Light Travel Time Calculator
Compute how long light takes to travel a given distance in a perfect vacuum using the defined speed of light (299,792.458 km/s)—useful for astronomy, communications latency limits, and physics homework.
Assumes straight-line travel through an ideal vacuum. Gravity wells, atmospheric refraction, and relativistic effects can lengthen real-world propagation times.
Examples
- 299,792.458 km ⇒ 1.00 seconds (one light-second)
- 384,400 km ⇒ 1.28 seconds (Earth ↔ Moon)
- 149,600,000 km ⇒ 499.0 seconds (Sunlight to Earth)
- 1,000 km fiber span ⇒ 0.00334 seconds vacuum (≈0.00499 s in glass, n ≈ 1.49)
FAQ
What speed of light does the calculator use?
It uses c = 299,792.458 kilometers per second, the constant defined by the International System of Units.
How do I account for travel through fiber or other media?
Multiply the vacuum time by the medium's refractive index. Single-mode fiber with n ≈ 1.468 stretches a 1 ms vacuum trip to about 1.468 ms.
Can I enter distances in miles or astronomical units?
Convert first: 1 mile = 1.60934 km and 1 astronomical unit = 149,597,870.7 km. Then enter the kilometer value.
Is this suitable for estimating network latency?
Yes, as a theoretical lower bound. Real latency includes switching, routing, encoding, and medium delays that add on top of the physical light travel time.
Additional Information
- Formula: Travel time (seconds) = Distance (km) ÷ 299,792.458 km/s, the exact SI definition of the speed of light.
- Convert the result to milliseconds by multiplying by 1,000, to minutes by dividing by 60, or to hours by dividing by 3,600.
- For in-medium propagation (fiber, glass, air), multiply the vacuum result by the medium's refractive index to approximate the slower speed.