t.py

from kivy.app import App
from kivy.lang import Builder


KV = '''
<BorderBoxLayout@BoxLayout,BorderGridLayout@GridLayout,CanvasButton@Label+ButtonBehavior,BorderLabel@Label>:
    border_width: 1
    canvas.before:
        Line:
            rectangle: self.pos + self.size
            width: self.border_width or 1

<BorderBoxLayout,BorderGridLayout>:
    size: self.minimum_size
    size_hint: None, None

<CanvasButton>:
    size: "80dp", self.texture_size[1]
    font_size: "30sp"
    size_hint: None, None

FloatLayout:
    BorderBoxLayout:
        pos_hint: {'center': (.5, .5)}
        orientation: 'vertical'
        size_hint: None, None
        border_width: 4
        BorderLabel:
            id: weight
            size_hint_y: None
            height: "30sp"
            font_size: "30sp"
            border_width: 2
        BorderGridLayout:
            cols: 3
            border_widh: 2
            CanvasButton:
                text: "7"
            CanvasButton:
                text: "8"
            CanvasButton:
                text: "9"
            CanvasButton:
                text: "4"
            CanvasButton:
                text: "5"
            CanvasButton:
                text: "6"
            CanvasButton:
                text: "1"
            CanvasButton:
                text: "2"
            CanvasButton:
                text: "3"
            CanvasButton:
                text: "0"
            CanvasButton:
                text: "lbs"
            CanvasButton:
                text: "«"

        BorderBoxLayout:
            border_width: 2
            size_hint_x: 1
            CanvasButton:
                text: "EXIT"
                size_hint_x: 1
            CanvasButton:
                text: "ENTER"
                size_hint_x: 1

'''

class Application(App):
    def build(self):
        return Builder.load_string(KV)


if __name__ == "__main__":
    Application().run()