Compound Interest, Part 4: The Hidden Opportunity Cost Of Debt

Compound Interest, Part 4: The Hidden Opportunity Cost Of Debt

A well-camouflaged owl, illustrating the hidden and hard-to-spot nature of the opportunity cost of debt.
A well-camouflaged owl, one of nature’s most seldom-seen, stealthy, and deadly predators.
Photo by Kdsphotos, CC0 1.0 license

This is the fourth installment of a 5-part series examining the massive impact compound interest can have on your journey to financial freedom.

Previously in the series we introduced the phenomenon of compound interest, explored how it can serve as a rocket to riches capable of powering us through the millionaire barrier, and discovered how it can serve as the best double agent the world has ever seen.

By now you should be familiar with how to calculate the total amount of interest incurred over the life of a loan when you purchase something on credit, whether that be in the form of a credit card, auto loan, student loan, or a mortgage.

Adding interest costs to the original loan amount is a much more accurate method of determining the total cost of a purchase than is considering just the loan amount itself. Yet correctly calculating the TRUE cost of debt requires one final piece of the compound interest puzzle, which we haven’t yet addressed.

Today we’re taking a look at how compound interest creates a well-camouflaged, deadly cost of debt which has an outsized impact on your ability to escape the rat race and obtain financial freedom. This secret exists in the form of opportunity cost.

What Is Opportunity Cost?

Opportunity cost consists of the value one gives up in order to pursue an alternative course of action. Uhh… say what? Let’s try that again, in English this time.

Let’s say you’re an hourly employee who comes down with the flu, but you’ve already used up all of your vacation and sick days for the year. You can call in sick, but you won’t be paid if you do. Still, you feel absolutely terrible, so you make the decision to call in to your supervisor to let them know you’re ill and won’t be in for the day.

The opportunity cost of calling in sick in this scenario consists of two amounts:

  1. The wages you would have otherwise earned had you dragged yourself in to work.
  2. The future money you could have made with those wages had you invested them.

You gave up both the current value of those lost wages AND their future earning potential in order to pursue an alternative course of action (rest).

Loan Interest – The Predator You Can See

Now that we’ve defined the concept of opportunity cost, let’s explore how compound interest contributes to an additional, hidden cost of debt beyond simple loan interest.

Consider a scenario where you purchase a new, top-of-the-line TV for $1,200 using your credit card, which possesses the national average interest rate of 16.15%. You set a goal of paying off this balance in 12 months, which our familiar credit card payoff calculator tells us will require monthly payments of $108.96:

Credit Card Payoff Calculator results displaying the interest cost of purchasing a TV on credit.

You diligently make these payments and pay off the TV a year later, paying $107.53 in interest on the balance along the way.

The average person would believe they purchased a TV for $1,200, but by now you know better. You factor in the effects of compound interest on the purchase and know that your TV cost a total of $1,307.53 ($1,200 + $107.53), 9% more than the original purchase price.

But don’t rest on your laurels just yet, because we still haven’t calculated the full cost of your new TV. What’s missing? You guessed it – we still have to factor in what else you could have done with the $107.53 which you paid in credit card interest.

The Stealthy Second Element of Opportunity Cost

After reading part 2 of this series, you’re now well aware of the power that compound interest has to grow exponentially over time in a savings or investment account.

So let’s assume that if you hadn’t been losing a portion of each credit card payment to compound interest, you would have taken that same amount and invested it instead at the same time that you made your credit card payment every month.

The opportunity cost of your TV purchase consists in part of the money you would have otherwise earned via compound interest had you saved or invested the cash that you were instead losing by way of compound interest on your credit card balance.

Foregone Future Value – A Seldom Seen Danger

Calculating this amount would be a quite monotonous and time-consuming exercise involving the use of multiple online calculator types were it not for the existence of this handy cost of debt calculator, which does all of the heavy lifting for us.

Plugging in our credit card balance, rate, and payment info from our TV purchase above (use monthly payments of $108.975 rather than $108.96 to resolve a rounding discrepancy) and using the historical annual average of 7% for our hypothetical stock market investments returns the below results:

Cost of Debt Calculator results displaying the opportunity cost of debt incurred by purchasing a TV on credit.

We see per the Foregone Future Value column above that had you instead invested the $107.87 which you paid in credit card interest, it would have grown to a total of $112.86 after one year. So you’re out not only the original $107.87 lost to credit card interest, but the chance to keep that money AND earn an additional $4.99 in interest ($112.86 future value – $107.87 starting balance).

The total opportunity cost of the debt you incurred by purchasing the TV on credit is therefore the $107.87 you lost to credit card interest, plus the $112.86 you could have had in the form of an investment account balance – $220.73.

This is 18.4% more than the original purchase price of the TV, a hard-to-spot and stealthy surcharge.

Mortgage Opportunity Cost – The Silent Killer

An 18.4% cost increase is bad any way you slice it, but it pales in comparison to what happens when we calculate the opportunity cost of even larger loans over longer periods of time after giving compound interest a longer timeframe with which to work.

For example, let’s calculate the opportunity cost of paying 4% interest on a $100,000 mortgage in monthly payments spread over 30 years. Our mortgage loan amortization schedule calculator tells us that this will require monthly payments of $477.42 for 30 years and will result in a total of $71,869.51 in interest paid:

Mortgage Loan Amortization Calculator results indicating monthly payment and total interest paid.

If that $71,869.51 of interest were not otherwise lining the pockets of our mortgage lender, we could have saved and invested it instead. The decision to forego this in favor of borrowing money to purchase this home represents the opportunity cost of this mortgage.

Plugging our loan information and a 7% rate of return on our hypothetical investments into our cost-of-debt calculator (enter monthly payment as $477.415299 to avoid rounding errors), we discover that if we had invested the money that we are otherwise losing to mortgage interest on a monthly basis, it would have grown to a total of $307,405.37 after 30 years:

Cost of Debt Calculator results displaying the total opportunity cost of debt incurred in the form of a $100,000 mortgage.

This means that the cost of our mortgage in this scenario is far, far higher than the original $100,000 price tag considered by most. First, we have the $71,869.51 we’ve already signed away in the name of mortgage interest, a 71.9% increase over the original $100,000 loan.

Then we have the $307,405.37 we would have accumulated in our investment account thanks to contributions and compound interest growth over 30 years had we not signed that $71,869.51 away. This triples the original mortgage amount, a cost increase of 307.4%.

Adding the two together we find that the total opportunity cost of the mortgage is a mind-numbing $379,274.88, a 379.3% increase over the original $100,000 balance. This number represents the total cost of purchasing something which we could not afford outright and had to purchase on credit.

The Piercing Talons Of Debt Opportunity Cost

Projecting interest owed AND calculating the opportunity cost of that interest are essential to understanding the true cost of purchasing something on credit and falling into the clutches of debt.

In the above example, the couple considering the $100,000 mortgage may very well have made a different decision had they known the true price tag would amount to $479,274.88.

Informing your purchasing decisions with this information has the potential to dramatically change your long-term financial picture. Just one large-scale purchase avoided or modified can have an impact of hundreds of thousands of dollars.

Identify The Hidden Costs Of Your Own Debt

Using what you’ve learned in this article and the below compound interest calculators, see if you can pick out the hidden opportunity costs of your own debt:

  • Compound Interest Calculator – Use to calculate the growth of a starting balance over time at a given interest rate (select “View Report” for year-by-year details).
  • Debt Payment Principal & Interest Calculator – Use to calculate what portion of a debt payment is applied to principal vs. interest based on loan balance and interest rate.
  • Credit Card Payoff Calculator – Use to view credit card payoff timeline, total interest incurred, and principal vs. interest portions of payments.
  • Mortgage Loan Amortization Schedule Calculator – Use to calculate the total interest owed over a given mortgage term, as well as the impact of making additional payments (select “show amortization schedule” for year-by-year details).
  • Cost of Debt Calculator – Use to calculate the opportunity cost of debt payments in the form of the interest you could have otherwise earned with the money you are losing to interest payments on debt (scroll down to see payment-by-payment details).

Extra Bonus – Did you know that owls have three different eyelids? Or that the world’s smallest owl is only 5-6 inches tall? Brush up on your owl trivia here and here to impress your family and friends with both your financial and feathered wisdom.

Up Next – Compound Interest, Part 5: Benjamin Franklin’s 200-Year Experiment

Join The Conversation!

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I don’t know that the true cost of the mortgage includes the full opportunity cost of investing in the markets because you hade an appreciating asset (hopefully). You’d have take some of that appreciation off of the opportunity cost.

In any case, it is a good exercise for people to understand the total cost of the mortgage over its lifetime. That is in the closing documents but by that time it is too late.

I’d add that there is not only opportunity cost of missed investments but missed adventures, more time with family, more time for self development. Debt really takes a lot away from us and it’s best to rid yourself of it ASAP.


I would say the biggest flaw in this argument (albeit a good one, thank you), is you are neglecting to account for the individual “needing” that item to begin with. So if we run around saving all of our money all the time we could become quite rich but would not have clothes on our back or a roof over our head.

This argument is sound with frivolous purchases, but one may argue at some point you will want SOME kind of TV, and live SOMEwhere. So regardless of the cost and interest, through that period if you didn’t buy that house, you could be renting and paying more than the cost of the house plus interest. That gives you another opportunity cost consideration, on whether you rent or take a loan out for a house.

But that wouldn’t make your point, and your point was a good one.

Thanks! Great Article!

Chris @ Duke of Dollars

Awesome explanation my friend.

I just signed up and look forward to the future posts in the series. I love the concept of being able to put that money to work, instead of putting it into a mortgage.

Personally, I’d include the opportunity cost of the mortgage, compare that to the opportunity cost of renting, and then see the difference in the years after the mortgage is paid off.

I haven’t done any calculations, but I believe that once the mortgage is paid off, the opportunity cost will change a bit because you now increased your net worth, you now have no rent for the rest of your life and that can go to investments. It is a bit hard to just compare only a mortgage without thinking of the cost of renting because we all need a place to live :).

Wonderful concept and one I plan to add to the toolbox when comparing things in the future – thank you!


Great post. You did a nice job of breaking down the true cost of a mortgage and lost opportunity. It costs money to live in your own house or if you rent. Most people would not be able to ever buy a house if not for a mortgage. I don’t see any issue with taking out a mortgage if you truly want to own a house. Just take out a 15 year mortgage instead of a 30 and buy a smaller house than you can afford.

Dr. Cory S. Fawcett

Great concept. You could even extend the opportunity cost figures on to retirement, because if you invested the money, you would likely keep it invested after the debt was paid.

In my book The Doctors Guide to Eliminating Debt, I did something similar. I compared the cost of paying the house off in 30 years vs 7 years. The extra interest means a doctor would have to see another 17,000 patients to pay for it. That is another opportunity cost.

Dr. Cory S. Fawcett
Prescription for Financial Success


If I wasn’t paying my $1200 mortgage id be paying $1900 in rent for a similar house so it saves us. We have zero debt other then our house. We’ve had out house for 1 year this month and have paid off 4 years of the mortgage no way am I handing them 150k in interest! Most people say don’t pay off the house early instead invest in the stock market but we are pretty safe and paying off our house is a garanteed return with a sense of accomplishment at the end.


Im trying to figure out what to do, save up money for 3 years and pay for a house in full or invest some of that money during that 3 years and have less money to put down on the house, but I will start earning compounding interest on my investment. What do you think?