Chronological age vs biological age.
Your chronological age is fixed — the number of times Earth has orbited the sun since you were born. Your biological age is the condition your cells and organ systems are actually in, and it can run years ahead of or behind the calendar. Two 50-year-olds can have biological ages a decade apart. The gap between the two numbers is the signal: it is largely driven by modifiable factors, and unlike your birthday, it can move.
"Biological age clocks" are the algorithms that estimate that second number. They fall into a few families depending on what they read and what they were trained to predict.
The major clocks compared
Horvath Clock (2013)
1st gen · multi-tissueInput: DNA methylation (353 CpG sites)
Measures: Chronological age · needs a methylation lab
The original epigenetic clock. Trained to predict calendar age across almost any tissue with remarkable accuracy. Because it was trained on chronological age, it is a weaker predictor of mortality than the clocks that came after it, but it proved the core idea: the methylome carries an age signal.
Hannum Clock (2013)
1st gen · bloodInput: DNA methylation (71 CpG sites)
Measures: Chronological age · needs a methylation lab
Published alongside Horvath, trained specifically on whole blood. Similar philosophy — predict calendar age from methylation — and similar limitation as a healthspan marker.
PhenoAge (Levine 2018)
2nd gen · phenotypicInput: DNA methylation — OR 9 routine blood markers
Measures: Biological age (mortality-weighted) · computable from a blood test
The breakthrough that made biological age useful. Levine first built a "phenotypic age" from nine standard blood markers (albumin, creatinine, glucose, hs-CRP, lymphocyte %, MCV, red-cell distribution width, alkaline phosphatase, white-cell count) trained against mortality, then trained a DNAm version on top of it. The blood-based version is the one you can actually compute from a normal lab panel.
Phenotypic Age (PhenoAge) Calculator →DNAm GrimAge (Lu 2019)
2nd gen · mortalityInput: DNA methylation (DNAm surrogates of 7 plasma proteins + pack-years)
Measures: Time-to-death risk · needs a methylation lab
Currently the strongest single predictor of all-cause mortality and time-to-disease among the major clocks. Instead of predicting age directly, GrimAge predicts methylation-based surrogates of plasma proteins (like GDF-15, PAI-1) and smoking pack-years, then converts the composite into "years." It requires a methylation array — there is no blood-panel shortcut.
DunedinPACE (Belsky 2022)
3rd gen · pace of agingInput: DNA methylation
Measures: Rate of aging (years per calendar year) · needs a methylation lab
Different question entirely. Rather than "how old is your body right now," DunedinPACE estimates how fast you are aging — a value near 1.0 means one biological year per calendar year, above 1.0 means faster. Built from the Dunedin birth cohort that tracked the decline of 19 organ systems over two decades. Best suited to measuring whether an intervention is slowing your rate of aging.
Telomere length
Legacy markerInput: Blood (qPCR or flow-FISH)
Measures: Replicative aging (weak) · needs a methylation lab
The original "aging biomarker" hype. Telomeres do shorten with age, but the measurement is noisy and only loosely tracks health outcomes at the individual level. Largely superseded by methylation clocks for biological-age estimation.
Which can you actually calculate yourself?
This is the practical fork. The true epigenetic clocks — Horvath, Hannum, GrimAge, DunedinPACE — all read DNA methylation, which means a saliva or blood sample run on a methylation array by a lab. There is no way to compute them from numbers you type into a calculator, and any site claiming to "calculate your GrimAge" from a questionnaire is not measuring methylation.
Two routes are genuinely free and self-serve:
- Blood-based PhenoAge. Levine’s phenotypic age was originally derived from nine routine blood markers, before any methylation was involved. If you have a recent metabolic panel and CBC, you can compute it directly.
- Lifestyle estimate. If you do not have bloodwork handy, a lifestyle-weighted questionnaire gives a fast first pass at the same question using the habits that drive most of the variance — sleep, fitness, diet, smoking, alcohol, stress.
How to get a real epigenetic test.
If you want the methylation-based clocks, several consumer labs report them from a blood spot or saliva sample, typically for $200–$500:
- TruDiagnostic (TruAge) — reports PhenoAge, GrimAge, and DunedinPACE from a finger-prick blood spot; the most research-aligned consumer option.
- Elysium Index — saliva-based, reports a methylation age and a pace-of-aging metric.
- myDNAge — saliva or blood, based on the Horvath/PhenoAge lineage.
One caveat worth knowing: test–retest noise on a single methylation result can be a year or more, so a single reading is a baseline, not a verdict. The clocks are most useful tracked over time against a stable protocol — which is exactly what DunedinPACE was designed for.
Frequently asked questions
Is there an epigenetic age calculator I can use for free?+
Not from your own data alone — true epigenetic (DNA methylation) clocks like Horvath, GrimAge, and DunedinPACE require a methylation array run on a saliva or blood sample by a lab, so there is no way to compute them from numbers you type in. What you CAN compute for free is the blood-based Levine PhenoAge (from nine standard lab markers) and a lifestyle-based biological age estimate. Both are on this site.
What is the difference between GrimAge, PhenoAge, and the Horvath clock?+
They answer slightly different questions. The Horvath and Hannum clocks (2013) were trained to predict your chronological age from methylation. PhenoAge (Levine 2018) and GrimAge (Lu 2019) were trained against health outcomes and mortality, so they are far better at telling you whether your body is aging faster or slower than your years. GrimAge is currently the strongest mortality predictor; PhenoAge is the only one of the three with a version you can compute from a routine blood panel.
Can I calculate my Levine biological age without a DNA test?+
Yes. The original Levine PhenoAge was built from nine ordinary blood markers — albumin, creatinine, glucose, hs-CRP, lymphocyte percentage, mean cell volume, red-cell distribution width, alkaline phosphatase, and white-cell count — plus your chronological age. If you have a recent comprehensive metabolic panel and CBC, our Phenotypic Age calculator applies the published Levine equation to give you a blood-based biological age.
Which biological age clock is the most accurate?+
For predicting mortality and time-to-disease, DNAm GrimAge and the pace-of-aging measure DunedinPACE are currently the best-validated. For accessibility, the blood-based Levine PhenoAge is the most practical because you can get it from a standard lab panel. "Most accurate" depends on the question: GrimAge for risk, DunedinPACE for tracking whether an intervention is working, PhenoAge for a cheap, repeatable baseline.
How do I get my epigenetic age tested?+
Commercial methylation tests include TruDiagnostic (which reports PhenoAge, GrimAge, and DunedinPACE from a blood spot), Elysium Index, and myDNAge (saliva). They typically cost $200–$500. For a free first pass, compute your blood-based PhenoAge from labs you may already have, or use the lifestyle biological-age estimator.
Can biological age be reversed?+
The clocks can move in both directions, and several small trials have shown measured reductions in epigenetic age or pace of aging with sustained lifestyle change (sleep, exercise, diet, smoking cessation). The honest caveat: most of these studies are small and short. The robust takeaway is that biological age is far more modifiable than chronological age, and the gap between them is the part worth working on.
Assembled from the primary literature — Horvath (2013), Hannum et al. (2013), Levine et al. (2018, Aging), Lu et al. (2019, Aging, DNAm GrimAge), and Belsky et al. (2022, eLife, DunedinPACE) — plus standard reviews of epigenetic aging. Educational only; not medical advice or a diagnostic tool.