Geologic Time Scale Calculator
Look up the geologic period, era, and eon for any age in millions of years ago (Ma) using the international geologic time scale.
About this calculator
Geologists divide Earth's 4.6-billion-year history into a nested hierarchy of named intervals -- eons, eras, and periods -- defined by boundaries in the international chronostratigraphic chart maintained by the International Commission on Stratigraphy. This calculator takes an age in millions of years ago (Ma) and looks up which period, era, and eon that age falls into, along with where the age sits within its period's own span. The boundary dates used here (for example, the Cretaceous-Paleogene boundary at 66 Ma marking the dinosaur-ending extinction event, or the Cambrian's start at roughly 538.8 Ma marking the "Cambrian explosion" of complex animal life) reflect the currently published international time scale.
Everything before the Cambrian -- roughly the first four billion years of Earth's history, encompassing the Hadean, Archean, and Proterozoic eons -- is grouped here informally as "Precambrian" for simplicity, since it covers such a vast span with comparatively coarse internal subdivisions. The calculator also reports what fraction of Earth's total history (4,600 Ma) a given age represents, which is a useful way to put deep time in perspective: the entire span of the dinosaurs, for instance, occupies only a few percent of Earth's total history, and recorded human history occupies far too small a fraction to register at this scale.
Inputs
Results
Geologic Period
Jurassic
Era
Mesozoic
How to Use This Calculator
- Enter the Age in Ma (millions of years ago) for the rock, fossil, or event you are studying.
- Review the Geologic Period, Era, and Eon — the calculator places your age in the geologic time scale.
- Check Period Start and End dates and Position in Period to understand relative placement.
- Use Earth Age Percentage to put the event in context of Earth's 4.6-billion-year history.
- Compare Period Duration to understand whether your rock formed during a long or short interval.
What each input means
- Age (Ma — millions of years ago)
- Enter the age in millions of years ago (Ma). Earth formed ~4,600 Ma ago. Dinosaurs: ~66–230 Ma.
How this is calculated
Worked example, using the default values
- Identify Input ParametersAge (Ma — millions of years ago) = 150 = 1 input(s) provided
- Calculate Geologic PeriodGeologic Period = matchedPeriod.nameJurassic = Jurassic
- Calculate EraMesozoic = Mesozoic
- Calculate EonPhanerozoic = Phanerozoic
- Calculate Input AgeInput Age = 150150 = 150
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
What age range corresponds to the age of the dinosaurs?
Dinosaurs are generally considered to have lived from roughly 230 million years ago (in the Triassic period) until the end-Cretaceous extinction event at 66 million years ago -- spanning the Triassic, Jurassic, and Cretaceous periods within the Mesozoic era. Entering an age anywhere in that range will place you within one of those three periods.
Why is 'Precambrian' treated as one big block instead of being subdivided?
The Precambrian actually comprises three formal eons of its own (Hadean, Archean, and Proterozoic) spanning roughly the first four billion years of Earth's history, but this calculator groups them under a single simplified label because that span, while enormous in duration, has comparatively few widely known named subdivisions compared to the much more finely divided Phanerozoic eon that follows it.
How is the geologic period boundary at 66 million years ago significant?
66 Ma marks the Cretaceous-Paleogene (K-Pg) boundary, associated with the mass extinction event -- widely attributed to an asteroid impact -- that ended the age of non-avian dinosaurs and closed out the Mesozoic era. It is one of the most well-known boundaries in the geologic time scale.
What does the 'position in period' output actually measure?
It shows how far through the matched period your entered age falls, as a percentage, measured from the older boundary (period start) toward the younger boundary (period end). A position near 0% means the age sits near the beginning of that period; a position near 100% means it sits near the period's end, closest to the present.
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