Speed of Light Delay Calculator
One-way and round-trip light-time across interplanetary distances. c = 299,792,458 m/s exactly (SI defining constant).
Result
- Distance225,000,000 km (1.5040 AU; 0.00002 ly)
- Speed of light c299,792,458 m/s (exact SI defining constant)
- One-way time12 min 30.5 s
- Round-trip time25 min 1.0 s
- In seconds (one-way)750.5192 s
- In minutes (one-way)12.5087 min
- In hours (one-way)0.2085 hr
Step-by-step
- c = 299,792,458 m/s (exact, CGPM 1983 — defining constant of the metre).
- One-way light time = distance / c = 225,000,000 km / 299,792.458 km/s = 750.5192 s.
- Round-trip = 2 × one-way = 25 min 1.0 s.
How to use this calculator
- Pick a destination from the preset list — covers Moon, Sun, all planets at closest/average/farthest, plus Voyager 1 and the nearest star system.
- Or switch to custom-distance mode and enter any distance in km.
- Read the one-way time as a sanity check; round-trip is what matters for any back-and-forth communication.
About this calculator
Light is fast but not instant. From Earth to the Moon: 1.28 seconds. To the Sun: 8 minutes 19 seconds. To Mars: 3 to 22 minutes depending on orbital position (closest is ~54M km at opposition, farthest is ~401M km when Mars is on the opposite side of the Sun). The round-trip time is what NASA cares about for spacecraft commanding: a rover on Mars at average distance has a ~25-minute command-confirm loop. Voyager 1, the most distant human-made object, currently sits about 165 AU from Earth (≈ 22.9 light-hours each way for round-trip telemetry). The speed of light c = 299,792,458 m/s is the defining constant of the metre since the 1983 CGPM redefinition — the metre is now defined as "the distance light travels in 1/299,792,458 of a second", so c is exact by definition rather than measured.