusively in particle physics and theoretical work involving sub-nuclear dimensions.
The attometer was defined using the SI prefix atto- (from Danish atten, meaning eighteen), adopted by the Conférence Générale des Poids et Mesures (CGPM) in 1964 to denote one quintillionth (10⁻¹⁸).
Attometers appear in particle physics literature when describing distances comparable to or smaller than nuclear dimensions, including the range of weak nuclear interactions and theoretical Planck-scale ratios. They are rarely encountered outside specialised scientific work.
SI prefix atto- adopted in 1964; the attometer became the standard unit for sub-femtometer measurements in high-energy physics from the late 20th century onward.
Kilometer to Attometer conversion formula
The relationship between kilometers and attometers:
To convert kilometers to attometers, multiply the value in kilometers by 1e+21. To reverse, multiply attometers by 1e-21.
How to use this converter
Type a value into the calculator. The result in attometers updates as you type. Tap a quick value, copy the result with one click, or use the swap arrow to jump straight to the Attometer to Kilometer converter for the reverse direction.
Step-by-step: convert kilometers to attometers
- Write down the value in kilometers (km).
- Multiply that value by the factor 1e+21.
- The product is the equivalent value in attometers (am).
- To reverse, multiply the attometer value by 1e-21.
Worked examples
Example 1 — Convert 1 km to am:
1 × 1e+21 = 1e+21 am
Example 2 — Convert 100 km to am:
100 × 1e+21 = 1e+23 am
Real-world example — Kilometres to wavelengths
One kilometer equals one trillion attometers — a conversion physics teachers use to convey the gulf between everyday geographic and atomic scales.
1 km × 1e+21 = 1e+21 am
Real-world example — Geographic to wavelength scale
One kilometer equals one trillion attometers — illustrating the 12-order-of-magnitude span between geographic distance and atomic-feature scales.
1 km × 1e+21 = 1e+21 am
Kilometer to Attometer conversion table
Standard reference values for converting kilometers to attometers:
| Kilometer [km] | Attometer [am] |
|---|---|
| 0.01 | 1e+19 |
| 0.1 | 1e+20 |
| 1 | 1e+21 |
| 2 | 2e+21 |
| 3 | 3e+21 |
| 4 | 4e+21 |
| 5 | 5e+21 |
| 10 | 1e+22 |
| 20 | 2e+22 |
| 30 | 3e+22 |
| 40 | 4e+22 |
| 50 | 5e+22 |
| 100 | 1e+23 |
| 500 | 5e+23 |
| 1000 | 1e+24 |
Frequently asked questions
How many attometers is 1 kilometer?
How do I convert kilometers to attometers?
How do I convert attometers back to kilometers?
How many attometers is 100 kilometers?
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Atomic / Physics (6 units)
Typographic (3 units)
Sources & references
Conversion factor (1 km = 1e+21 am) verified against the following authoritative sources:
- BIPM — The International System of Units (SI Brochure 9th ed.)
Official BIPM publication defining the seven SI base units (including the meter) and the rules for their use. The global authority on units of measurement.
- NIST — Guide to the SI
US National Institute of Standards and Technology reference covering the SI base and derived units with definitions and usage rules for US technical practice.
- NIST Special Publication 811 — Guide for the Use of the International System of Units
Detailed NIST guide covering exact conversion factors between SI and US customary units along with formatting and rounding conventions.
- NIST — Refinement of values for the yard and pound (Federal Register 1959)
The treaty (signed by US
- International Hydrographic Organization — Resolution on the Nautical Mile
International authority that standardised the nautical mile at exactly 1852 m in 1929 — the value adopted worldwide for sea and air navigation.