Is the speed of light the same in all materials?

No. The value c = 299,792,458 m/s applies only in a perfect vacuum. In glass the effective speed is about c/1.5 (roughly 200,000 km/s). In water it is c/1.33. This slowing is described by the refractive index n, so v = c/n.

What is an AU and how long does it take light to cross one?

One Astronomical Unit (AU) is the mean Earth-Sun distance: 149,597,870.7 km. Light takes 499.0 seconds (8 min 19 s) to cover 1 AU. Pluto at 39.5 AU averages about 5.5 hours of light-travel time from the Sun.

How long does light take to cross the Milky Way?

The Milky Way galaxy is roughly 100,000 light-years in diameter. Light therefore takes about 100,000 years to cross it from edge to edge. Even at the speed of light, intergalactic travel on human timescales is impossible without relativistic time dilation.

Speed of Light Calculator

Q: How long does light take to travel from the Sun to the Earth?

Light takes about 499.0 seconds (8 minutes 19 seconds) to travel 1 AU (149,597,870.7 km). This means you are always seeing the Sun as it was over 8 minutes ago.

Q: What is the speed of light in km/s?

The speed of light is 299,792.458 km/s, which rounds to about 300,000 km/s. This is the value most commonly cited in astronomy textbooks.

Q: How long does light take to travel from the Earth to the Moon?

The Moon averages 384,400 km from Earth. Light covers this in about 1.282 seconds, so the round-trip delay for a laser reflected off the Moon is roughly 2.56 seconds.

Q: How does the speed of light compare to Mach 1?

Sound at sea level (20 degrees C) travels at about 343 m/s. Light is roughly 874,030 times faster, equivalent to Mach 874,030. In the time it takes sound to cross a 10-metre room, light has already circled the Earth nearly 25 times.

Find how long light takes to cross any distance, or how far light travels in any time span, with full unit conversion.

Distance

Time Duration

Exact value of c in common units:

Light Travel Time

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Nanoseconds (ns)

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Microseconds (µs)

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Milliseconds (ms)

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Seconds (s)

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Minutes (min)

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Hours (hr)

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Days

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Years

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Input Distance (m)

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Input Distance (km)

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Input Distance (AU)

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Distance Covered

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Metres (m)

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Kilometres (km)

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Miles (mi)

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Astronomical Units (AU)

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Light-years (ly)

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Input Time (seconds)

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c in m/s

299,792,458 m/s

c in km/s

299,792.458 km/s

c in km/h

1,079,252,848.8 km/h

c in mph

670,616,629.4 mph

c in ft/s

983,571,056.4 ft/s

c in knots

582,749,918.4 kn

Mach equivalent

Mach 874,030

💡 What is the Speed of Light Calculator?

The speed of light calculator is a tool for converting between the speed of light constant c and practical physical quantities: travel time for a beam of light over any distance, the distance light covers in any time interval, and the value of c expressed in every common speed unit. It is used in physics education, astronomy, telecommunications engineering, and wherever light-speed propagation matters.

The speed of light in a vacuum is exactly 299,792,458 metres per second. This value is not a rounded measurement: since 1983 the SI definition of the metre is based on c, making it a defined constant with no measurement uncertainty. In everyday terms, light travels about 300,000 km per second, circles the Earth roughly 7.49 times in a single second, and covers the Earth-Moon distance in just 1.28 seconds.

Practical use cases for this calculator include: estimating signal latency in satellite communications (a geostationary satellite at 35,786 km adds about 240 ms one-way delay), understanding why astronomers describe objects as "X light-years away" (that distance is how far light travels in X years), and converting c into non-SI units for engineering problems involving radar, GPS, and optical fibre systems.

The three modes cover different problem types. Travel Time mode answers questions such as "how long does it take light to reach Mars?" Distance Covered mode answers "how far does light travel in one microsecond?" (useful in GPS timing, where 1 ns corresponds to about 30 cm of path-length uncertainty). Speed Units mode is a reference table showing c in m/s, km/s, km/h, mph, ft/s, knots, and Mach number at sea level.

📐 Formula

t = d ÷ c | d = c × t

t = light travel time (seconds)

d = distance (metres)

c = speed of light in vacuum = 299,792,458 m/s (exact SI constant)

Example 1: Earth to Sun = 149,597,870,700 m ÷ 299,792,458 m/s = 499.0 s (8 min 19 s)

Example 2: c in 1 ns = 299,792,458 × 1 × 10^-9 = 0.2998 m (~30 cm per nanosecond)

📖 How to Use This Calculator

Steps

Select a calculation mode - Choose Travel Time to find how long light takes to cross a distance, Distance Covered to find how far light travels in a time span, or Speed Units to see c in every common unit.

Enter your value - Type the distance or time into the input box. Use the dropdown to pick the unit (metres, AU, light-years, seconds, years, etc.).

Read the results - The calculator instantly shows the primary answer and a full breakdown across all relevant units so you can compare directly.

💡 Example Calculations

Example 1 - Earth to Sun

How long does it take light to travel from the Sun to the Earth?

Earth-Sun distance = 1 AU = 149,597,870.7 km = 149,597,870,700 m.

t = d / c = 149,597,870,700 / 299,792,458 = 499.005 s.

499 s = 8 minutes 19 seconds. The Sun you see is always just over 8 minutes in the past.

Travel time = 499.0 s (8 min 19 s)

Try this example →

Example 2 - GPS Signal Propagation Delay

GPS satellites orbit at 20,200 km. What is the one-way signal delay?

Distance = 20,200 km = 20,200,000 m.

t = 20,200,000 / 299,792,458 = 0.06738 s = 67.38 ms.

GPS receivers correct for this propagation delay plus atmospheric and relativistic effects to achieve sub-metre position accuracy.

One-way signal delay = 67.38 ms

Try this example →

Example 3 - Distance Light Covers in One Nanosecond

How far does light travel in 1 nanosecond? (Critical in digital circuit design)

1 ns = 1 x 10 to the power of minus 9 seconds.

d = c x t = 299,792,458 x 0.000000001 = 0.2998 m.

Light travels about 30 cm per nanosecond. A 3 GHz CPU clock period is 0.33 ns, meaning signal paths longer than about 10 cm become a timing concern in high-speed chip layout.

Distance in 1 ns = 0.2998 m (about 30 cm)

Try this example →

❓ Frequently Asked Questions

What is the speed of light in m/s?+

The speed of light in a vacuum is exactly 299,792,458 metres per second. This is a defined constant: since 1983 the SI metre is defined so that c has precisely this value, with no measurement uncertainty attached.

How long does light take to travel from the Sun to the Earth?+

Light takes about 499.0 seconds, or 8 minutes 19 seconds, to travel 1 AU (149,597,870.7 km). This means the Sun you see is always over 8 minutes in the past. If the Sun suddenly vanished, you would not know for more than 8 minutes.

What is the speed of light in km/s?+

The speed of light is 299,792.458 km/s, commonly rounded to 300,000 km/s in textbooks. In one second, light covers a distance equivalent to about 7.49 times the Earth's circumference (40,075 km).

How long does light take to travel from the Earth to the Moon?+

The Moon averages 384,400 km from Earth. Light takes about 1.282 seconds one-way, so a round-trip laser pulse takes about 2.56 seconds. Lunar Laser Ranging experiments use this principle to measure the Earth-Moon distance to millimetre precision.

How far does light travel in one year (one light-year)?+

One light-year equals 9,460,730,472,580.8 km (about 9.461 trillion km) or 63,241 AU. It is the standard unit for stellar and intergalactic distances. The nearest star, Proxima Centauri, is 4.24 light-years away.

Is the speed of light the same in water and glass as in vacuum?+

No. In any transparent medium, light slows to v = c / n, where n is the refractive index. Water has n about 1.33 (light travels at roughly 225,000 km/s), and glass has n about 1.5 (about 200,000 km/s). This slowing causes refraction. The c in this calculator is the vacuum value.

How does the speed of light compare to Mach 1?+

Sound at sea level and 20 degrees C travels at 343 m/s. The speed of light is about 874,030 times faster, equivalent to Mach 874,030. While a commercial jet flies at roughly Mach 0.85, light covers the same distance in about one microsecond per 300 metres.

What is a light-second in kilometres?+

A light-second is 299,792.458 km, or about 7.49 times the circumference of the Earth. The Moon at 384,400 km is 1.28 light-seconds away. The geostationary orbit belt at 35,786 km is about 0.12 light-seconds from Earth's surface.

What is the speed of light in mph?+

The speed of light equals 670,616,629.4 mph. By comparison, the Parker Solar Probe, the fastest human-made object, reached about 692,000 km/h (430,000 mph) at its closest solar approach, which is still only 0.064% of the speed of light.

Why is the speed of light called c?+

The symbol c comes from the Latin word celeritas, meaning swiftness or speed. It was popularised by Albert Einstein in his 1905 paper on special relativity, where the mass-energy equivalence E = mc squared made c one of the most famous constants in physics.

How long does it take light to cross the Milky Way?+

The Milky Way is about 100,000 light-years in diameter. Light takes 100,000 years to cross it edge to edge. Even the nearest large galaxy, Andromeda, is 2.537 million light-years away, so light from there takes 2.537 million years to reach Earth.

How does this calculator handle very large or very small distances?+

The calculator accepts distances from fractions of a metre all the way to parsecs and beyond. Use the unit dropdown to select the appropriate scale (m, km, AU, ly, pc) and enter numbers in standard decimal form. Results are displayed in the most human-readable unit alongside a full multi-unit breakdown.

🔗 Related Calculators

📌 Quick Tips

💡The speed of light in a vacuum is exactly 299,792,458 m/s by SI definition since 1983. It is a defined constant, not a measured value.

💡One light-year is the distance light travels in one Julian year (365.25 days): about 9.461 trillion km or 63,241 AU.

💡Light takes 1.282 seconds to travel from the Earth to the Moon, and 8.317 minutes to travel from the Sun to the Earth.

💡The speed of light in glass or water is less than c due to the refractive index (roughly c/1.5 in glass). This calculator uses the vacuum value c.

💡In digital electronics, light travels about 30 cm per nanosecond in vacuum. This physical limit constrains high-speed chip design.