Radiometric Age Calculator
Radioactive Decay Is a Clock That Never Needs Resetting
A radioactive isotope decays at a fixed, statistically predictable rate regardless of temperature, pressure, or chemical environment — which is exactly what makes it useful as a geologic clock. Measure how much of the original ("parent") isotope remains, or the ratio of parent to its decay product ("daughter"), and the decay rate converts that measurement directly into an age.
The Formula
Where the parent/daughter ratio is known:
N ÷ N₀ = P ÷ (P + D)
N₀ is the original amount of parent isotope, N is the amount remaining today. When you know the ratio of remaining parent (P) to accumulated daughter (D) instead of a direct remaining-fraction measurement, that ratio converts to a remaining fraction first, then feeds into the same age equation.
Why Half-Life Choice Depends on What You're Dating
- Matching isotope to timescale — a short half-life isotope like Carbon-14 is only useful for material younger than roughly 50,000 years, since almost none of the parent isotope remains beyond that; dating a billion-year-old rock requires an isotope with a correspondingly long half-life.
- Cross-checking ages — geologists often date the same sample using two different isotope systems; agreement between the two builds confidence in the result.
- Establishing the geologic timescale — the boundaries between geologic periods and eras are themselves defined using radiometric dates from key rock units worldwide.
- Archaeological and paleoclimate dating — Carbon-14 dating of organic material underpins much of the chronology used in archaeology and recent paleoclimate reconstruction.
Common Radiometric Dating Isotopes
| Isotope System | Half-Life | Typical Use |
|---|---|---|
| Carbon-14 | ~5,730 years | Organic material, up to ~50,000 years |
| Potassium-40 | ~1.25 billion years | Volcanic rocks, millions to billions of years |
| Uranium-238 | ~4.47 billion years | Zircon crystals, oldest rocks on Earth |
| Rubidium-87 | ~48.8 billion years | Very old igneous and metamorphic rocks |
Half-life values are established physical constants; enter the appropriate half-life for your isotope system into the calculator below in whatever time units you prefer.
Remaining Fraction After N Half-Lives
| Half-Lives Elapsed | Fraction Remaining |
|---|---|
| 1 | 50% |
| 2 | 25% |
| 3 | 12.5% |
| 4 | 6.25% |
| 5 | 3.125% |
| 10 | ~0.098% |
How to Use This Calculator
- Choose a mode: By Remaining Fraction or By Parent/Daughter Ratio.
- Enter the Half-Life of the isotope you're using (e.g. years).
- For the first mode, enter the Remaining Fraction N/N₀, a value between 0 and 1.
- For the second mode, enter the Parent/Daughter Ratio instead.
- Select Calculate to get the age in the same time units as the half-life entered, with the full logarithmic breakdown.
Related Calculations
Correct field bed measurements with the Stratigraphic Thickness Calculator for a fuller picture of a rock sequence's history.