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Hubble's Law Recession Velocity Calculator

Calculate a galaxy's recession velocity or distance using Hubble's Law.

Result

Recession Velocity
7,000 km/s

Formula: v = H₀ × d. Hubble's Law describes how distant galaxies recede from us due to the expansion of space itself, with recession velocity increasing linearly with distance - it was one of the first major pieces of observational evidence for an expanding universe.

About the Hubble's Law Calculator

This calculator finds the recession velocity of a distant galaxy using Hubble's Law, the direct proportional relationship between a galaxy's distance and how fast the expansion of space carries it away from us. It is a starting point for exploring the observational evidence for an expanding universe.

How It Works

You enter a galaxy's distance in megaparsecs and a value for the Hubble constant in km/s per Mpc, with commonly cited estimates ranging from about 67 to 74. The calculator multiplies the two together to produce a recession velocity in kilometers per second, reflecting how fast that galaxy moves away from us due to the expansion of space itself, not motion through space.

v = H0 x d, where H0 is the Hubble constant in km/s/Mpc and d is distance in megaparsecs.

Formula & Methodology

The relationship is directly proportional, so by hand you only multiply the chosen Hubble constant by the distance in Mpc to get velocity in km/s. Because the formula is linear, doubling the distance doubles the recession velocity for a fixed Hubble constant, and changing the Hubble constant scales every distance's velocity by the same factor.

Examples

A galaxy at 100 Mpc

With H0 = 70 km/s/Mpc and distance 100 Mpc: v = 70 x 100 = 7,000 km/s.

The same distance with a different Hubble constant

Using the same 100 Mpc distance but H0 = 73 km/s/Mpc: v = 73 x 100 = 7,300 km/s, a 300 km/s difference that comes purely from the choice of Hubble constant.

Advantages

  • Lets users directly compare how different accepted values of the Hubble constant change the predicted recession velocity for the same galaxy.
  • Turns a one-line proportional formula into an instant lookup, useful for checking astronomy homework or lecture examples.
  • Makes the linear nature of Hubble's Law visually obvious by letting users vary distance and see velocity scale proportionally.

Common Mistakes

  • Treating recession velocity from Hubble's Law as ordinary motion through space, when it actually reflects the expansion of space itself carrying galaxies apart.
  • Using a Hubble constant value without noting which measurement method it came from, then being surprised results differ from a calculation using a different assumed H0.
  • Applying the simple linear formula to extremely distant, high-redshift galaxies, where cosmological effects make the distance-velocity relationship more complicated than direct proportionality.

Edge Cases to Watch For

  • Distance and the Hubble constant are not clamped or validated, so a negative distance produces a negative, physically meaningless velocity.
  • Hubble's Law is a reliable linear approximation mainly for relatively nearby galaxies; at very large cosmological distances the distance-velocity relationship becomes more complex than this simple multiplication captures.
  • Because the Hubble constant is measured differently by different methods, a discrepancy known as the Hubble tension, the result is only as accurate as the chosen input, and shifting within the commonly cited 67 to 74 range changes every computed velocity by up to about 10 percent.

Common Use Cases

  • Astronomy students learning how Hubble's Law provides evidence for an expanding universe.
  • Educators demonstrating how the choice of Hubble constant affects distance and velocity calculations amid the Hubble tension debate.
  • Science communicators or hobbyists estimating rough recession velocities for known galaxy distances.
Written & fact-checked by the Calculateus TeamLast updated August 5, 2026How we verify our formulas

Frequently asked questions

Why is there still debate over the exact value of the Hubble constant?

Different measurement methods (observing nearby standard candles like Cepheid variables and supernovae versus analyzing the cosmic microwave background from the early universe) currently give slightly different values, a discrepancy known as the "Hubble tension" - this is one of the more actively researched open questions in modern cosmology, since it may hint at physics beyond the current standard cosmological model.

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Conclusion

Hubble's Law reduces to a single multiplication, but that simplicity is exactly what made it such a direct piece of early evidence for cosmic expansion. This calculator makes it easy to see how sensitive the predicted velocity is to the still-debated exact value of the Hubble constant.