Use Hubble's Law to find a galaxy's recession velocity from its distance, then see that speed as a percent of light, its approximate redshift, and the Hubble time — the universe's expansion-based age estimate.
Quick Facts
Method
Hubble's Law: v = H0 x d
Recession velocity scales linearly with distance. Hubble time (1/H0) is about 977.8/H0 billion years.
Results
Calculated
Recession velocity
—
v = H0 x d
Speed as % of light
—
v / c x 100
Approximate redshift (z)
—
Low-z Doppler approximation, z ≈ v/c
Hubble time
—
1/H0, age estimate at constant expansion
Ready
Enter a distance and Hubble constant, then press Calculate.
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How this calculator works
This tool applies Hubble's Law, v = H0 x d, where v is a galaxy's recession velocity, d is its distance, and H0 is the Hubble constant — the current expansion rate of the universe. Enter a distance and a value for H0, then click Calculate to see the recession velocity, its size relative to the speed of light, an approximate redshift, and the Hubble time. Click Clear to reset to the defaults.
Understanding the inputs
Distance can be entered in megaparsecs (Mpc, the standard astronomical unit used with H0), million light-years, or billion light-years — the calculator converts internally to Mpc before applying the formula. The Hubble constant is typically quoted between about 67 and 74 km/s/Mpc; the exact value is an active area of research (the "Hubble tension" between early-universe and local measurements), so the field defaults to 70 km/s/Mpc as a common round-number estimate.
Interpreting the results
Recession velocity and its percent of light speed show how fast the object is receding due to cosmic expansion (not through motion in space, but because space itself is stretching). The redshift estimate uses the simple non-relativistic relation z ≈ v/c, which is accurate only when v is much smaller than c; the status panel flags when that assumption is getting strained. The Hubble time is independent of distance — it only depends on H0 — and represents how old the universe would be if it had always expanded at today's rate.
Frequently Asked Questions
What is Hubble's Law?
Hubble's Law states that a galaxy's recession velocity is proportional to its distance: v = H0 x d, where H0 is the Hubble constant. Discovered by Edwin Hubble in 1929, it is the primary observational evidence that the universe is expanding.
What is the Hubble constant?
The Hubble constant (H0) measures the current expansion rate of the universe in kilometers per second per megaparsec. Its exact value is debated: cosmic microwave background data give about 67.4 km/s/Mpc, while local supernova measurements give about 73 km/s/Mpc, a gap known as the Hubble tension.
How is redshift related to recession velocity?
For speeds much smaller than light, redshift z is approximately v/c, the non-relativistic Doppler approximation. At larger recession speeds this simple relation breaks down and the full relativistic and cosmological redshift formulas are needed instead.
What is the Hubble time?
The Hubble time is 1/H0, the age the universe would have if it had always expanded at its current rate. It equals about 977.8/H0 billion years, roughly 13.97 billion years for H0 = 70 km/s/Mpc, close to but not identical to the universe's measured age of about 13.8 billion years.