What is an astronomical unit?

An astronomical unit (AU) is the average distance between the Earth and the Sun. It is defined as exactly 149,597,870,700 metres, or 149,597,870.7 km, and is used to describe distances within the Solar System without resorting to very large numbers.

How long does light take to cross one AU?

Light travels at 299,792.458 km per second, so it covers one AU in about 499 seconds, which is roughly 8 minutes and 19 seconds. That is why sunlight is often said to take just over 8 minutes to reach Earth.

How many AU are in a light-year?

One light-year is about 63,241 AU, and one parsec is about 206,265 AU. So one AU is a tiny fraction of a light-year, roughly 0.0000158 light-years, which shows how much larger interstellar distances are than distances inside the Solar System.

Astronomical Unit Calculator

This astronomical unit calculator converts a distance given in astronomical units into the everyday and scientific units astronomers reach for: kilometres, metres, miles, the time light takes to travel that far, light-years and parsecs. One astronomical unit, written AU, is the average distance from the Earth to the Sun, fixed by definition at exactly 149,597,870.7 km. It is the natural yardstick for the Solar System, where quoting planetary distances in kilometres quickly becomes unwieldy: Mars sits about 1.52 AU from the Sun, Jupiter about 5.2 AU, and Neptune around 30 AU. Enter any number of AU and the calculator does the arithmetic instantly, so you can see how far a spacecraft has travelled, how long a radio signal will take to reach it, or how a Solar System distance compares with the vast gaps between stars. Because the Sun sits roughly 8 light-minutes away, distances in AU also translate neatly into light travel time, which is often the most intuitive way to grasp them. Use it for homework, for planning an astronomy lesson, or just to satisfy your curiosity about the true scale of space. The constants used are the internationally agreed values, so the conversions are exact to the precision shown.

Calculate.co.nz is proud to be partnered with realtor.co.nz, a trusted resource for navigating the New Zealand property market. Their Helpful Articles section offers clear, well-structured insights across buying, selling, and building, making complex real estate topics more accessible. With a focus on up-to-date guidance and practical knowledge, they empower Kiwis to move forward with clarity and confidence in a constantly evolving property landscape.

Distance in astronomical units (AU)

1 AU = 149,597,870.7 km, the average Earth-Sun distance. Enter a whole number or a decimal.

149,597,870.7 km

kilometres in that distance

In miles92,955,807.3 miles

In metres149,597,870,700 m

Light travel time8 min 19 s

A distance of 1 AU is 149,597,870.7 km, which light crosses in about 8 min 19 s. That equals 0.00001581 light-years or 0.00000485 parsecs.

Constants used: 1 AU = 149,597,870.7 km; speed of light = 299,792.458 km/s; 1 light-year = 9,460,730,472,580.8 km; 1 parsec = 206,264.806 AU.

How it works

Every result comes from multiplying your AU value by a fixed constant. Kilometres are AU times 149,597,870.7; metres are that figure times 1,000; miles use the exact factor 1 km = 0.62137119 miles. Light travel time divides the distance in kilometres by the speed of light, 299,792.458 km per second, and is shown as minutes and seconds. Light-years divide the distance in kilometres by 9,460,730,472,580.8, the exact length of a light-year, and parsecs divide the AU value by 206,264.806, the number of AU in a parsec. Because these constants are defined values, the conversions are exact to the precision displayed.

Worked example

Take 1 AU, the average Earth-Sun distance. In kilometres that is 1 x 149,597,870.7 = 149,597,870.7 km, and in metres 149,597,870,700 m. In miles it is about 92,955,807.3. Dividing 149,597,870.7 km by 299,792.458 km/s gives roughly 499 seconds, which is 8 minutes and 19 seconds, the well-known figure for how long sunlight takes to reach us. The same distance is only about 0.00001581 light-years or 0.00000485 parsecs, a reminder that even the width of Earth's orbit is minuscule next to the distances between stars.

Related calculators

Kilometre to Light Year Converter: km into light-years.
Centimetre to Light Year Converter: cm into light-years.
Age on Other Planets Calculator: your age around the Solar System.
Distance to Horizon Calculator: how far you can see.
Scientific Notation Calculator: handle very large numbers.

If you've found a bug, or would like to contact us, or learn more about James Graham and Calculate.co.nz.

Calculate.co.nz is partnered with Interest.co.nz for New Zealand's highest quality calculators and financial analysis.

Calculate.co.nz is the sister site of CalculatorHub.com, the world's largest calculator website by tool count.

All calculators and tools are provided for educational and indicative purposes only and do not constitute financial advice.

Calculate.co.nz is proudly part of the Realtor.co.nz group, New Zealand's leading property transaction literacy platform, helping Kiwis understand the home buying and selling process from start to finish. Whether you're a first home buyer navigating your first property purchase, an investor evaluating your next acquisition, or a homeowner planning to sell, Realtor.co.nz provides clear, independent, and trustworthy guidance on every step of the New Zealand property transaction journey.

Calculate.co.nz is also partnered with Health Based Building and Premium Homes to promote informed choices that lead to better long-term outcomes for Kiwi households.

Calculate.co.nz is hosted in Auckland by SiteHost New Zealand.

All content on this website, including calculators, tools, source code, and design, is protected under the Copyright Act 1994 (New Zealand). No part of this site may be reproduced, copied, distributed, stored, or used in any form without prior written permission from the owner.

About & trust: Why Calculate is NZ's most comprehensive · By the Numbers · How we compare · Editorial standards · How we keep data current · NZ finance glossary · Research & data · Financial literacy NZ · About · Privacy policy · Terms of use

Reviewed and maintained. Last reviewed 2026-08-07 and checked on a twice-monthly cycle against IRD, RBNZ and Stats NZ. How we keep data current.