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Light-Second to Kilometer Converter

Enter a value and choose the conversion direction.

How it is calculated

The conversion uses one fixed factor, the distance light covers in a vacuum in one second: 1 ls = 299,792.458 km. Light-seconds to kilometres is km = ls × 299,792.458, and kilometres to light-seconds is ls = km ÷ 299,792.458. Choose the direction first, because the same number means very different things in each. Kilometre results are shown to three decimals and light-second results to six. When a non-zero result is too small for that precision, it is shown in scientific notation (such as 3.3356 × 10⁻¹³) instead of a misleading 0.000. Negative values return nothing, and zero returns zero. The tool also prints the factor and a one-line statement of the conversion, which you can copy as written.

Worked example

The average Earth–Sun distance is about 149,597,870.7 km. In the kilometres-to-light-seconds direction the tool divides: 149,597,870.7 ÷ 299,792.458 = 499.004784 light-seconds, which is about 8 minutes and 19 seconds (499.0 ÷ 60). So sunlight takes roughly that long to reach us. Going the other way, 1.28 light-seconds × 299,792.458 gives 383,734.346 km, close to the Earth–Moon distance.

Try this example →

Common mistakes

  • Leaving the direction on light-seconds to kilometres when the number you typed is already in kilometres. Entering 384,400 then returns 115,240,220,855.200 km instead of about 1.282220 light-seconds.
  • Confusing a light-second with a light-year. A light-year is about 31.6 million times larger than a light-second, so mixing them up by name gives answers off by that factor.
  • Rounding the factor to 300,000 km/s in your own check. This gives a result about 0.07% off the tool's exact value.
  • Reading a result such as 3.3356 × 10⁻¹³ as an error. It is scientific notation for a number with twelve zeros after the decimal point, which the tool uses for very small non-zero values.
  • Typing a negative number to indicate direction of travel. The tool accepts only zero or positive distances and returns nothing otherwise.

About this calculator

A light-second is the distance light travels through a vacuum in one second — exactly 299,792.458 kilometers, since the speed of light (c) is a defined constant rather than a measured one. Light-based distance units like the light-second, light-minute, and light-year are used throughout astronomy because they scale naturally with how long it takes light (and therefore any signal or observation) to cross a given distance.

This converter switches between light-seconds and kilometers in either direction using that exact conversion factor. It's handy for astronomy students and space enthusiasts putting solar-system distances into perspective — the Moon is about 1.28 light-seconds away, the Sun about 499 — and for anyone working with spacecraft communication delays, where light-seconds double as a direct measure of signal lag to a probe or satellite.

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