The Ultimate Personal Finance Cheatsheet

# ── 50/30/20 Rule ──
# 50% Needs (essential expenses)
# 30% Wants (lifestyle, entertainment)
# 20% Savings & Investments (future goals)

def budget_503020(monthly_income: float):
    needs = monthly_income * 0.50   # Rent, food, utilities, transport, insurance
    wants = monthly_income * 0.30   # Dining out, shopping, subscriptions, travel
    savings = monthly_income * 0.20  # SIP, FD, PPF, emergency fund

    print(f"Monthly Income: ₹{monthly_income:,.0f}")
    print(f"  Needs (50%): ₹{needs:,.0f}")
    print(f"  Wants (30%): ₹{wants:,.0f}")
    print(f"  Savings (20%): ₹{savings:,.0f}")
    return {"needs": needs, "wants": wants, "savings": savings}

# ── Emergency Fund Calculator ──
def emergency_fund(monthly_expenses: float, months: int = 6):
    target = monthly_expenses * months
    monthly_saving = monthly_expenses * 0.2  # Save 20% of needs
    time_to_build = target / monthly_saving
    return round(target), round(time_to_build)

# ── Financial Freedom Number ──
def fire_number(monthly_expenses: float, return_rate: float = 0.07):
    """25x annual expenses at 4% safe withdrawal rate."""
    annual = monthly_expenses * 12
    return round(annual * 25)

def goal_calculator(target_amount, years, expected_return):
    """Calculate monthly SIP needed to reach a financial goal."""
    annual_return = expected_return / 100
    monthly_return = annual_return / 12
    n = years * 12

    if monthly_return == 0:
        monthly_sip = target_amount / n
    else:
        # Future Value of Annuity: FV = PMT * ((1+r)^n - 1) / r * (1+r)
        monthly_sip = target_amount / (
            (((1 + monthly_return) ** n - 1) / monthly_return) * (1 + monthly_return)
        )

    total_invested = monthly_sip * n
    returns_earned = target_amount - total_invested
    return round(monthly_sip), round(total_invested), round(returns_earned)

# Example: ₹50 lakh goal in 15 years at 12% expected returns
sip, invested, gains = goal_calculator(5000000, 15, 12)
print(f"Monthly SIP: ₹{sip:,}")
print(f"Total Invested: ₹{invested:,}")
print(f"Wealth Gained: ₹{gains:,}")

import matplotlib.pyplot as plt

def compound_growth(principal, rate, years, monthly_addition=0):
    """Calculate compound growth with optional monthly SIP."""
    monthly_rate = rate / 100 / 12
    n = years * 12
    future_value = principal * (1 + monthly_rate) ** n

    if monthly_addition > 0:
        future_value += monthly_addition * (
            ((1 + monthly_rate) ** n - 1) / monthly_rate
        ) * (1 + monthly_rate)

    return round(future_value)

# ── The Magic of Starting Early ──
# Person A starts ₹5,000 SIP at age 25 (12% returns, till age 55 = 30 years)
a = compound_growth(0, 12, 30, 5000)
# Person B starts ₹10,000 SIP at age 35 (12% returns, till age 55 = 20 years)
b = compound_growth(0, 12, 20, 10000)

print(f"Person A (₹5K from 25): ₹{a/100000:.2f}L")
print(f"Person B (₹10K from 35): ₹{b/100000:.2f}L")
# A: ~₹1.76Cr, B: ~₹99.9L
# A invested HALF the money but got 76% MORE!