Cell styles

Cells can be styled, their background, border, font of the content and many other options.

Fluent vs properties

The style can be set through properties or through fluent API. Both style produce same result.

// Set style through property
ws.Cell("A1").Style.Font.FontSize = 20;
ws.Cell("A1").Style.Font.FontName = "Arial";

// Set style using fluent API



Background color


Cell border

You can set a border of a cell.

// Default color is black

Number format

Excel has an ability to format a value in a cell through a built-in number format or through a custom number format code.

Number format code in a cell format dialog Number format options in a ribbon

ClosedXML can set format of a cell or a range through IXLStyle.NumberFormat property. The property can be found on interfaces of ranges, e.g. IXLCell, IXLCells or IXLRangeBase. The built-in style is set by setting a (in theory well known) number format id. Custom style is set by setting a number format code.

Built-in number format

To use built-in number format, first determine a number format id. ClosedXML contains a subset of common format ids (0..49) in a static class XLPredefinedFormat or you can just pick an integer from OpenXML SDK documentation NumberingFormat Class.


The built-in formats are identified in an excel only as an id, but actual formatting for for the built-in formats differs depending on a locale of the Excel.

As an example, built-in format 20 (XLPredefinedFormat.DateTime.Hour24Minutes) uses formatting string h:mm for en-US locale, but hh:mm in en-GB locale. That means, if the same workbook is opened in US Excel, Excel will display 8:25, but if the same workbook is opened in UK Excel, it will display 08:25.

en-US and en-GB both use english, so the built-in format can differ even for the same language in different territories, not just different languages.

Some locales also have more built-in formats than others, e.g th-th locale has some ids that are over 80.

You can set a built-in number format by setting a NumberFormatId property like this: ws.Cell("A1").Style.NumberFormat.NumberFormatId = (int)XLPredefinedFormat.Number.PercentInteger

ClosedXML doesn’t contain all possible values for all locales. You either have to look through specification (ECMA-376 18.8.30), but it is not exhaustive.

The other option just change the value in the saves excel (/xl/styles.xml, tag xf, attribute numFmtId) and observe results in different locales. On Windows, you can change locale through Settings dialogue:

  • Open Settings

  • Go to Time & language on the left panel

  • Open *Language & region *in the right panel

  • Change Regional format

  • Restart Excel

Custom number format

Number format can be more tailored through a custom format code. All formats, that are not built-in, use custom format code, e.g. if you specify in the format in Excel dialogue to display value as a number with 5 decimal digits and to use 1000 serparator, Excel will store the format in the file as #,##0.00000.


The format code can be rather complicated, but is well described in official documentation Number format codes or the chapter 18.8.31 of the ECMA-376.

using var wb = new XLWorkbook();
var ws = wb.AddWorksheet();
var format = "#,##0.0; (#,##0.000)";

ws.Cell("A1").Value = 10;
ws.Cell("A1").Style.NumberFormat.Format = format;

ws.Cell("A2").Value = -10;
ws.Cell("A2").Style.NumberFormat.Format = format;
The output of the code sample

Neither built-in number format nor custom number format change the value of a cell. The value stays the same, only presentation of the value changes. It is possible to get formatted string IXLCell.GetFormattedString() method.


The format code in the xlsx file must be in some cases escaped, but ClosedXML doesn’t do that at the moment. If you encounter a problem that value isn’t formatted correcty, set the format in an Excel, save the xlsx file, change the xlsx extension to zip and use correctly escaped format code from /xl/styles.xml file in the zip.