Python Mortgage Calculator

Build a mortgage calculator that computes monthly payments, generates amortisation schedules, and compares different deposit and rate scenarios. Essential for anyone buying property.

def mortgage_calculator(property_price, deposit_pct, interest_rate, term_years):
    """Calculate monthly mortgage payment and total cost."""
    deposit = property_price * (deposit_pct / 100)
    loan = property_price - deposit
    monthly_rate = interest_rate / 100 / 12
    num_payments = term_years * 12

    if monthly_rate == 0:
        monthly_payment = loan / num_payments
    else:
        monthly_payment = loan * (monthly_rate * (1 + monthly_rate)**num_payments) / \
                         ((1 + monthly_rate)**num_payments - 1)

    total_paid = monthly_payment * num_payments
    total_interest = total_paid - loan

    return {
        "property_price": property_price,
        "deposit": deposit,
        "loan_amount": loan,
        "monthly_payment": monthly_payment,
        "total_paid": total_paid,
        "total_interest": total_interest,
        "ltv": (loan / property_price) * 100
    }

result = mortgage_calculator(300000, 10, 4.5, 25)
for k, v in result.items():
    print(f"{k}: {v:,.2f}" if isinstance(v, float) else f"{k}: {v}")

import pandas as pd

def amortisation_schedule(loan, annual_rate, years):
    """Generate full amortisation schedule."""
    monthly_rate = annual_rate / 100 / 12
    num_payments = years * 12
    monthly_payment = loan * (monthly_rate * (1 + monthly_rate)**num_payments) / \
                     ((1 + monthly_rate)**num_payments - 1)

    schedule = []
    balance = loan

    for month in range(1, num_payments + 1):
        interest_payment = balance * monthly_rate
        principal_payment = monthly_payment - interest_payment
        balance -= principal_payment

        schedule.append({
            "Month": month,
            "Payment": round(monthly_payment, 2),
            "Principal": round(principal_payment, 2),
            "Interest": round(interest_payment, 2),
            "Balance": round(max(balance, 0), 2)
        })

    return pd.DataFrame(schedule)

schedule = amortisation_schedule(270000, 4.5, 25)
print(schedule.head(12))

# Year 1 summary
y1 = schedule[:12]
print(f"\nYear 1: Principal paid = £{y1['Principal'].sum():,.2f}")
print(f"Year 1: Interest paid = £{y1['Interest'].sum():,.2f}")

import matplotlib.pyplot as plt

# Compare different deposit sizes
deposits = [5, 10, 15, 20, 25]
results = []

for dep in deposits:
    r = mortgage_calculator(300000, dep, 4.5, 25)
    results.append({
        "Deposit %": f"{dep}%",
        "Deposit": f"£{r['deposit']:,.0f}",
        "Monthly": f"£{r['monthly_payment']:,.2f}",
        "Total Interest": f"£{r['total_interest']:,.0f}",
        "LTV": f"{r['ltv']:.0f}%"
    })

df = pd.DataFrame(results)
print(df.to_string(index=False))

# Plot monthly payment vs interest rate
rates = [x/10 for x in range(20, 80)]  # 2.0% to 7.9%
payments = [mortgage_calculator(300000, 10, r, 25)["monthly_payment"] for r in rates]

plt.figure(figsize=(10, 6))
plt.plot(rates, payments, linewidth=2, color="blue")
plt.xlabel("Interest Rate (%)")
plt.ylabel("Monthly Payment (£)")
plt.title("Monthly Payment vs Interest Rate (£300k, 10% deposit, 25yr)")
plt.grid(True, alpha=0.3)
plt.savefig("mortgage_rates.png", dpi=150)
plt.show()