title: "ValueError: could not convert string to float" when converting input
body: <p>Recently I have been working on a code and got stuck for days on this error.
  Basically the program calculates how many calories you have to eat everyday. I have to take input from an entry and I don't know how to convert that input(it's a string by default) into a float to start using the numbers.
  I am using Python 3 with Tkinter.</p>
  
  <p>Here is the code:</p>
  
  <pre><code>from tkinter import *
  
  root = Tk()
  root.geometry("1000x500")
  root.resizable(FALSE, FALSE)
  root.title("BMI Calculator")
  
  def calc(args):
  
      def BMI_temp(args):
          print(str(boyage))
          BMI = IntVar()
          BMI = 66.5 + (13.75 * float(boykg)) + (5.003 * float(boycm)) - (6.755 * float(boyage))
          bmi_temp = Label(root, text="This is how many calories you have to eat if you have a non-active life: " + str(float(BMI)))
          bmi_temp.grid(row=3, sticky=W)
  
      def boy_age_fnct(args):
          boy_age_entry.focus_set()
          boy_cm_entry.delete(0, "end")
          boy_age.grid(row=2, sticky=W)
          boy_age_entry.grid(row=2, column=1)
          boy_age_entry.bind("&lt;Return&gt;", BMI_temp)
  
      def boy_cm_fnct(args):
          boy_cm_entry.focus_set()
          boy_kg_entry.delete(0, "end")
          boy_cm.grid(row=1, sticky=W)
          boy_cm_entry.grid(row=1, column=1)
          boy_cm_entry.bind("&lt;Return&gt;", boy_age_fnct)
  
      boy_kg_entry.focus_set()
      temp = boygirle.get()
      gender = temp.title()
      welcome.destroy()
      hello_lbl.destroy()
      boygirle.destroy()
      boygirlq.destroy()
  
      if gender[0] == 'B':
          boy_kg.grid(row=0, sticky=W)
          boy_kg_entry.grid(row=0, column=1)
          boy_kg_entry.bind("&lt;Return&gt;", boy_cm_fnct)
  
      boyage = boy_age_entry.get()
      boycm = boy_cm_entry.get()
      boykg = boy_kg_entry.get()
  
  def hello(args):
      name_user = name_entry.get()
      name2 = name_user.title()
      name_entry.delete(0, "end")
      hello = "Hello " + name2 + "!"
      hello_lbl["text"] = hello
      hello_lbl.grid(row=2, sticky=W)
      btn_cont.grid(row=3, sticky=W)
      name.destroy()
      name_entry.destroy()
      btn_cont.focus_set()
  
  def BMI():
      btn_cont.destroy()
      boygirlq.grid(row=3, sticky=W)
      boygirle.grid(row=3, column=0, ipadx=35)
      boygirle.bind("&lt;Return&gt;", calc)
      boygirle.focus_set()
  
  welcome = Label(root, text="Hello! This is a BMR calculator. It tells you how many calories you have to eat!", font="System 14 bold")
  name = Label(root, text="Please enter your name:", font="System 12")
  hello_lbl = Label(root, font="System 14")
  boygirlq = Label(root, text="Are you a boy or a girl?", font="System 12 bold")
  boy_kg = Label(root, text="Please enter your weight(in kg):", font="System 12 bold")
  boy_cm = Label(root, text="Please enter your height(in cm):", font="System 12")
  boy_age = Label(root, text="Please enter your age(in years):", font="System 12")
  
  btn_cont = Button(root, text="Continue", font="Helvetica 12", command=BMI, relief=RAISED)
  
  boy_kg_entry = Entry(root, font="System 12", relief=SUNKEN)
  boy_cm_entry = Entry(root, font="System 12", relief=SUNKEN)
  boy_age_entry = Entry(root, font="System 12", relief=SUNKEN)
  name_entry = Entry(root, font="System 12", relief=SUNKEN)
  boygirle = Entry(root, font="System 12", relief=SUNKEN)
  name_entry.bind("&lt;Return&gt;", hello)
  name_entry.focus_set()
  
  welcome.grid(row=0, columnspan=2, ipadx=200)
  name.grid(row=1, sticky=W)
  name_entry.grid(row=1, column=0)
  
  root.mainloop()
  </code></pre>
  
  <p>I tried all the methods I found on the internet but nothing worked.</p>