About

Astronomical distances defy human intuition. An error of one decimal place when converting from pc (parsecs) to ly (light-years) can misplace a star by 3.26 light-years - the difference between Alpha Centauri and empty void. This calculator performs exact conversions between 16 distance units used in astrophysics, cosmology, and space navigation. All conversion factors trace to the IAU 2012 exact definition: 1 AU = 149,597,870,700 m, and the NIST value of c = 299,792,458 m/s. The tool assumes flat Euclidean space. It does not account for cosmological expansion (comoving vs. proper distance) or relativistic corrections. For redshift-dependent distances, use a cosmology calculator instead.

Precision matters in orbital mechanics. NASA's Mars Climate Orbiter was lost because of a unit mismatch between lbf·s and N·s. This tool eliminates that class of error for distance units. Results are displayed in both decimal and scientific notation. Pro tip: when working with extragalactic distances, prefer Mpc (megaparsecs) to avoid coefficients exceeding 10²⁵.

Formulas

All conversions use a base-unit pivot through meters. Given a value V in source unit S to be converted to target unit T:

V_T = V_S × F_SF_T

Where F_S is the number of meters in one unit of S, and F_T is the number of meters in one unit of T.

The parsec is derived from the astronomical unit via the exact relation:

1 pc = 648,000π AU = 648,000π × 149,597,870,700 m

The light-year uses the Julian year (365.25 days exactly):

1 ly = c × 365.25 × 86,400 s = 9,460,730,472,580,800 m

Where c = 299,792,458 m/s (exact, SI definition). V_S = input value in source units. V_T = output value in target units. F = conversion factor (meters per unit).

Reference Data

Unit	Symbol	Meters (exact or best)	Relative to 1 AU	Common Use
Meter	m	1	6.6846 × 10⁻¹²	SI base unit
Kilometer	km	1,000	6.6846 × 10⁻⁹	Planetary radii, altitudes
Mile	mi	1,609.344	1.0758 × 10⁻⁸	US/UK informal
Astronomical Unit	AU	1.495978707 × 10¹¹	1	Solar system distances
Light-second	ls	2.99792458 × 10⁸	2.0040 × 10⁻³	Earth - Moon delay
Light-minute	lm	1.79875475 × 10¹⁰	1.2024 × 10⁻¹	Sun - Earth light travel
Light-hour	lh	1.07925285 × 10¹²	7.2143	Outer solar system
Light-day	ld	2.59020684 × 10¹³	1.7314 × 10²	Kuiper belt scale
Light-year	ly	9.4607304726 × 10¹⁵	6.3241 × 10⁴	Stellar distances (popular)
Parsec	pc	3.0856775815 × 10¹⁶	2.0626 × 10⁵	Stellar parallax measurements
Kiloparsec	kpc	3.0856775815 × 10¹⁹	2.0626 × 10⁸	Galactic structure
Megaparsec	Mpc	3.0856775815 × 10²²	2.0626 × 10¹¹	Hubble constant, galaxy clusters
Gigaparsec	Gpc	3.0856775815 × 10²⁵	2.0626 × 10¹⁴	Observable universe scale
Lunar Distance	LD	3.844 × 10⁸	2.5696 × 10⁻³	Near-Earth asteroid tracking
Solar Radius	R_⊙	6.9634 × 10⁸	4.6542 × 10⁻³	Stellar astrophysics
Earth Radius	R_⊕	6.371 × 10⁶	4.2588 × 10⁻⁵	Exoplanet sizes, orbital mechanics

Frequently Asked Questions

A light-year (ly) is the distance light travels in one Julian year (365.25 days): 9.461 × 10¹⁵ m. A parsec (pc) is the distance at which 1 AU subtends an angle of 1 arcsecond: 3.086 × 10¹⁶ m, or about 3.2616 ly. Professional astronomers prefer parsecs because parallax measurements yield distances directly in parsecs. Popular science uses light-years because the concept is more intuitive.

This tool converts between static distance units in flat Euclidean space. Cosmological distances (comoving distance, luminosity distance, angular diameter distance) depend on the Hubble constant H₀, matter density Ω_m, and dark energy density Ω_Λ. Those require integrating the Friedmann equations, which is a fundamentally different calculation. A result in Mpc from this tool is a proper distance at a fixed epoch, not a comoving distance.

The value 1 AU = 149,597,870,700 m is the exact IAU 2012 definition (Resolution B2). Before 2012, the AU was defined dynamically via the gravitational parameter and had uncertainty. The current definition is a fixed conventional value, so there is zero uncertainty in the AU-to-meter conversion factor itself.

JavaScript uses IEEE 754 double-precision floating point, which provides about 15 - 17 significant decimal digits and a range up to approximately 1.8 × 10³⁰⁸. Converting 1 Gpc to meters yields ~3.09 × 10²⁵, well within range. However, converting very small fractions of meters to gigaparsecs may lose precision below 10⁻¹⁵ relative accuracy. The tool displays results in scientific notation when the value exceeds 10⁶ or falls below 10⁻³.

No. The mean Earth - Moon distance is approximately 384,400 km, but the actual distance varies from about 356,500 km (perigee) to 406,700 km (apogee). The value 384,400 km used here is the conventional mean. Similarly, the Solar Radius (695,700 km, IAU 2015 nominal) and Earth Radius (6,371 km, mean volumetric) are nominal values. For high-precision ephemeris work, use JPL Horizons data.

Yes. The input accepts standard scientific notation such as 1.5e11 or 3.08E16. The parser handles both lowercase "e" and uppercase "E". Negative exponents like 5.2e-4 are also supported. This is essential for astronomical values that routinely span dozens of orders of magnitude.