Light Travel Time Calculator

How it is calculated

Enter a distance in kilometres. The tool divides it by the speed of light in vacuum: t (s) = distance ÷ 299,792.458, and then t (min) = t (s) ÷ 60. Both values are rounded to two decimals. The distance must be greater than zero.

The result is the one-way time. For a radio signal sent to a probe and answered, double it, and add any processing time on board. The tool assumes straight-line travel through empty space. Light passing through air, water or glass moves more slowly, so the figure does not apply to cables or fibre, where signals take noticeably longer than the vacuum value. Very short distances produce a zero in the minutes column because of the two-decimal rounding; use the seconds figure for them.

Common mistakes

• Entering the distance in miles or metres. The field is in kilometres, and 93 million miles typed as 93000000 gives a result about 38% too small.
• Using the answer as the round-trip delay. It is one way only; double it for a command and reply.
• Applying the result to fibre-optic or cable signals. Light in glass is slower than in vacuum, so real delays are longer.
• Reading 0.02 minutes as zero. Short distances show tiny minute values, so the seconds figure is the useful one.
• Entering a distance in light-years. Convert it to kilometres first, or the time will be wildly too small.