What speed of light value does this tool use?

The tool uses the exact defined value of 299,792.458 km/s (approximately 186,282 miles per second), which is the 1983 BIPM definition of the speed of light in a vacuum.

What is an Astronomical Unit (AU)?

One AU is the mean distance from Earth to the Sun, defined by the IAU as exactly 149,597,870.7 km. Light covers this distance in about 8 minutes and 20 seconds.

How many km is one light-year?

One light-year is exactly 9,460,730,472,580.8 km — the distance light travels in one Julian year (365.25 days) at 299,792.458 km/s.

How many km is one parsec?

One parsec equals 3.085677581×10^13 km, or about 3.26156 light-years. It is defined as the distance at which one AU subtends an angle of one arcsecond.

Why does it matter how long light takes to travel?

When you look at a star 100 light-years away, you are seeing it as it was 100 years ago. The farther the object, the older the image you observe. Light travel time is the universe's built-in time machine.

Can I use this for distances within Earth?

Yes. Enter a distance in km — for example, 40,075 km for Earth's circumference — and the tool will show that light circles Earth in about 0.134 seconds.

How long would it take a spacecraft to travel 49 light-years?

This calculator always uses the speed of light, which gives 49 years. For a slower craft, divide the distance by its speed: at 10% of light speed the trip takes 490 years, and at Voyager 1's roughly 17 km/s it takes about 860,000 years. The same rule works closer to home, since a car at 100 km/h would need 3,840 hours (160 days) to cover the 384,000 km to the Moon. Time felt by a crew near light speed is a separate question for the Time Dilation Calculator.

How long does light take to reach the Moon from Earth?

About 1.28 seconds one way at the average distance of 384,400 km, or about 2.56 seconds for a round-trip signal. Because the Moon's distance varies from about 356,500 to 406,700 km, the one-way time ranges from about 1.19 to 1.36 seconds.

How long does light take to get from Mars to Earth right now?

Between about 3 and 22 minutes, depending on where both planets are in their orbits. At the January 12, 2025 close approach Mars was 0.642 AU (96 million km) away, a 5 min 20 s light delay. Use the planet table on this page and pick any date from 1800 to 2050 to see the distance and the one-way and round-trip delay.

How long does a signal take to reach Voyager 1?

About one day. NASA expects Voyager 1 to reach one light-day from Earth, about 25.9 billion km or 173 AU, on November 18, 2026, so a command then takes 24 hours to arrive and a reply another 24 hours. The distance still swings by about ±1 AU (±8 light-minutes) over the year as Earth orbits the Sun.

Why is the observable universe 93 billion light-years across if it is only 13.8 billion years old?

Because space expanded while the light was traveling. The oldest light we see, the cosmic microwave background, has traveled for about 13.8 billion years, but the matter that emitted it is now about 46 billion light-years away. Distance ÷ c works for the solar system and our galaxy; at cosmological distances light travel time and present-day distance are different numbers, so this calculator does not treat the observable-universe radius as a travel time.

How long does sunlight take to reach Earth?

About 8 minutes 19 seconds (499.0 s) at the mean distance of 1 AU, 149,597,870.7 km. It ranges from about 8 min 11 s in early January at perihelion to about 8 min 27 s in early July at aphelion.

Light Travel Time Calculator

Convert distance to light travel time or elapsed time to distance, including one-way and round-trip signal paths, see the light delay from Earth to every planet on any date, and compare with spacecraft speeds.

Light travel time = distance ÷ 299,792.458 km/s.

Enter Distance

Calculation Mode

Distance

Unit

Signal Path

Time for light to travel from sender to destination.

Common Distances

Enter a distance above and press Calculate.

Light Time from Earth to Each Planet on a Date

Distances change daily. Computed from JPL approximate planetary elements (1800–2050), accurate to well under 1% of the distance.

Date (UTC)

Planet	Distance	One way	Round trip	Load

Method, scope, and privacy

Calculations use the exact vacuum speed of light, 299,792.458 km/s. Round trip means a signal travels the same distance out and back; in time-to-distance mode, the entered round-trip time is divided by two to find one-way range.

This is an ideal vacuum propagation result. Atmosphere, media, orbital motion, routing, equipment, processing, and relativistic spacecraft effects are not modeled. All inputs stay in your browser and no calculation request is sent to an external service.

Definitions: BIPM definition of the metre and c, IAU 2012 Resolution B2 (astronomical unit), NIST SI length conversions, and JPL approximate positions of the planets.

Reference: Light Travel Times for Famous Distances

Journey	Distance	Light Travel Time
Earth to Moon (average)	384,400 km	1.28 s
Earth to Sun (1 AU)	149,597,871 km	8 min 19 s
Sun to Mercury (mean)	57,909,227 km	3 min 13 s
Sun to Venus (mean)	108,209,475 km	6 min 1 s
Sun to Mars (mean)	227,943,822 km	12 min 40 s
Earth to Mars (closest possible)	54,600,000 km	3 min 2 s
Earth to Mars (farthest)	401,000,000 km	22 min 18 s
Sun to Jupiter (mean)	778,340,817 km	43 min 16 s
Sun to Saturn (mean)	1.427 × 10⁹ km	1 h 19 min
Sun to Uranus (mean)	2.871 × 10⁹ km	2 h 40 min
Sun to Neptune (mean)	4.498 × 10⁹ km	4 h 10 min
Sun to Pluto (mean, 39.5 AU)	5.909 × 10⁹ km	5 h 29 min
Earth to Voyager 1 (Nov 18, 2026)	2.590 × 10¹⁰ km	24 h 0 min
Sun to Proxima Centauri (4.2465 ly)	4.017 × 10¹³ km	4.25 years
Sun to Sirius (8.6 ly)	8.136 × 10¹³ km	8.60 years
Milky Way diameter (~100,000 ly)	9.461 × 10¹⁷ km	100,000 years
Andromeda Galaxy (2.537 million ly)	2.400 × 10¹⁹ km	2,537,000 years

Beyond our galaxy simple distance ÷ c stops working: the cosmic microwave background has traveled about 13.8 billion years, yet its source is now about 46 billion light-years away because space expanded meanwhile.

Quick answer

Light travel time = distance ÷ 299,792.458 km/s. A light-year is a distance (9.46 trillion km), so light needs exactly 49 years to cross 49 light-years, while a probe at Voyager 1's roughly 17 km/s would need about 860,000 years.

Examples

Distance	Light travel time (one way)
Earth to Moon, 384,400 km	1.28 seconds
Earth to Sun, 1 AU	8 min 19 s
Earth to Mars, 54.6 to 401 million km	3 to 22 minutes
Proxima Centauri, 4.2465 ly	4.25 years
49 light-years	49 years

Enter any distance in km, miles, AU, light-years or parsecs to get one-way or round-trip light time.

Summary

The Light Travel Time Calculator tells you how long it takes light — traveling at 299,792.458 km/s — to cover any distance you specify. Enter the distance in kilometers, astronomical units (AU), light-years, or parsecs and the tool converts it to travel time across every useful time scale: seconds, minutes, hours, days, and years. A built-in preset table covers the most iconic astronomical benchmarks, from Earth to the Moon and the planets out to the Andromeda Galaxy, and a dated table gives the current light delay from Earth to each planet.

How it works

Choose distance-to-time or time-to-distance mode.
Enter a distance or elapsed time and select its unit.
Choose a one-way light path or round-trip signal delay.
The tool converts your input to kilometers using exact IAU constants.
It divides the distance by the speed of light (299,792.458 km/s) to get travel time in seconds.
Results include decimal units plus a readable composite duration or converted distance.
Use the preset table to instantly load famous astronomical distances.
For planets, pick a date: the tool computes Earth-to-planet distance from JPL's approximate Keplerian elements (valid 1800–2050) and converts it to one-way and round-trip light time.

Use cases

Understand how "old" the light from distant stars actually is.
Teaching or studying the scale of the solar system and universe.
Comparing communication delays for hypothetical interstellar travel.
Visualizing why real-time communication with Mars probes is impossible.
Converting between distance units used in astronomy.
Checking how long ago light left a galaxy you are observing tonight.
Building intuition for cosmological distances in science writing.
Exploring how delays grow non-linearly as distances reach parsec scales.

Frequently Asked Questions

Last updated: 2026-10-09 · Reviewed by Nham Vu