# Importing a spreadsheet

**Category:** [Data Tables](https://help.totalctrl.app/hc/totalctrl/totalctrl-help-center/en-US/categories/data-tables)
**Updated:** 2026-10-04

## Nothing is written until you have seen it

Choosing a file does not change your table. The upload is read, the column types are guessed, and you get a preview: every column, what it will be imported as, and the first twenty-five rows. Only **Import** writes anything.
## What the guesses are based on

Each column is given the narrowest type that *every* value survives. One “N/A” in a column of numbers is enough to keep the whole column as text, because the alternative is silently blanking the odd one out.

- `1,200` becomes the number 1200, so the column can be sorted and totalled.
- Dates are normalized however they were written — `04 Mar 2026`, `4/3/2026` and `2026-03-04` all land the same way.
- A column whose values repeat inside a small vocabulary becomes a **single select**, which is what gives you a dropdown filter instead of a text box.
- A column of only 1s and 0s stays **numbers**. It is genuinely ambiguous, and reading a quantity as yes/no would destroy it — change it in the preview if it is a flag.

## Every guess is yours to overrule

In the preview you can rename any column, change its type, send it into a column you already have, or leave it out entirely.
## Importing into a table that already has data

Map each column to **an existing column** and the rows are appended. Mapping to *a new column* instead creates a second column with the same name, which is almost never what you want.
## If the file is bigger than the table can hold

The rows that fit are imported and the rest are reported. You get a partial, coherent table rather than losing the whole upload, and you can bring in the remainder once the limit is raised.
## Undoing an import

An import is a single entry in **History**. Undoing it removes exactly the rows it added and leaves everything that was there before untouched.

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## Related Articles

- [Archiving and deleting a table](https://help.totalctrl.app/en-US/articles/data-tables-archive)
- [Exporting a table](https://help.totalctrl.app/en-US/articles/data-tables-export)
- [History and undo](https://help.totalctrl.app/en-US/articles/data-tables-history)
- [Getting started with Data Tables](https://help.totalctrl.app/en-US/articles/data-tables-getting-started)
- [Working in the grid](https://help.totalctrl.app/en-US/articles/data-tables-grid)

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