What is Hubble's Law?

Hubble's Law states that the recession velocity of a galaxy is proportional to its distance: v = H₀ × d, where H₀ is the Hubble constant (roughly 70 km/s/Mpc). It describes the expansion of the universe.

What value of H₀ should I use?

The Hubble constant is not perfectly known. Planck CMB measurements give ~67.4 km/s/Mpc; the Hubble Space Telescope distance-ladder gives ~73.0 km/s/Mpc. The tool lets you choose either preset or enter your own value.

What is a megaparsec (Mpc)?

A megaparsec is 1,000,000 parsecs, or about 3.086 × 10²² meters (roughly 3.26 million light-years). Cosmological distances are typically quoted in Mpc.

How is redshift (z) related to recession velocity?

For non-relativistic recession speeds (v ≪ c), z ≈ v/c. This tool uses that approximation, which is valid for v below about 10% of the speed of light (distances below ~430 Mpc at H₀ = 70).

Does Hubble's Law break down at large distances?

Yes. At very large distances (high redshifts), the linear approximation breaks down and full relativistic cosmological models are needed. This tool is most accurate for z ≤ 0.2.

Hubble Law Calculator

Compute galaxy recession velocity from distance or distance from recession velocity using Hubble's Law (v = H₀ × d).

Solve for

Distance (Mpc)

1 Mpc = 3.086 × 10²² m ≈ 3.26 million light-years

Hubble Constant H₀ (km/s/Mpc)

km/s/Mpc

Result

Enter values and press Calculate.

H₀ Comparison

Same input, different published Hubble constant values.

Results appear after calculation.

H₀	Source	Velocity (km/s)	z

Summary

Hubble's Law (v = H₀ × d) links a galaxy's recession velocity to its distance from us, reflecting the expansion of the universe. This calculator solves for either quantity given the other, using any Hubble constant you specify. It also reports the implied redshift z and lets you compare results side-by-side for multiple H₀ values.

How it works

Select the solve mode: calculate velocity from distance, or distance from velocity.
Enter the known value (distance in Mpc or velocity in km/s).
Choose a Hubble constant value (H₀) or enter a custom one.
Read the computed result and the implied redshift (z) in the output panel.
Use the table to compare results across different H₀ values at once.

Use cases

Estimate how fast a galaxy at a known redshift is receding from us.
Convert a measured recession velocity to a cosmological distance in megaparsecs.
Compare recession velocities under different published Hubble constant values.
Cross-check textbook cosmology homework problems involving Hubble's Law.
Understand the scale of the observable universe by experimenting with large distances.
Demonstrate the relationship between redshift and distance in an astronomy class.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu