Light-Speed Delay Calculator
Plain JavaScriptCalculate how long a signal takes to travel at light speed — one-way or round-trip. Choose a common destination preset or enter a custom distance.
Inputs
Tip: Round-trip is useful for “command + reply” scenarios.
If filled, this overrides the preset.
Assumptions & notes speed of light, presets, variability
Uses the speed of light in vacuum: c = 299,792,458 m/s.
This ignores routing, processing, atmospheric effects and relativity (fine for a simple delay estimate).
Planet distances vary massively with orbital position. Presets are “typical / illustrative” values, not live ephemeris. If you need precision for a specific date, use a custom distance.
Quick intuition checks common examples
- Earth → Moon is about 1.3 seconds one-way.
- Earth → Sun is about 8 minutes 20 seconds one-way.
- Earth → Mars is typically minutes one-way (varies a lot).
Results
Selected distance (normalised)
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Signal delay
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In seconds (raw)
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Permalink (copies current settings)
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FAQ what is “round-trip”?
One-way is the time for a signal to reach the destination at light speed.
Round-trip (RTLT) is the time to send a command and receive a reply at light speed. In reality you also add processing time, routing, and sometimes store-and-forward delays.
Round-trip (RTLT) is the time to send a command and receive a reply at light speed. In reality you also add processing time, routing, and sometimes store-and-forward delays.