Geologic Time Scale Converter
Convert a time in millions of years ago (Ma) to its corresponding geologic eon, era, period, and epoch — or look up a named interval to find its age range.
Mode 1: Years → Geologic Period
Mode 2: Geologic Period → Age Range
Results will appear here.
Formula & Scale
Age lookup: Given an age t in Ma (millions of years ago), the calculator finds the most specific geologic interval where:
interval.start ≥ t ≥ interval.end
Priority: Epoch > Period > Era > Eon.
Position within interval:
Position (%) = (start − t) / (start − end) × 100
Unit conversions:
1 Ma = 1,000,000 years = 1,000 ka
1 Ga = 1,000 Ma = 1,000,000,000 years
Assumptions & References
- Boundary ages follow the International Chronostratigraphic Chart v2023/09 (Cohen et al., 2013; updated) published by the International Commission on Stratigraphy (ICS).
- Earth's age is taken as ~4,600 Ma (4.6 Ga) based on radiometric dating of meteorites and lunar samples.
- The Anthropocene is treated as informal; its base (~1950 CE, ~0.00007 Ma) reflects the proposed GSSP at Crawford Lake, Canada (Anthropocene Working Group, 2023).
- The Carboniferous is treated as a single period (as used internationally); North American usage splits it into Mississippian and Pennsylvanian sub-periods.
- Pre-Cambrian eons (Hadean, Archean, Proterozoic) are not subdivided into periods/epochs in this tool due to the informal nature of many pre-Cambrian subdivisions.
- All ages are in Ma (mega-annum) = millions of years before present (J2000 / 1950 CE baseline per ICS convention).
- Reference: Cohen, K.M., Finney, S.C., Gibbard, P.L. & Fan, J.-X. (2013, updated). The ICS International Chronostratigraphic Chart. Episodes 36: 199–204. stratigraphy.org/chart