Einstein's time dilation — how much slower a moving clock runs, the Lorentz factor, and the time difference.
How to use this tool
As a fraction of light speed, a percentage of it, or in km/s — light speed is 299,792.458 km/s.
The time measured by a stationary observer — seconds, minutes, hours, days or years.
Gamma (γ) is the big number: moving time is stationary time divided by γ.
The time difference underneath is how much younger the traveller stays over that interval.
Formula & worked example
Worked example: at 0.6 c, γ = 1 ÷ √(1 − 0.36) = 1 ÷ 0.8 = 1.25. Ten years on Earth pass as 10 ÷ 1.25 = 8 years on board — the traveller ages 2 years less. At 0.99 c, γ ≈ 7.09, so 10 Earth years become just 1.41 years on board.
Limitations — read this before you rely on it
This is special-relativistic time dilation for constant velocity in flat spacetime. It ignores acceleration, gravity (which causes its own time dilation — GPS satellites must correct for both), and the fact that massive objects cannot reach c.
Frequently asked questions
Moving clocks run slow compared with stationary ones — a prediction of Einstein's special relativity, confirmed by atomic clocks on aeroplanes and by the extended lives of fast-moving particles.
Gamma (γ) measures how strong the effect is: γ = 1 ÷ √(1 − v²/c²). At everyday speeds γ is essentially 1, so nothing seems to change; at 0.6 c it is 1.25, and it explodes towards infinity as speed approaches c.
GPS satellites move fast (special relativity slows their clocks) and sit in weaker gravity (general relativity speeds them up). Ignoring both would throw positions off by kilometres within a day.
In a sense, yes: travelling near light speed and returning leaves you younger than people who stayed behind — the twin paradox. Travelling to the past is a different, unsolved question.
As speed approaches c, γ approaches infinity, so the energy needed to accelerate further also approaches infinity. Massless particles like photons travel at c; everything with mass stays below it.