Hubble's Law Calculator
Find a galaxy's recession velocity from distance.
Default 70
Results update as you type.
How it works
Hubble's law is the observation that led to our understanding of an expanding universe, and this calculator applies it. You enter a galaxy's distance in megaparsecs and, optionally, the Hubble constant, and it returns the galaxy's recessional velocity and that velocity as a fraction of the speed of light. The law, discovered by Edwin Hubble in 1929, states that the farther a galaxy is, the faster it appears to move away from us, with velocity directly proportional to distance. The constant of proportionality is the Hubble constant, whose value is around 70 kilometers per second per megaparsec, though its precise measurement is an active area of research with some tension between methods. This simple relationship is profound: it implies the universe is expanding, with space itself stretching and carrying galaxies apart, and running it backward leads to the Big Bang. The recessional velocity is not motion through space but the expansion of space between us and distant galaxies. For nearby galaxies the calculation is straightforward, while for very distant ones, where the velocity becomes a large fraction of light speed, relativistic effects and the details of cosmic expansion make the simple formula an approximation. This calculator uses the classic linear law, ideal for education and estimates. Enter the distance and Hubble constant to find the recessional velocity.
v = H₀ × d
Hubble's law states that a galaxy's recessional velocity is proportional to its distance, with the Hubble constant as the proportionality factor. Multiplying the distance in megaparsecs by the Hubble constant in kilometers per second per megaparsec gives the velocity at which the galaxy recedes due to the expansion of the universe.
Source:Cosmology
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter distance.
- 2
Adjust the options
Adjust hubble constant (h₀) if they apply — sensible defaults are already set.
- 3
Read your result
Recessional velocity updates instantly as you type — no button to press.
Frequently asked questions
What is Hubble's law?
Hubble's law states that galaxies recede from us at a velocity proportional to their distance: the farther away a galaxy is, the faster it moves away. The proportionality constant is the Hubble constant. Discovered by Edwin Hubble in 1929, this relationship is direct evidence that the universe is expanding.
What is the Hubble constant?
The Hubble constant is the rate of cosmic expansion, expressed in kilometers per second per megaparsec, meaning how much faster a galaxy recedes for each megaparsec of distance. Its value is about 70, though different measurement methods give slightly different results, a discrepancy known as the Hubble tension that astronomers are actively investigating.
Does recessional velocity mean galaxies move through space?
Not exactly. The recession is due to the expansion of space itself, which stretches the distances between galaxies, rather than galaxies moving through space in the ordinary sense. This is why distant galaxies can recede faster than light without violating relativity: it is space expanding, not objects traveling through it.
How does Hubble's law support the Big Bang?
If galaxies are all moving apart with velocity proportional to distance, then running the expansion backward in time brings everything together at a single point in the finite past. This extrapolation is a key piece of evidence for the Big Bang theory, the idea that the universe began in an extremely hot, dense state and has been expanding ever since.
When does the simple formula break down?
For nearby galaxies the linear law works well, but for very distant objects, where recessional velocity becomes a large fraction of light speed, relativistic cosmology and the changing expansion rate over cosmic time make the simple formula an approximation. Precise work at large distances requires the full framework of general relativity and cosmological models.