Mortgage Calculator – Master Your Home Loan Numbers

To calculate your monthly mortgage repayment for a Principal and Interest loan, use the formula:

Where:

• M = Monthly repayment
• P = Amount borrowed (principal)
• r = Monthly interest rate (annual rate ÷ 12)
• n = Total number of payments (loan term in years × 12)

Example Calculation:

Given:

• Amount borrowed (P) = $400,000
• Annual interest rate (r) = 6.38% (0.0638, monthly = 0.005317)
• Loan term = 30 years (n = 360)
• Repayment frequency = Monthly

Step 1: Compute the monthly repayment:

After 30 years, your monthly repayment would be approximately $2,496.52.

Key Insights:

• The repayment covers both principal and interest, reducing the loan balance over time.
• For Interest Only loans, you’d pay $2,126.80/month (P × r), but the principal stays the same.
• Longer terms lower monthly payments but increase total interest paid.

The loan term impacts your monthly repayments and total interest paid. A shorter term increases monthly payments but reduces interest, while a longer term does the opposite.

Comparison Example: Different Loan Terms

Given:

• Amount borrowed (P) = $400,000
• Annual interest rate (r) = 6.38% (0.005317 monthly)
• Repayment frequency = Monthly

Case 1: 15-year term (n = 180)

Case 2: 30-year term (n = 360)

Results:

1. 15 years → $3,451.62/month (Total interest ~$221,291)
2. 30 years → $2,496.52/month (Total interest ~$498,747)

Key Insights:

• Shorter terms mean higher payments but save significantly on interest.
• Doubling the term (15 to 30 years) cuts the monthly payment by ~27% but more than doubles the interest.
• Choose a term based on affordability and long-term cost goals.

Principal and Interest (P&I) repayments reduce your loan balance over time, while Interest Only (IO) payments cover only the interest, leaving the principal unchanged until the IO period ends.

Example Comparison:

Given:

• Amount borrowed (P) = $400,000
• Interest rate = 6.38% (0.005317 monthly)
• Term = 30 years (n = 360)

P&I:

IO:

Results:

1. P&I → $2,496.52/month (loan paid off in 30 years)
2. IO → $2,126.80/month (principal remains $400,000 after IO period)

Key Insights:

• P&I builds equity; IO delays principal reduction, often used by investors.
• IO payments are lower but require a plan to pay the principal later.
• After an IO period (e.g., 5 years), P&I payments jump as the term shortens.

A higher interest rate increases both your monthly repayment and the total interest paid over the loan term.

Comparison Example:

Given:

• Amount borrowed (P) = $400,000
• Loan term = 30 years (n = 360)

Case 1: 6.38% (0.005317 monthly)

Total paid = $2,496.52 × 360 = $898,747 (Interest ~$498,747)

Case 2: 4.38% (0.00365 monthly)

Total paid = $2,108.64 × 360 = $759,110 (Interest ~$359,110)

Results:

1. 6.38% → $2,496.52/month, $498,747 interest
2. 4.38% → $2,108.64/month, $359,110 interest

Key Insights:

• A 2% rate drop saves ~$139,637 in interest over 30 years.
• Small rate differences compound into big savings or costs over time.
• Locking in a lower rate can significantly reduce your mortgage burden.

Extra repayments reduce the principal faster, shortening the loan term and saving interest.

Example Calculation:

Given:

• Amount borrowed (P) = $400,000
• Interest rate = 6.38% (0.005317 monthly)
• Term = 30 years (n = 360)
• Standard repayment = $2,496.52

Add $500/month (Total = $2,996.52):

New term calculated iteratively: ~23 years (276 months)

Total paid = $2,996.52 × 276 = $827,039 (Interest ~$427,039)

Results:

• Standard: 30 years, $498,747 interest
• Extra $500: 23 years, $427,039 interest (saves ~$71,708, 7 years)

Key Insights:

• Extra payments early in the loan save the most interest.
• Even small increases can cut years off your mortgage.
• Check with your lender for prepayment penalties or restrictions.

To find your borrowing capacity, reverse the repayment formula based on what you can afford:

Example Calculation:

Given:

• Affordable repayment (M) = $2,000
• Interest rate = 6.38% (0.005317 monthly)
• Term = 30 years (n = 360)

You can borrow approximately $320,000.

Key Insights:

• Higher affordable payments or lower rates increase borrowing power.
• Include fees or buffers in your budget for a realistic estimate.
• Lenders also consider income, debt, and credit score.

Refinancing replaces your current loan with a new one, often to get a lower rate or adjust terms, affecting repayments and total cost.

Example Calculation:

Given:

• Current loan: $400,000, 6.38%, 25 years left, $2,596.52/month
• Refinance to: 4.38%, 25 years

New repayment:

Results:

• Old: $2,596.52/month, ~$378,956 interest
• New: $2,233.40/month, ~$270,020 interest (saves ~$108,936)

Key Insights:

• Lower rates via refinancing save money long-term.
• Fees (e.g., $1,000-$3,000) may apply—ensure savings outweigh costs.
• Shortening the term during refinance accelerates payoff.

A lump-sum payment reduces the principal, lowering interest and potentially the term if repayments stay the same.

Example Calculation:

Given:

• Amount owing = $400,000
• Interest rate = 6.38%
• Repayment = $2,496.52 (30 years)
• Lump sum = $20,000 after 5 years

After 5 years, balance ~$385,000. Lump sum reduces it to $365,000. New term with $2,496.52/month: ~27 years.

Interest saved: ~$50,000.

Key Insights:

• Lump sums early maximize savings due to interest reduction.
• Adjust repayments post-payment to shorten the term further.
• Verify your loan allows penalty-free lump sums.

Fees (e.g., monthly service fees) increase your effective repayment cost but don’t reduce the principal.

Example Calculation:

Given:

• Amount borrowed = $400,000
• Interest rate = 6.38%
• Term = 30 years
• Fees = $10/month

P&I repayment = $2,496.52

Total repayment = $2,496.52 + $10 = $2,506.52

Total fees = $10 × 360 = $3,600

Results:

• Without fees: $898,747 total
• With fees: $902,347 (extra $3,600)

Key Insights:

• Fees add up over time—$3,600 is ~1% of the loan here.
• Compare loans with fee structures to minimize costs.
• Some fees may be tax-deductible (e.g., for Investment loans).

An offset account reduces the principal used to calculate interest, lowering your total interest without changing repayments.

Example Calculation:

Given:

• Loan = $400,000
• Interest rate = 6.38%
• Repayment = $2,496.52 (30 years)
• Offset balance = $20,000

Interest calculated on $400,000 – $20,000 = $380,000:

Monthly interest = $380,000 × 0.005317 = $2,020.46 (vs. $2,126.80)

Term shortens to ~28 years, saving ~$40,000 in interest.

Key Insights:

• Every dollar in the offset saves interest.
• Flexible access to funds makes it ideal for savings.
• Not all loans offer offsets—check availability.

Your credit score influences the interest rate and loan approval. A higher score often secures lower rates.

Comparison Example:

Given:

• Amount borrowed = $400,000
• Term = 30 years

Case 1: High score, 6.38% → $2,496.52/month, $498,747 interest

Case 2: Lower score, 7.38% → $2,764.09/month, $595,072 interest

Results:

• 1% higher rate adds ~$267/month, ~$96,325 total.

Key Insights:

• Improving your score before applying can save thousands.
• Lenders may also adjust terms or require higher deposits for lower scores.
• Check your score annually to maintain borrowing power.

Owner-Occupied loans typically have lower rates and fees, while Investment loans may offer tax benefits but higher costs.

Comparison Example:

Given:

• Amount borrowed = $400,000
• Term = 30 years

Owner-Occupied: 6.38% → $2,496.52/month

Investment: 6.88% → $2,630.28/month

Results:

• Investment loan costs ~$134/month more, ~$48,240 extra interest.

Key Insights:

• Investment loans may allow interest deductions (tax advice needed).
• Owner-Occupied loans prioritize affordability for homeowners.
• Compare rates specific to your loan purpose.