Set size (height and width) for elements in layout (Constraint layout) with percent (%) in Android Studio version 3.1.2.

Create new project with Application name: My Application; Minimum SDK: API 27Android 8.1 (Oreo).

In app->res->layout->activity_main.xml write the code:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <android.support.constraint.Guideline
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/right_guideline"
        app:layout_constraintGuide_percent=".80"
        android:orientation="vertical"/>

    <android.support.constraint.Guideline
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/left_guideline"
        app:layout_constraintGuide_percent=".20"
        android:orientation="vertical"/>

    <android.support.constraint.Guideline
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/top_guideline"
        app:layout_constraintGuide_percent=".20"
        android:orientation="horizontal"/>

    <android.support.constraint.Guideline
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/bottom_guideline"
        app:layout_constraintGuide_percent=".80"
        android:orientation="horizontal"/>

    <Button
        android:id="@+id/button"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_marginBottom="8dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginTop="8dp"
        android:background="@android:color/holo_red_dark"
        android:text="Button"
        android:textColor="@android:color/white"
        android:textSize="24sp"
        app:layout_constraintBottom_toBottomOf="@+id/bottom_guideline"
        app:layout_constraintLeft_toLeftOf="@+id/left_guideline"
        app:layout_constraintRight_toRightOf="@+id/right_guideline"
        app:layout_constraintTop_toTopOf="@+id/top_guideline" />

</android.support.constraint.ConstraintLayout>

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