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Import customers from a spreadsheet

Add many customers at once from a CSV file. Covers the columns, the rules, how to fix errors and what happens when you confirm.

5 min read · Updated November 10, 2026

Screenshot of the Slate partner dashboard: Import customers

You can add many customers at once by uploading a CSV file, a spreadsheet saved as plain text. It is the quickest way to register a list you already have.

You'll need: a CSV file with one row per customer, and each customer's own ID from your system. The file can hold businesses or people, but not both.

Import a file

Open Import

Select Customers in the menu, then Import at the top right.

The Customers page with the Import button and its Customers option numbered
  1. Import: opens a short menu.
  2. Customers: imports a list of customers. The other option, Financial data, is covered in Import financial data.

Choose the customer type

Select Business or Person. One file holds one type.

The question What type of customer are you uploading with Person and Business cards numbered
  1. Person: individuals.
  2. Business: companies. We use this one below.

Get the template (optional)

Select Download Template. It saves a sample CSV with the right column names and ten made-up rows. Delete the sample rows and add your own.

The Upload the customers box with the Download Template button, the upload area and the Use Example button numbered
  1. Download Template: a sample file for the type you chose.
  2. Upload a file: drag your CSV here, or select the area to browse. Files must end in .csv and be under 10 MB.
  3. Use Example: loads the sample rows without a file. It only appears in sandbox.

Upload your file

Drop your CSV on the upload area. A message at the top says whether it was read, for example Parsed with 1 error(s).

Check the preview

The Customer upload preview shows what Slate understood.

The Customer upload preview showing one validation error and four valid customers, with the Import 4 Customers button
  1. Validation Errors: rows Slate cannot import. Each line names the row and the problem.
  2. Valid customers: the rows that will be imported. Scroll sideways to see every column.
  3. Import 4 Customers: imports the valid rows. The number matches the list.

Select Import

Slate adds the customers one at a time. A message at the top says imported 4 customers.

The message imported 4 customers at the top of the Customers page

What goes in a business file

The first row must hold these column names, spelled exactly like this, including capital letters.

Required:

  • externalId: your own ID for the customer. It must be unique.
  • productType: capital or credit_line. Required if your company offers both. If you offer one, you can leave it empty.

Optional:

  • legalName, dba (trading name) and website.
  • type: CORPORATION, SOLE_PROPRIETORSHIP or PARTNERSHIP.
  • address1, address2, city, state, postalCode and country (CA or US). Slate saves the address only if address1, city, state, postalCode and country are all filled in.
  • contactEmail, contactPhone and contactFullName. See the note below.

Only externalId is truly needed, so you can import a list of IDs now and attach data later.

What goes in a person file

Required:

  • externalId, firstName, lastName.
  • address1, city, state, postalCode and country (CA or US).
  • birthdate, written as YYYY-MM-DD, for example 1988-04-23.
  • productType: same rule as above.

Optional: middleName, address2, email and phone.

Phone numbers use the international format with a plus sign, for example +14165550100.

Fix the errors

Slate names the row and the problem. Row numbers count your data rows, so the first customer under the column names is Row 1.

The example below fails because productType says line_of_credit.

Row 4: Product must be one of: capital, credit_line

Fix that cell, save, and upload the file again. Slate may show a new problem on the next try if the row has more than one.

If something goes wrong

"Invalid file type"

The file does not end in .csv. In Excel or Numbers, choose Save as or Export and pick CSV.

"Duplicated External ID"

A customer with that externalId already exists. If you upload a file twice, every row fails this way.

The message there were some errors while importing the customers, listing four Duplicated External ID lines

Remove the rows that already imported, or give the new ones fresh IDs.

Some rows imported and some did not

Slate adds rows one at a time, so earlier rows stay even if a later one is rejected. The message lists each rejected row.

Where are my customers?

Open Customers and search for one by name or ID. New customers show Not underwritten until Slate has data on them.

The Customers page searched for Lakeview, showing Lakeview Plumbing Ltd. numbered
  1. The imported customer: shown with Mississauga, ON as its location and Not underwritten as its status.

Next, give Slate their numbers: Import financial data.

Learn more about Slate Signals.

Still stuck? Email [email protected] with your account email and the screen you are on. We reply within 24 business hours.