How this calculator works
Length along the direction of relative motion contracts by the inverse Lorentz factor. Proper length is measured with both endpoints at rest. The moving length requires endpoint positions measured simultaneously in the observer frame, not a photograph with unequal light travel times. Transverse dimensions are unchanged in this ideal inertial comparison. Speed is a magnitude below c. Equal lengths imply zero relative speed, while a measured moving length larger than proper length is inconsistent with the model.
Formula
L = L0/gamma = L0 sqrt(1-v²/c²); v = c sqrt(1-(L/L0)²).
Worked example
A 10 m object moving at 0.6c has Lorentz factor 1.25 and observed length 8 m. A measured ratio of 8 m to 10 m implies speed 0.6c.
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Frequently asked questions
Does the object feel compressed?
No. In its own inertial rest frame it retains its proper length.
Is this what a camera sees?
A photograph includes different light travel times. Length contraction is a simultaneous-coordinate measurement.
Does width contract too?
Only the component parallel to the relative motion is contracted in this setup.