Enter the time that passes on a moving clock and its speed as a percentage of light speed to find, with special relativity, how much time passes for an observer at rest.
Last updated: November 8, 2026
Moving clocks run slow in special relativity
In Einstein's special relativity, a clock moving fast runs slower as seen by an observer at rest: • Δt = Δt₀ ÷ √(1 − v² ÷ c²) = γ × Δt₀ • Δt₀ time on the moving clock (proper time), γ Lorentz factor, c ≈ 299,792 km/s Example: if 1 year passes on a spaceship travelling at 90% of light speed, γ = 1 ÷ √(1 − 0.81) ≈ 2.29. About 2.29 years pass on Earth. At everyday speeds the effect is tiny but measurable: GPS satellites would drift by kilometres a day without relativity corrections. This calculator leaves out the effect of gravity (general relativity).
Common questions about time dilation
γ = 1 ÷ √(1 − v²/c²). It tells you how many times slower a moving clock runs for an observer at rest. At 90% of c, γ ≈ 2.29.
Other tools you may find useful
Any unit (seconds, years…).
Percent of light speed, below 100.