What is the Hubble time?

The Hubble time is t_H = 1/H₀ — the time it would take the universe to reach its current size if it had always expanded at its present rate. It is a useful order-of-magnitude estimate for the age of the universe. The actual age depends on the full expansion history (matter, dark energy, radiation content) and is typically about 4–7% less than the Hubble time for the standard ΛCDM model.

Why is the Hubble time only an estimate of the universe's age?

The real universe did not expand at a constant rate. It decelerated early on (dominated by matter and radiation) and is now accelerating (dominated by dark energy). The Hubble time 1/H₀ ignores this history. For Planck 2018 parameters (H₀ = 67.4), the Hubble time is ~14.5 Gyr while the true ΛCDM age is ~13.8 Gyr — about 5% lower.

What is the Hubble tension?

The Hubble tension is a statistically significant discrepancy between the value of H₀ inferred from the early universe (CMB, ~67 km/s/Mpc) and measurements using the local distance ladder (Cepheids + Type Ia supernovae, ~73 km/s/Mpc). The two values imply Hubble times of ~14.5 Gyr vs. ~13.4 Gyr. The tension remains unresolved as of 2025.

How do I convert H₀ from km/s/Mpc to s⁻¹?

Divide the km/s/Mpc value by the number of km in one megaparsec: 1 Mpc = 3.085677581 × 10¹⁹ km. So H₀ = 70 km/s/Mpc → 70 / 3.085677581 × 10¹⁹ ≈ 2.269 × 10⁻¹⁸ s⁻¹. The Hubble time is then 1 / (2.269 × 10⁻¹⁸) ≈ 4.41 × 10¹⁷ s ≈ 13.97 Gyr.

What H₀ value should I use?

For early-universe/CMB-based estimates use H₀ = 67.4 km/s/Mpc (Planck 2018). For local distance-ladder estimates use ~73 km/s/Mpc (Riess et al. SH0ES). Both are shown as quick presets. The "true" value is actively debated.

Hubble Time Calculator

Enter the Hubble constant (H₀) in km/s/Mpc to get the Hubble time — the characteristic age of the universe — in gigayears, megayears, and seconds.

Hubble Constant Input

H₀ — Hubble constant (km/s/Mpc)

Typical range: 50–100 km/s/Mpc

Quick presets

Formula

t_H = 1 / H₀

where H₀ is converted from km/s/Mpc to s⁻¹ using
1 Mpc = 3.085677581 × 10¹⁹ km

Hubble Time Results

Enter a Hubble constant value to see results.

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Summary

The Hubble time is defined as the reciprocal of the Hubble constant (t_H = 1/H₀) and gives a characteristic timescale for the expansion of the universe — roughly equal to the age of a universe that has always expanded at its current rate. This calculator converts H₀ from the standard observational unit (km/s/Mpc) into the Hubble time expressed in gigayears, megayears, and seconds, using the exact unit conversion: 1 Mpc = 3.085677581 × 10¹⁹ km. The current best-fit values range from about 67 km/s/Mpc (Planck CMB) to 73 km/s/Mpc (distance-ladder measurements), yielding Hubble times between roughly 13.4 and 14.6 Gyr.

How it works

Enter the Hubble constant H₀ in km/s/Mpc. The current accepted range is roughly 67–73 km/s/Mpc.
The calculator computes t_H = 1/H₀ by converting H₀ from km/s/Mpc to s⁻¹ using 1 Mpc = 3.085677581 × 10¹⁹ km.
The result in seconds is then divided by 3.15576 × 10¹⁶ s/Gyr to give gigayears.
Results are shown in gigayears (Gyr), megayears (Myr), and raw seconds simultaneously.
Use the quick-preset buttons for common H₀ values from major cosmological surveys.

Use cases

Estimate the characteristic age of the universe for a given Hubble constant.
Compare age estimates from the Planck CMB value vs. local distance-ladder measurements.
Convert H₀ from km/s/Mpc to inverse seconds for physics equations.
Explore how the Hubble tension affects the inferred age of the universe.
Verify cosmological time-scale calculations in coursework or research.
Understand the relationship between expansion rate and cosmic age.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu