Time Dilation Calculator

The Lorentz factor, written gamma, measures how much time, length, and mass change at relativistic speeds. It equals one divided by the square root of one minus the velocity squared over the speed of light squared, and it grows without bound as speed approaches light speed.

At the speed of light the Lorentz factor would require dividing by zero and becomes infinite, so no object with mass can reach or exceed it. The calculator therefore only accepts velocities below 100 percent.

Proper time is the time measured by a clock in the moving object's own frame of reference. You enter it here, and the calculator multiplies it by the Lorentz factor to give the longer dilated time seen by a stationary observer.

It is real and has been measured many times. GPS satellites move fast enough that their onboard atomic clocks would drift by microseconds per day from special relativity alone; without correcting for it (and for gravitational effects) positions would be off by kilometers within hours. Muons created high in the atmosphere reach the ground in far greater numbers than their short half-life allows, because their internal clock runs slow at near-light speed. The 1971 Hafele-Keating experiment flew atomic clocks around the world and recorded exactly the time differences special and general relativity predict.