What is the default text size on Android?

This will return default size of text on button in pixels.

Kotlin

val size = Button(this).textSize

Java

float size = new Button(this).getTextSize();

In general:

Three "default" textSize values:

 - 14sp
 - 18sp
 - 22sp

These values are defined within the following TextAppearances:

 - TextAppearance.Small
 - TextAppearance.Medium
 - TextAppearance.Large

More information about Typography can be found in the design guidelines

Related to your question:

If you don't set a custom textSize or textAppearance, TextAppearance.Small will be used.

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Update: Material design:

New guidelines related to font and typefaces. The standard rule of 14sp remains (body).

Examples how to set textappearances

AppCompat version:

android:textAppearance="@style/TextAppearance.AppCompat.Body"

Lollipop and up version:

android:textAppearance="@android:style/TextAppearance.Material.Body"

Looks like someone else found it: What are the default font characteristics in Android ?

There someone discovered the default text size, for TextViews (which use TextAppearance.Small) it's 14sp.