Python Program - Compound Interest WITH Monthly Contributions/Deposit

Program to calculate compound interest with monthly contribution at the end of the month.

Formula taken from TheCalculatorSite.com:
Compound interest for principal:
P(1+r/n)^(nt)
Future value of a series:
PMT × (((1 + r/n)^nt - 1) / (r/n))

Step 1: Get Data From User

Ask the user to put in the required data:

principalinput = input("Enter principal: ")
annualrateinput = input("Enter annual rate: ")
numberoftimescompoundedinput = input("Enter number of times that the interest is compounded per year: ") yearsinput = input("Time in years: ")
monthlycontributioninput = input("Enter monthly contribution amount: ")

Step 2: Convert Data

Data is entered as a strings which must be converted into integers

# Convert entered input from strings into integers
principal = int(principalinput)
annualrate = (int(annualrateinput))/100
numberoftimescompounded = int(numberoftimescompoundedinput)
years = int(yearsinput)
monthlycontribution = int(monthlycontributioninput)

Step 3: Calculate the Compound Interest Plus the Principal

First, calculate the compound interest plus the principal:

# calculate compound interest plus the principal
preliminarynumber = (1 + (annualrate/numberoftimescompounded))
# print ("Preliminary number:", preliminarynumber)
raisedtopower = (numberoftimescompounded * years)
# print ("Raised to power:", raisedtopower)
compoundinterestplusprincipal = principal * (preliminarynumber**raisedtopower)
print("The compound interest plus the principal is: ", compoundinterestplusprincipal)

Step 4: Calculate the Future Value

Then calculate the future value with deposits

# Now calculate the future value with deposits made at the end of the period
# Using formula: Monthly Payment × ( ( ( (1 + r/n)^(nt) ) - 1 ) / (r/n) )
# r = annual interest rate
# n = number of compounds per period (usually in months)
# t = time the money is invested (usually in years)

oneplus = (1+(annualrate/numberoftimescompounded))
raisedtopower2 = ((numberoftimescompounded*years))
ratedividedbynumberoftimes = annualrate/numberoftimescompounded
halfdone = (((oneplus**raisedtopower2)-1)/ratedividedbynumberoftimes)
futurevaluewithdeposits = monthlycontribution*halfdone
print ("Future value with deposits: ",futurevaluewithdeposits)
totalamount = compoundinterestplusprincipal + futurevaluewithdeposits
print ("Total Amount:", totalamount)

Step 5: Code

# Program to calculate compound interest with monthly contribution at end of month

# First calculate the compound interest for principal using formula: A = P (1 + r/n)**(nt)
# r = annual interest rate
# n = number of compounds per period (usually in months)
# t = time

principalinput = input("Enter principal: ")
annualrateinput = input("Enter annual rate: ")
numberoftimescompoundedinput = input("Enter number of times that the interest is compounded per year: ")
yearsinput = input("Time in years: ")
monthlycontributioninput = input("Enter monthly contribution amount: ")

# Convert entered input from strings into integers
principal = int(principalinput)
annualrate = (int(annualrateinput))/100
numberoftimescompounded = int(numberoftimescompoundedinput)
years = int(yearsinput)
monthlycontribution = int(monthlycontributioninput)

print ("The principal entered is: ", principal)
print ("The annual rate in decimal form is: ", annualrate)
print ("The number of times it will be compounded per year is: ", numberoftimescompounded)
print ("The number of years it will be compounded: ", years)
print ("The monthly contribution is: ", monthlycontribution)

# calculate compound interest plus the principal
preliminarynumber = (1 + (annualrate/numberoftimescompounded))
# print ("Preliminary number:", preliminarynumber)
raisedtopower = (numberoftimescompounded * years)
# print ("Raised to power:", raisedtopower)

compoundinterestplusprincipal = principal * (preliminarynumber**raisedtopower)

print("The compound interest plus the principal is: ", compoundinterestplusprincipal)

# Now calculate the future value with deposits made at the end of the period
# Using formula: Monthly Payment × ( ( ( (1 + r/n)^(nt) ) - 1 ) / (r/n) )
# r = annual interest rate
# n = number of compounds per period (usually in months)
# t = time the money is invested (usually in years)

oneplus = (1+(annualrate/numberoftimescompounded))
raisedtopower2 = ((numberoftimescompounded*years))
ratedividedbynumberoftimes = annualrate/numberoftimescompounded

halfdone = (((oneplus**raisedtopower2)-1)/ratedividedbynumberoftimes)
futurevaluewithdeposits = monthlycontribution*halfdone
print ("Future value with deposits: ",futurevaluewithdeposits)

totalamount = compoundinterestplusprincipal + futurevaluewithdeposits
print ("Total Amount:", totalamount)