Loan Calculator using PyQt5 in Python

Last Updated : 31 Jan, 2023

In this article, we will see how we can create a loan calculator using PyQt5, below is an image that shows how is the loan calculator will look like :

Loan calculator user interface

PyQt5 is cross-platform GUI toolkit, a set of python bindings for Qt v5. One can develop an interactive desktop application with so much ease because of the tools and simplicity provided by this library

GUI implementation steps :
1. Create a heading label that display the calculator name
2. Create label and line edit pair for interest rate, label to show what user has to enter and line edit to enter text
3. Similarly create a pair for number of year and for amount
4. Create a push button to calculate
5. Create a label to show the calculated monthly payment
6. Create a label to show the calculated total amount

Back end implementation steps :
1. Make the line edit to accept only the number as input
2. Add action to the push button
3. Inside the push button action get the text of the line edits
4. Check if the line edit text is empty or zero then return so that function will not execute further
5. Convert the text value into integer
6. Calculate the monthly amount and set this value to the label
7. Calculate total amount from the monthly amount and show this value through label

Below is the implementation :

Python3

# importing required libraries
from PyQt5.QtWidgets import *
from PyQt5 import QtCore, QtGui
from PyQt5.QtGui import *
from PyQt5.QtCore import *

import sys


class Window(QMainWindow):
  
    # constructor
    def __init__(self):
        super().__init__()

        # setting title
        self.setWindowTitle("Python ")

        # width of window
        self.w_width = 400

        # height of window
        self.w_height = 500

        # setting geometry
        self.setGeometry(100, 100, self.w_width, self.w_height)

        # calling method
        self.UiComponents()

        # showing all the widgets
        self.show()

    # method for adding components
    def UiComponents(self):
        # creating head label
        head = QLabel("Loan Calculator", self)

        # setting geometry to the head
        head.setGeometry(0, 10, 400, 60)

        # font
        font = QFont('Times', 15)
        font.setBold(True)
        font.setItalic(True)
        font.setUnderline(True)

        # setting font to the head
        head.setFont(font)

        # setting alignment of the head
        head.setAlignment(Qt.AlignCenter)

        # setting color effect to the head
        color = QGraphicsColorizeEffect(self)
        color.setColor(Qt.darkCyan)
        head.setGraphicsEffect(color)

        # creating a interest label
        i_label = QLabel("Annual Interest", self)

        # setting properties to the interest label
        i_label.setAlignment(Qt.AlignCenter)
        i_label.setGeometry(20, 100, 170, 40)
        i_label.setStyleSheet("QLabel"
                              "{"
                              "border : 2px solid black;"
                              "background : rgba(70, 70, 70, 35);"
                              "}")
        i_label.setFont(QFont('Times', 9))

        # creating a QLineEdit object to get the interest
        self.rate = QLineEdit(self)

        # accepting only number as input
        onlyInt = QIntValidator()
        self.rate.setValidator(onlyInt)

        # setting properties to the rate line edit
        self.rate.setGeometry(200, 100, 180, 40)
        self.rate.setAlignment(Qt.AlignCenter)
        self.rate.setFont(QFont('Times', 9))


        # creating a number of years label
        n_label = QLabel("Years ", self)

        # setting properties to the years label
        n_label.setAlignment(Qt.AlignCenter)
        n_label.setGeometry(20, 150, 170, 40)
        n_label.setStyleSheet("QLabel"
                              "{"
                              "border : 2px solid black;"
                              "background : rgba(70, 70, 70, 35);"
                              "}")
        n_label.setFont(QFont('Times', 9))

        # creating a QLineEdit object to get the years
        self.years = QLineEdit(self)

        # accepting only number as input
        onlyInt = QIntValidator()
        self.years.setValidator(onlyInt)

        # setting properties to the rate line edit
        self.years.setGeometry(200, 150, 180, 40)
        self.years.setAlignment(Qt.AlignCenter)
        self.years.setFont(QFont('Times', 9))

        # creating a loan amount label
        a_label = QLabel("Amount", self)

        # setting properties to the amount label
        a_label.setAlignment(Qt.AlignCenter)
        a_label.setGeometry(20, 200, 170, 40)
        a_label.setStyleSheet("QLabel"
                              "{"
                              "border : 2px solid black;"
                              "background : rgba(70, 70, 70, 35);"
                              "}")
        a_label.setFont(QFont('Times', 9))

        # creating a QLineEdit object to get the amount
        self.amount = QLineEdit(self)

        # accepting only number as input
        onlyInt = QIntValidator()
        self.amount.setValidator(onlyInt)

        # setting properties to the rate line edit
        self.amount.setGeometry(200, 200, 180, 40)
        self.amount.setAlignment(Qt.AlignCenter)
        self.amount.setFont(QFont('Times', 9))


        # creating a push button
        calculate = QPushButton("Compute Payment", self)

        # setting geometry to the push button
        calculate.setGeometry(125, 270, 150, 40)

        # adding action to the calculate button
        calculate.clicked.connect(self.calculate_action)

        # creating a label to show monthly payment
        self.m_payment = QLabel(self)

        # setting properties to m payment label
        self.m_payment.setAlignment(Qt.AlignCenter)
        self.m_payment.setGeometry(50, 340, 300, 60)
        self.m_payment.setStyleSheet("QLabel"
                                     "{"
                                     "border : 3px solid black;"
                                     "background : white;"
                                     "}")
        self.m_payment.setFont(QFont('Arial', 11))

        # creating a label to show monthly payment
        self.y_payment = QLabel(self)

        # setting properties to y payment label
        self.y_payment.setAlignment(Qt.AlignCenter)
        self.y_payment.setGeometry(50, 410, 300, 60)
        self.y_payment.setStyleSheet("QLabel"
                                     "{"
                                     "border : 3px solid black;"
                                     "background : white;"
                                     "}")
        self.y_payment.setFont(QFont('Arial', 11))

    # method for calculating monthly 
    # and annually payments
    def calculate_action(self):

        # getting annual interest rate
        annualInterestRate = self.rate.text()

        # if there is no number is entered
        if len(annualInterestRate) == 0 or annualInterestRate == '0':
            return

        # getting number of years
        numberOfYears = self.years.text()

        # if there is no number is entered
        if len(numberOfYears) == 0 or numberOfYears == '0':
            return

        # getting loan amount
        loanAmount = self.amount.text()

        # if there is no number is entered
        if len(loanAmount) == 0 or loanAmount == '0':
            return

        # converting text to int
        annualInterestRate = int(annualInterestRate)
        numberOfYears = int(numberOfYears)
        loanAmount = int(loanAmount)

        # getting monthly interest rate
        monthlyInterestRate = annualInterestRate / 1200

        # calculating monthly payment
        monthlyPayment = loanAmount * monthlyInterestRate / (1 - 1 / (1 + monthlyInterestRate) ** (numberOfYears * 12))

        # setting formatting
        monthlyPayment = "{:.2f}".format(monthlyPayment)

        # setting text to the label
        self.m_payment.setText("Monthly Payment : " + str(monthlyPayment))

        # getting total payment
        totalPayment = float(monthlyPayment) * 12 * numberOfYears
        totalPayment = "{:.2f}".format(totalPayment)

        # setting text to the label
        self.y_payment.setText("Total Payment : " + str(totalPayment))



# create pyqt5 app
App = QApplication(sys.argv)

# create the instance of our Window
window = Window()

# start the app
sys.exit(App.exec())