from tkinter import *


class inch_to_mm:
    def __init__(self, inch):
        self.inch = inch

        frame= Frame(inch)
        frame.pack()
        Label(frame,text = 'Size in inches').grid(row = 0, column = 0)
        self.I_var = DoubleVar()
        Entry(frame, textvariable= self.I_var).grid(row=0,column=1)
        Label(frame, text = 'Size in millimeters').grid(row=1,column=0)
        self.result_var=DoubleVar()
        Label(frame,textvariable=self.result_var).grid(row=1, column=1)
        button = Button(frame,text = 'Find millimeters', command =self.convert)
        button.grid(row=2, columnspan=2)

    def convert(self):
        mm = self.inch * 25.4
        return mm






root = Tk()
root.wm_title('Inch to mm')
Size = inch_to_mm(root)
root.mainloop()