Geological Formation Age Calculator
This calculator determines the age of a geological formation using the radiometric decay equation t = (1/lambda) x ln(1 + D/P). Select a decay system, enter the measured daughter-to-parent isotope ratio, and the calculator returns the age in years and million years (Ma). Decay constants are the IUPAC-recommended values used internationally in geochronology. This is intended for educational use; published ages from real samples require specialist laboratory data and isochron analysis.
Radiometric age equation
t = (1 / lambda) x ln(1 + D/P)
Where t = age (years), lambda = radioactive decay constant of the parent isotope (per year), D = radiogenic daughter isotope concentration, P = remaining parent isotope concentration. Half-life = ln(2) / lambda. Source: Steiger and Jager (1977), Earth and Planetary Science Letters; Jaffey et al. (1971), Physical Review C.
IUPAC decay constants used
| System | Decay Constant (lambda, per year) | Half-life (billion years) |
|---|---|---|
| K-40 to Ar-40 | 5.81 x 10^-11 | 11.93 Ga |
| Rb-87 to Sr-87 | 1.42 x 10^-11 | 48.8 Ga |
| U-238 to Pb-206 | 1.55125 x 10^-10 | 4.468 Ga |
| U-235 to Pb-207 | 9.8485 x 10^-10 | 0.704 Ga |
| Sm-147 to Nd-143 | 6.54 x 10^-12 | 106 Ga |
Geological formation age calculator: frequently asked questions
How does radiometric dating of geological formations work?
Radiometric dating measures the ratio of parent to daughter isotope in a rock or mineral. Using the known radioactive decay constant (lambda), the age is calculated from: t = (1/lambda) x ln(1 + D/P), where D is the daughter isotope concentration and P is the parent isotope concentration. This assumes the system has been closed since crystallisation.
What is the K-Ar (potassium-argon) dating method?
K-Ar dating measures the decay of potassium-40 to argon-40. The decay constant for K-40 to Ar-40 is 5.81 x 10^-11 per year (IUPAC). It is used for volcanic rocks and minerals such as hornblende, biotite, and feldspar, typically for ages from 100,000 to 4.5 billion years.
What is U-Pb dating and when is it used?
Uranium-lead dating uses two decay chains: U-238 to Pb-206 (lambda = 1.55125 x 10^-10/yr) and U-235 to Pb-207 (lambda = 9.8485 x 10^-10/yr). Zircon is the primary mineral dated using U-Pb because it incorporates uranium but excludes lead at crystallisation. U-Pb is the most accurate geochronometer for ancient crustal rocks.
What daughter-to-parent ratio is used in the formula?
The daughter-to-parent ratio (D/P) is measured by mass spectrometry (TIMS or LA-ICP-MS). For K-Ar, it is the Ar-40 content (minus any initial atmospheric argon) divided by the remaining K-40. For U-Pb, it is Pb-206/U-238. These measurements require specialist laboratory equipment.
What is the concordia diagram in U-Pb dating?
The concordia diagram plots Pb-206/U-238 against Pb-207/U-235 ratios. A system that has remained closed plots on the concordia curve, and the intersection gives the crystallisation age. Discordant samples that have lost lead through later metamorphism or alteration plot off the curve, allowing interpretation of both crystallisation and alteration ages.
Official sources
- USGS Geochronology: USGS Geochronology Program.
- IUPAC recommendations: Steiger, R.H. and Jager, E. (1977), Earth and Planetary Science Letters 36: 359-362. Decay constants for geochronology.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.