Astronomical Distance Converter (ly / pc / AU / km)
Astronomical distances span more than twenty orders of magnitude — from the Earth–Moon distance (10⁸ m) to the edge of the observable universe (10²⁶ m). Enter any value and the tool converts it simultaneously to metres, kilometres, miles, Earth radii, Earth–Moon distances, astronomical units (AU), light-seconds / minutes / hours / days, light-years, parsecs and megaparsecs, using the IAU 2012 / 2015 anchor definitions. Common-object presets (Moon, Mars, Proxima Centauri, Sgr A*, Andromeda) fill the field with one click.
Common objects (one-click)
Please enter a valid non-negative distance.
Anchors: 1 au = 149 597 870 700 m (IAU 2012); 1 ly = c × Julian year = 9.461 × 10¹⁵ m; 1 pc = 648 000/π au; c = 299 792 458 m/s exactly.
Formula
1 au = 149 597 870 700 m (IAU 2012 Resolution B2) 1 ly = c × Julian year = 299 792 458 m/s × 365.25 × 86 400 s = 9 460 730 472 580 800 m 1 pc = 648 000 / π au ≈ 3.085 677 581 × 10¹⁶ m (IAU 2015 Resolution B2) Speed of light c = 299 792 458 m/s (exact, 1983 SI definition)
- · The astronomical unit (au) was redefined as exactly 149 597 870 700 m by IAU 2012 Resolution B2; previously it was tied to the mean Earth–Sun distance. The classroom approximation "1 au ≈ 1.496 × 10¹¹ m" still works.
- · A light-year uses the Julian year (365.25 d × 86 400 s = 31 557 600 s), not a tropical or civil year — so 1 ly = c × Julian year = 9.461 × 10¹⁵ m is an exact, dimensionless-by-convention value.
- · Parsec definition: the distance d at which one astronomical unit (the Earth–Sun baseline) subtends an angle of one arcsecond. Conversion: 1 pc = 648 000/π au ≈ 3.262 ly ≈ 3.086 × 10¹⁶ m. Professional astronomy quotes distances in parsec / kpc / Mpc rather than light-years.
- · Light-seconds / minutes / hours / days are also length units — "1 light-second" = the distance light travels in one second ≈ 3 × 10⁸ m. The Earth–Moon distance is ≈ 1.28 light-seconds, Earth–Sun is 8.32 light-minutes, Neptune at opposition is ≈ 4.2 light-hours.
- · Useful checkpoints: Earth–Moon = 384 400 km; 1 au = 8.32 light-minutes; outer Solar System (heliopause) ≈ 121 au; Proxima Centauri = 4.2465 ly; Milky Way diameter ≈ 100 000 ly = 30.7 kpc; Andromeda Galaxy (M31) ≈ 2.537 Mly = 0.778 Mpc.
- · For nearby stars (< 100 pc) parallax (Gaia mission) gives distance directly; for distant galaxies Hubble's law via redshift is used. The comoving radius of the observable universe is ≈ 46.5 Gly ≈ 14.3 Gpc — not 13.8 Gyr × c (cosmic expansion has stretched the light path).
- · References: IAU 2012 Resolution B2 (au), IAU 2015 Resolution B2 (parsec), CODATA 2018 (c as a defined constant), NASA Earth–Moon fact sheet.
Frequently asked
Why do both light-years and parsecs exist — which one should I use?
Both are entrenched, for different reasons. The light-year (ly) is intuitive for non-specialists — the number ties directly to light speed and time, so textbooks, popular science and ad copy default to it. The parsec (pc) is more convenient for working astronomers because it falls directly out of parallax: invert the parallax angle in arcseconds and you have the distance in parsecs, so converting Hipparcos / Gaia data (e.g. parallax 4 mas → 250 pc) is one division. Professional journals (ApJ, A&A, MNRAS) quote pc / kpc / Mpc almost exclusively. The conversion is 1 pc = 3.262 ly, so swapping is trivial — the tool shows both so you can pick whichever fits your audience.
Is a light-year a unit of time or distance?
It is a unit of *distance*, despite the "year" in the name — specifically, it is the distance light travels in one year. 1 ly = c × Julian year = 9.461 × 10¹⁵ m ≈ 9.461 trillion km. So "Proxima Centauri is 4.2465 ly away" means that a beam of light leaving Earth would take 4.2465 years to reach it — but the "4.2465 ly" itself is a length, not a time. The common sci-fi line "we are 5 light-years away" is describing distance; how long the ship actually takes to get there depends on its speed (the light-year only says "5 years at light speed").
Isn't the observable universe just 13.8 Gly across (the age of the universe times c)?
No — this is a common classroom slip. 13.8 Gyr is the age of the universe; multiplied by c, it gives only the *light-travel* distance ("how far did light move through space, ignoring expansion"). But space has been expanding the whole time, so the emitter is now much farther away. The current (comoving) distance to the most distant light we can see — the cosmic microwave background — is about 46.5 Gly (≈ 14.3 Gpc). In other words: that CMB photon was emitted from material now ~46.5 Gly away, even though only 13.8 Gyr have elapsed. The tool's Mpc tile lines up with this scale (46.5 Gly = 14 277 Mpc). Primary sources: Planck 2018 cosmological parameters; J. R. Gott et al., ApJ 624 (2005).
Related tools
Length Unit Converter
Cm, m, km, in, ft, yd, mi — common length units, converted instantly.
Temperature Converter
Convert between Celsius (°C), Fahrenheit (°F) and Kelvin (K) instantly.
Shoe Size Converter (US / UK / EU / CM)
Enter your foot length or a size in any system to see the equivalent US / UK / EU and centimetre sizes.
Paper Size Reference
Pick a paper size (A0–A10, B / C series, Letter, Legal, card sizes) to see its mm / cm / inch / pixel dimensions.
Number Base Converter (Binary / Octal / Decimal / Hex)
Convert a number simultaneously between binary, octal, decimal and hexadecimal — useful for programmers, students and electronics work.
Aspect Ratio & Screen Size Calculator
From an aspect ratio and one side, find the other; or from a diagonal and ratio, get real width and height (4:3, 16:9, 21:9 …).
Data Storage Unit Converter (B/KB/MB/GB/TB)
Convert between bytes, KB, MB, GB, TB and PB — distinguishing decimal (1000-base, drive labels) from binary (1024-base, OS-reported).
RGB ↔ HEX ↔ HSL Color Converter
Convert between HEX (#RRGGBB), RGB(0–255) and HSL(0–360, 0–100, 0–100) instantly — with a live colour preview swatch.
Tire Size Calculator (Spec / Diameter / Speedometer Error)
Convert a P-metric tire spec (e.g. 225/65R17) into sidewall height, overall diameter, circumference and revolutions per km — and compare against a reference size to see speedometer error.
Energy Unit Converter (J / kJ / cal / kcal / kWh / BTU / Wh)
Convert between joules, kilojoules, calories, kilocalories, kilowatt-hours, BTU and watt-hours in a single view — for nutrition labels, electricity bills, heating and fuel comparisons.
Pressure Unit Converter
Convert between Pa, kPa, bar, mbar, atm, psi, mmHg, Torr and inHg in a single view — all the everyday pressure units.
Torque Unit Converter
Convert between N·m, kgf·m, kgf·cm, lbf·ft, lbf·in and ozf·in — all the torque units used on torque wrenches, bicycle bolts and car service specs.
Angle Unit Converter
Convert between degrees, radians, gradians, turns, arcminutes, arcseconds, milliradians and NATO mil — with live sin/cos/tan values for reference.
Fuel Economy Converter (MPG ⇄ L/100km)
Convert between L/100km, km/L, US MPG and UK (imperial) MPG — compare car efficiency figures across regions without arithmetic.
Power Unit Converter
Convert between W, kW, MW, mechanical horsepower (hp), metric horsepower (PS), BTU/hr, ft·lbf/s, cal/s, kcal/h and refrigeration tons in one view.
px ↔ rem ↔ em Converter
Pick a root font-size (defaults to 16 px) and convert any CSS pixel, rem, em, percentage or point value into the others, with a reference table for common sizes.
Alcohol ABV ↔ Proof Converter & Standard Drinks Calculator
Convert alcohol by volume (ABV %) to US proof and UK proof, plus compute grams of pure alcohol and "standard drinks" per pour for US / UK / AU / HK.
Bandwidth & Throughput Converter (Mbps ↔ MB/s)
Convert network bandwidth (kbps, Mbps, Gbps) to file-transfer rate (KB/s, MB/s, MiB/s) and estimate how long a given file size takes.
Weight / Mass Unit Converter
Kilograms, grams, milligrams, pounds, ounces, stones, tonnes, jin and tael — common mass units, converted instantly.
Battery mAh ↔ Wh Converter (Power Bank Airline Rules)
Enter battery capacity (mAh) and voltage (V) to get watt-hours (Wh) instantly — with a live IATA airline check (100 Wh / 160 Wh cut-offs).
International Bra Size Converter
Enter underbust and bust (or pick a local size) to convert bra sizes across US, UK, EU, AU and JP standards.
International Ring Size Converter
Enter inner circumference (mm) or diameter to cross-reference US, UK, EU, JP and HK ring sizes.
Area Unit Converter
Square metres, hectares, acres, square feet, square miles and more — common area units, converted instantly.
Volume / Capacity Unit Converter
Litres, millilitres, cubic metres, US and imperial gallons, quarts, pints, fluid ounces, cups, tablespoons and teaspoons — converted instantly.
Speed Unit Converter
Convert freely between km/h, mph, m/s, ft/s, knots and Mach number.
Color Contrast Ratio (WCAG) Calculator
Enter foreground and background hex colors to compute the WCAG 2.x contrast ratio (1:1 to 21:1) and see whether the pair passes AA / AAA accessibility levels for normal and large text.
AWG Wire Size Calculator
Look up the diameter (mm), cross-sectional area (mm²), resistance per kilometre and safe ampacity for any American Wire Gauge (AWG) size — handy for low-voltage wiring design.
Frequency Unit Converter (Hz / kHz / MHz / GHz)
Convert between Hz, kHz, MHz, GHz, THz, RPM and BPM — one input fills every other unit instantly.
Force Unit Converter (N / kN / lbf / kgf / dyne)
Convert between newtons (N), kilonewtons (kN), pound-force (lbf), kilogram-force (kgf), dyne and ounce-force (ozf) — bridging SI, imperial and CGS force units used in physics and engineering.
Magnetic Flux Density Converter (T / mT / µT / Gauss)
Convert magnetic flux density between tesla (T), millitesla (mT), microtesla (µT), gauss (G) and milligauss (mG) — handy for comparing MRI fields, Earth field, speakers and lab magnets across very different scales.
Volumetric (Dimensional) Weight Calculator (Air / Courier)
Enter parcel length × width × height and DIM divisor (IATA air 6000 cm³/kg, express 5000, UPS/FedEx US 139 in³/lb) to compute volumetric weight and compare it with actual weight to find the chargeable weight.
Polar ↔ Cartesian Coordinate Converter
Convert Cartesian (x, y) ↔ polar (r, θ) coordinates in either direction with a degree/radian toggle — useful for math, physics, engineering and game-development work.
Map Scale Distance Converter
Enter a scale ratio (e.g. 1:25 000) and a measured map distance; the tool returns the real-world distance, or works in reverse from real distance to map distance — useful for hiking, map reading and town planning.