Redshift calculator
From redshift to recession speed and distance.
Because the universe expands, the light of distant galaxies is stretched — towards the red end of the spectrum. From this redshift z you can derive the recession speed and a rough distance. Enter a z value and see how fast a galaxy is receding from us.
Enter a redshift z
See how fast and how far a galaxy is.
Recession speed
179,875 km/s
60 % of light speed
Approx. distance
8,381 Mly
2,570 Mpc
Universe was
2× smaller
when the light left
Relativistic v from z; distance via Hubble law (H₀ = 70). Approximation — large z needs a full cosmological model.
What redshift tells us
For small values, v = c·z holds approximately. Via the Hubble constant (about 70 km/s per megaparsec) this yields a distance. The larger z, the farther — and the deeper we look into the universe’s past.
Limits of the approximation
For very large z (z > 1) the simple formula breaks down because relativistic and cosmological effects dominate. A full cosmological model is then required. Our calculator gives an illustrative order of magnitude.
Frequently asked questions
What does z = 1 mean?
The light’s wavelength has doubled. The light travelled for billions of years — we see the galaxy as it was when the universe was about half its current age.
Is redshift the same as the Doppler effect?
Related but not identical. Cosmological redshift arises from the expansion of space itself, not just motion through space.
Can z be greater than 1?
Yes. The most distant observed galaxies have z above 13 — their light dates to the first few hundred million years after the Big Bang.
Do galaxies move faster than light?
Very distant galaxies appear to recede faster than light — allowed because space itself expands, rather than the galaxy racing through space.
What is the Hubble constant?
It describes how fast the universe expands: about 70 km/s per megaparsec. Its exact value is debated in current research (the Hubble tension).
How accurate is the distance?
It is an approximation for small z. Precise cosmological distances require assumptions about matter and dark-energy density.
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