How big is the universe?
The observable universe is about 93 billion light years across — a radius of 46.5 billion light years — even though it is 13.787 billion years old. The universe as a whole may well be infinite; we have no way to measure past the part we can see.
Updated on October 7, 2026
The numbers
- Radius of the observable universe
- 46.5 billion light years (comoving)
- Diameter
- about 93 billion light years
- Age of the universe
- 13.787 ± 0.020 billion years
- Hubble constant (Planck 2018, CMB)
- 67.4 ± 0.5 km/s/Mpc
- Hubble constant (SH0ES, supernovae)
- 73.0 ± 1.0 km/s/Mpc
- Observable galaxies, order of magnitude
- hundreds of billions
How can it be wider than light has had time to cross?
Because space itself expanded while the light was in transit. The photons from the most distant galaxies we see left when the universe was young and have been travelling 13.8 billion years, but the places they came from have been carried much further away in the meantime. The 46.5 billion light year radius is a comoving distance: where those sources are now, not how far the light flew.
Nothing here breaks the speed limit. Light still moves at exactly 299,792,458 m/s locally. What has no speed limit is the stretching of space between distant objects, which is why galaxies far enough away are receding faster than light and will never be reachable.
The one number cosmologists cannot agree on
The age and the size both depend on the expansion rate, the Hubble constant, and there the field has a genuine problem. Measurements from the cosmic microwave background give 67.4 km/s/Mpc; measurements from nearby supernovae calibrated with Cepheid variables give about 73. The error bars do not overlap. Either one method has an unnoticed systematic error, or the standard cosmological model is incomplete. Any page that gives you a single confident figure for the size of the universe is glossing over this.
How to read this page
How to read the numbers here
Every value on "How big is the universe?" comes from the source named beside its section, never from an estimate of our own with no origin.
Where the data comes from
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Check the primary source
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More about “How big is the universe?”
How big the Hubble disagreement is
The supernova measurement (73.0 km/s/Mpc) sits about 8.3% above the cosmic-background measurement (67.4 km/s/Mpc). That sounds small, but it’s enough that the two methods’ error bars don’t touch — and that gap is exactly what "How big is the universe?" calls a problem with no agreed fix yet.
Words on this page
Astronomical unit
The solar system’s reference distance, fixed at exactly 149,597,870,700 metres since 2012, close to the average Earth–Sun distance. It saves writing every planet’s orbit in kilometres with ten digits.
Standard definition by IAU — International Astronomical Union · Astronomical unit
Standard uncertainty
The margin attached to a measured value, usually written in parentheses on the last digits. A constant that is "exact by definition" has zero standard uncertainty; a measured one, like the gravitational constant G, always carries some.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Standard uncertainty
Exoplanet
A planet orbiting a star other than the Sun. Most were found indirectly, by the star’s faint dimming as the planet crosses in front of it (transit) or by the gravitational wobble it causes in the star (radial velocity), not by direct image.
Standard definition by NASA Exoplanet Archive · Exoplanet
Launch window
The span of time in which a rocket can lift off and still reach its planned orbit or destination. For a trip to another planet that window can last only a few weeks and only returns at the next favourable alignment, so missing it costs months or years of waiting.
Mole
The SI counting unit for atoms, molecules or other entities: one mole is exactly 6.02214076 × 10²³ of them, by the definition fixed in 2019. It is what ties a mass in grams to a number of particles.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Mole