> For the complete documentation index, see [llms.txt](https://help.sbtinstruments.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.sbtinstruments.com/more/software/cell-size-cize.md).

# Cell size (CIZE)

What CIZE is, and how the QC test keeps it comparable.

CIZE is the average size of the cells your BactoBox® counts. It is given as the diameter of a sphere with the same volume as the cell, in µm. You get it with every measurement, next to cells/mL.

## How BactoBox® measures size

Each cell that passes the sensor gives an electrical signal, and the size of that signal scales with the volume of the cell. BactoBox® converts each signal to a diameter. CIZE is the average over all the cells it counts.

Because CIZE is a diameter, it encodes a volume. A CIZE of 2 µm is about eight times the volume of a CIZE of 1 µm.

## The QC test calibrates CIZE

The QC vial holds silica beads of a known size. In a QC test, BactoBox® measures the beads and uses their known size to calibrate the size signal of the mounted flow cell. That makes CIZE comparable between devices and between flow cells.

Run a QC test every week you use your BactoBox® to keep CIZE reliable. Your BactoBox® shows a reminder on the home screen when one is due. With [ready-made QC vials](/item-register/consumables/qc-vials.md) there is nothing to prepare. When you insert a new flow cell, BactoBox® requires a QC test before it measures, and that test calibrates the new flow cell. See [QC test](/maintenance/qc-test.md).

{% hint style="warning" %}

### CIZE from software older than v2026.09 is hidden

Earlier software did not calibrate CIZE, so those values cannot be compared with new ones. After the update, your BactoBox® hides them. [Contact us](https://sbtinstruments.com/contact) if you need them, and we will help you get them back.
{% endhint %}

## When CIZE reads wrong

* **Clumps.** A clump passes the sensor as one object, so it raises CIZE. See [Break up cell clumps](/more/how-to-guides/break-up-cell-clumps.md).
* **Too few cells.** CIZE needs a cells/mL value above the [limit of detection](/advanced/measurements-explained/limits-of-detection.md).

## Where you see CIZE

* On the result screen of your BactoBox®.
* In the *CIZE* column in *Access*. See [What the columns mean](/more/software/what-the-columns-mean.md).
* As a dashed line in growth curve plots. See [Plot a growth curve](/more/software/plot-a-growth-curve.md).


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