Hourwiz Explains · video notes

How Many Cells Are in the Human Body? Our count clock

From the Hourwiz video “Count your cells.” · Checked Oct 16, 2026

The short answer: about 28 to 37 trillion human cells, depending on the study and the body. Count them one per second, nonstop, and it takes about a million years (our math).

Three studies, three answers

Nobody has counted every cell in a body. Each study adds up published data for a reference body, and they use different methods and different bodies. So their answers differ.

• Why we say 28 to 37 trillion: that range covers every adult figure above. For a 70 kg adult alone, it's about 30 to 37 trillion.
• It isn't a trend. The figures went 37, then 30, then 36 trillion: different methods and different bodies, not a number that keeps shrinking.
• About the 2023 bodies: the 60 kg and child figures are built on the 70 kg model, which the authors were most confident in. The gap between them is about body size.
• These are human cells only. The bacteria living in and on you are counted separately (below).

Our count clock

Our math. One rule, never changed: 1 cell = 1 second, counted nonstop (no sleep, no breaks). That's 86,400 cells a day and 31,557,600 a year (the astronomers' 365.25-day year). Counting time in years = cells ÷ 31,557,600. A small Node script did the arithmetic on Oct 16, 2026, and it matched our hand calculation.

Our math: every adult estimate takes about a million years to count (0.89 to 1.18 million). That's three to four times as long as our species has existed: Homo sapiens is about 300,000 years old (Smithsonian).

• Counting only 8 hours a day instead of nonstop, the 30 trillion figure takes about 2.85 million years.
• No exact number. Every figure here is an estimate, so we always say “about”.

The core of the code

// 1 cell = 1 second, nonstop
// IAU year: 31,557,600 s
const JULIAN_YEAR_S = 365.25 * 86400;
// Smithsonian
const HOMO_SAPIENS_YEARS = 300_000;

// each published estimate
for (const r of RUNGS) {
  // 3.0e13 → 950,643 years
  const years =
    r.cells / JULIAN_YEAR_S;
  // → 3.17
  const xSapiens =
    years / HOMO_SAPIENS_YEARS;
}

An excerpt from our script, comments shortened. The full output is in the downloads below.

The bacteria: about 1 to 1, not 10 to 1

• The old claim: bacteria outnumber your cells at least 10 to 1. A 2016 essay in Cell calls that ratio “often presented as common knowledge”.
• Where it came from: a single rough, back-of-the-envelope estimate from the 1970s, which assumed the gut holds 1 liter and multiplied by a bacteria count per gram (Sender et al., 2016).
• The recount: about 38 trillion bacteria in the 70 kg reference man, against about 30 trillion human cells. That's about 1.3 to 1, “much closer to 1:1” (our check: 3.8 ÷ 3.0 = 1.27).
• It varies: the paper gives 1.3 with a 25% uncertainty, and across the other reference bodies it looked at, the ratio ran from 1.3 to 2.3. Same order of magnitude either way.
• Most of them live in the colon, and all of them together weigh about 0.2 kg: under half a pound (our conversion: 0.44 lb).
• A bathroom trip could flip it. In their 2016 preprint the researchers wrote that the numbers are close enough that “each defecation event may flip the ratio” in favor of human cells.

Most of your cells are red blood cells

• About 25 trillion red blood cells in the 70 kg reference man: 84% of the human cell count, more than 4 in 5 (Sender et al., 2016). Counting all the blood-forming cell types, it's about 90%.
• By count, not by weight. Red blood cells are among the smallest cells in the body (Sender et al., 2016). By mass, the body is dominated by muscle and fat cells (Hatton et al., 2023).
• A cross-check: in the 2023 count, red blood cells and platelets are about 29 trillion of the 36 trillion, about 81% (our math).
• Lined up edge to edge, 25 trillion red blood cells about 7.3 micrometers across would stretch about 182,000 km (113,000 miles): more than 4 times around Earth's equator and about halfway to the Moon (our math). Using the measured spread of sizes (6.75 to 7.79 micrometers), it's 4.2 to 4.9 laps.

Download our math

• Count clock (.csv, 1 KB): each estimate, the counting time in years, and the multiple of our species' age.
• Comparisons (.csv, 1 KB): the bacteria ratio, the red-blood-cell share, the bacteria weight, the red-blood-cell line-up, and the 8-hours-a-day aside.
• The script's run log (.txt, 3 KB): its own printout.

Edited for the web. In the count-clock file we removed our internal source-row column; in the comparisons file we removed stray quote marks from one note. The numbers are unchanged.

Images in the video

• Human blood under an electron microscope (1982): Bruce Wetzel & Harry Schaefer / National Cancer Institute (public domain)
• E. coli bacteria, magnified 10,000×: Eric Erbe, colorization: Christopher Pooley / USDA ARS (public domain). The colors are USDA's own. E. coli is one kind of bacteria that lives in the human gut (CDC).

Photos are shown unedited (cropped and resized only). The counter, the cartoon cells and everything else are drawn in code by Hourwiz.

Sources

Every fact on this page comes from these pages, opened on Oct 16, 2026. Things change: check the live page before you rely on it.

1. Bianconi et al., “An estimation of the number of cells in the human body”, Annals of Human Biology 40(6):463–471 (2013) About 37.2 trillion in a standard adult; added up organ by organ.
2. Sender, Fuchs & Milo, “Revised Estimates for the Number of Human and Bacteria Cells in the Body”, PLOS Biology 14(8): e1002533 (Aug 19, 2016) About 30 trillion human cells, about 38 trillion bacteria, about 1.3 to 1; 0.2 kg; red blood cells 84%; the 1970s estimate.
3. Sender, Fuchs & Milo, “Are We Really Vastly Outnumbered?”, Cell 164(3):337–340 (2016) “Much closer to 1:1”.
4. Sender, Fuchs & Milo, bioRxiv preprint (Jan 2016) The bathroom line. Coverage: Scientific American.
5. Hatton et al., “The human cell count and size distribution”, PNAS 120(39): e2303077120 (2023) About 36, 28 and 17 trillion; reference bodies; more than 1,500 sources; count vs mass.
6. IAU, measuring the universe A year of 365.25 days = 31,557,600 seconds.
7. Smithsonian National Museum of Natural History, Homo sapiens About 300,000 years ago.
8. BioNumbers (Harvard), red blood cell diameter 7.27 ± 0.52 micrometers (Silverman & Glick, 1969).
9. NASA NSSDC, Earth fact sheet Equatorial radius 6,378.137 km.
10. NASA NSSDC, Moon fact sheet 384,400 km.
11. NIST, unit conversions Exact mile. Pound: NIST SP 811, B.8.
12. CDC, about E. coli Found in the intestines of people and animals.

Our count clock is simple arithmetic on published estimates, rounded as shown. This page is about curiosity, not health or medical advice. The authors and agencies cited didn't make, review or endorse this page or the video.

No sponsor and no affiliate links on this page. Vendors are named so you can check them yourself, not as recommendations.

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