How to scale text buttons android?

i want to develop an app like a calculator. The problem is, that the text buttons have to be in the same order on every device. I tried it like this but i doesn't work:

<RelativeLayout xmlns:android="http://ift.tt/nIICcg"
xmlns:tools="http://ift.tt/LrGmb4" android:layout_width="fill_parent"
android:layout_height="fill_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity"
android:id="@+id/RelativeLayout1"
android:background="@drawable/neon_bg">



<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:text="New Button"
    android:id="@+id/button1"
    android:layout_alignParentTop="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />

<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:text="New Button"
    android:id="@+id/button2"
    android:layout_alignTop="@+id/button1"
    android:layout_toRightOf="@+id/button1"
    android:layout_toEndOf="@+id/button1"
    android:layout_weight="8"/>

<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:text="New Button"
    android:id="@+id/button3"
    android:layout_toEndOf="@+id/button2"
    android:layout_alignTop="@+id/button2"
    android:layout_toRightOf="@+id/button2" />

<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:text="New Button"
    android:id="@+id/button4"
    android:layout_toEndOf="@+id/button3"
    android:layout_alignTop="@+id/button3"
    android:layout_toRightOf="@+id/button3" />

<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:text="New Button"
    android:id="@+id/button5"
    android:layout_below="@+id/button1"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />

<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:text="New Button"
    android:id="@+id/button6"
    android:layout_alignBottom="@+id/button5"
    android:layout_alignLeft="@+id/button2"
    android:layout_alignStart="@+id/button2" />

<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:text="New Button"
    android:id="@+id/button7"
    android:layout_alignTop="@+id/button6"
    android:layout_alignLeft="@+id/button3"
    android:layout_alignStart="@+id/button3" />

<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:text="New Button"
    android:id="@+id/button8"
    android:layout_alignBottom="@+id/button7"
    android:layout_toRightOf="@+id/button7"
    android:layout_toEndOf="@+id/button7" />

<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:text="New Button"
    android:id="@+id/button9"
    android:layout_below="@+id/button5"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />

<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:text="New Button"
    android:id="@+id/button10"
    android:layout_alignBottom="@+id/button9"
    android:layout_alignLeft="@+id/button6"
    android:layout_alignStart="@+id/button6" />

<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:text="New Button"
    android:id="@+id/button11"
    android:layout_alignBottom="@+id/button10"
    android:layout_alignLeft="@+id/button7"
    android:layout_alignStart="@+id/button7" />

<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:text="New Button"
    android:id="@+id/button12"
    android:layout_alignBottom="@+id/button11"
    android:layout_toRightOf="@+id/button11"
    android:layout_toEndOf="@+id/button11" />

<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:id="@+id/button13"
    android:layout_below="@+id/button9"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />

<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:text="New Button"
    android:id="@+id/button14"
    android:layout_below="@+id/button10"
    android:layout_alignLeft="@+id/button10"
    android:layout_alignStart="@+id/button10" />

<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:text="New Button"
    android:id="@+id/button15"
    android:layout_below="@+id/button10"
    android:layout_toRightOf="@+id/button14"
    android:layout_toEndOf="@+id/button14" />

<Button
    android:layout_width="140dp"
    android:layout_height="75dp"
    android:text="New Button"
    android:id="@+id/button16"
    android:layout_below="@+id/button12"
    android:layout_toRightOf="@+id/button15"
    android:layout_toEndOf="@+id/button15" />

How can i let the button scale to the size required for the screen, so the buttons have the same size in relation?

Thanks