Your Population: a census of the human body
How many cells does a human have?
68 trillion cells
More than half of them are bacteria.
Your body has neighbours. Bacteria live on your skin and in your gut; many help with digestion and defence. This page does what any census does: it counts, sorts into categories, and asks who forms the majority.
01The tally sheet
| Population | Count | Share |
|---|---|---|
| Red blood cells Carry oxygen to every tissue; no nucleus, renewed about every 120 days. |
25 trillion | 37% |
| The rest of the human body Brain, liver, muscle, bone — everything we usually mean by body. |
5 trillion | 7% |
| Microbiome Bacteria living mostly in the colon, helping with digestion and immune function. |
38 trillion | 56% |
| Total | 68 trillion | 100% |
The values are rounded estimates; shares were computed from the totals and rounded separately.
02The counting chamber
Ten thousand dots. Each represents about 6.8 billion cells. Tap a category to isolate it.
The full population. Deep red: red blood cells. Ochre: the rest of the body. Green: the microbiome.
03What stays invisible
200 grams of tenants
The entire microbial population weighs about 200 grams, roughly a smartphone. The surprise is the numerical ratio: bacteria outnumber human cells.
The ten-to-one myth
The old ratio of ten bacteria to one human cell was revised. The modern estimate is about 1.3 bacteria per human cell.
A life in circulation
Over about 120 days, each red blood cell makes thousands of laps through the circulation. It is then recycled in the spleen, with its iron returning to the system.
04The history of a wrong number
- The 1970s
A hasty estimate, drawn from the contents of the large intestine, fixes the ratio at ten bacteria per human cell. Textbooks repeat it for four decades. - 2016
Ron Sender, Shai Fuchs and Ron Milo count again, tissue by tissue, and publish in PLOS Biology: 1.3 bacteria per human cell. - The lesson
A number can become familiar through repetition without ever becoming true. That is why censuses are redone.
05The count continues
Seconds since you opened this page: 0
Red blood cells your marrow has produced meanwhile: about 0
Heartbeats at an average 70 per minute: about 0
06Method note
The base estimate comes from the 2016 study by Ron Sender, Shai Fuchs and Ron Milo in PLOS Biology: about 30 trillion human cells and 38 trillion bacteria, with error margins in the tens of percent. For this page, I split the 30 trillion human cells into roughly 25 trillion red blood cells and 5 trillion for the rest of the body; this is a rounded visual model. Every share was recomputed from the totals. Red-cell production (about 2.4 million per second) and lifespan (about 120 days) come from the hematology literature. The heartbeat counter uses 70 beats per minute as a reference.