What this calculator does
Distant galaxies are moving away from us, and the further away they are the faster they recede. That simple proportionality is Hubble's law, and it is the observation that established the universe is expanding.
Inverting the Hubble constant gives a rough age for the universe. At 70 km/s/Mpc that works out to about 13.97 billion years, strikingly close to the modern figure of 13.8 billion, which is a reasonable outcome for such a crude calculation.
The formula
Multiply the Hubble constant by the distance in megaparsecs to get the recession velocity in kilometres per second. The Hubble time is the reciprocal of the constant, converted into years.
| Term | Meaning |
|---|---|
| Hubble constant (H0) | The current expansion rate, in kilometres per second per megaparsec. |
| Megaparsec (Mpc) | About 3.26 million light years, the standard unit for intergalactic distance. |
| Hubble time | One over the Hubble constant, an approximate age for the universe. |
The inputs explained
| Field | What to enter |
|---|---|
| Hubble constant (H0) (km/s/Mpc) | The Hubble constant. Measurements cluster around 67 to 74, and the disagreement between methods is an open problem. |
| Distance to galaxy (d) (Mpc) | The distance to the galaxy in megaparsecs. |
When to use it
Estimating a galaxy's distance
Running the relationship backwards from a measured redshift is the standard way distant galaxies are placed.
Understanding the age of the universe
The Hubble time gives an order-of-magnitude age without any detailed cosmological model.
Following the Hubble tension
Comparing results from different values of the constant shows how much the ongoing disagreement actually matters.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How fast is a galaxy receding at each distance?
Recession velocity at increasing distances.
| Distance | Recession velocity (v) | As a fraction of light speed |
|---|---|---|
| 100 Mpc | 7,000.0 km/s | 2.33% |
| 1,000 Mpc | 70,000.0 km/s | 23.3% |
| 4,000 Mpc | 280,000.0 km/s | 93.4% |
Questions
Are galaxies moving through space, or is space expanding?
Space itself is expanding, carrying galaxies with it. That distinction matters because it is why recession speeds can exceed light speed at great distances without violating relativity, which limits motion through space rather than the expansion of space.
Why do different measurements disagree?
Measurements from the cosmic microwave background give around 67, while those from nearby supernovae give around 73. The gap is larger than the stated uncertainties, which is called the Hubble tension and remains unresolved.
Why is the Hubble time close to the real age?
Partly coincidence. The Hubble time assumes the expansion rate has always been what it is now, which is not true. Early deceleration and later acceleration happen to roughly cancel, leaving the simple estimate close to the correct value.
Does this mean we are at the centre?
No. Every observer anywhere sees the same pattern of recession, because space is expanding uniformly everywhere. There is no centre to be at.
For the redshift that reveals this motion, see the redshift calculator. For the Doppler shift it resembles, see the Doppler effect calculator.