To calculate total interest, you multiply the principal by the interest rate and the time period for simple interest, or use a compound interest formula when interest builds on itself over time. The right formula depends on what type of debt or investment you’re working with: a savings account, a fixed-rate loan, or a credit card each calculate interest differently. Here’s how to handle each one.
Simple Interest Formula
Simple interest is the most straightforward calculation. You earn (or owe) interest only on the original principal, not on any interest that has already accumulated. The formula is:
Total Interest = Principal × Rate × Time
If you lend a friend $5,000 at 6% annual interest for 3 years, the total interest is $5,000 × 0.06 × 3 = $900. That’s it. The interest amount is the same each year ($300), and you’d receive $5,900 back at the end.
Simple interest shows up in some personal loans, short-term notes, and certain auto loans. It’s also useful as a quick mental estimate even when the actual calculation is more complex.
Compound Interest Formula
Compound interest is interest calculated on both the original principal and all previously accumulated interest. Over time, this snowball effect makes a significant difference. The formula is:
Total Interest = Principal × (1 + r/n)^(n×t) − Principal
In that formula, r is the annual interest rate as a decimal, n is how many times per year interest compounds, and t is the number of years. The expression (1 + r/n)^(n×t) gives you the future value of your money; subtracting the original principal leaves you with just the interest portion.
Say you deposit $10,000 in a savings account paying 5% interest, compounded monthly, for 10 years. Plug in the numbers: $10,000 × (1 + 0.05/12)^(12×10) = $16,470.09. Subtract your original $10,000, and you’ve earned $6,470.09 in total interest. With simple interest, the same deposit would have earned only $5,000.
How Compounding Frequency Changes the Result
The more frequently interest compounds, the more total interest you earn, though the gains get smaller as frequency increases. Consider $1,000,000 invested at 20% for one year:
- Annual compounding (once per year): $1,200,000, or $200,000 in interest
- Monthly compounding (12 times per year): $1,219,391, or $219,391 in interest
- Daily compounding (365 times per year): $1,221,336, or $221,336 in interest
Moving from annual to monthly compounding added about $19,000 in interest, but moving from monthly to daily added only about $2,000 more. In practice, the jump from annual to monthly compounding matters most. Beyond that, the differences shrink quickly.
The Rule of 72 for Quick Estimates
If you just want to know how long it takes your money to double, divide 72 by the annual interest rate. At 6% interest, your investment doubles in roughly 72 ÷ 6 = 12 years. At 9%, it doubles in about 8 years. This won’t give you a precise total interest figure, but it’s a handy shortcut for sizing up an investment or a debt.
Total Interest on an Amortized Loan
Mortgages, auto loans, and most student loans use amortization, meaning you make equal monthly payments that cover both principal and interest. Early payments are mostly interest; later payments are mostly principal. To find total interest on these loans, you need the monthly payment formula first.
The monthly payment formula is:
Monthly Payment = Loan Amount × [i × (1 + i)^n] / [(1 + i)^n − 1]
Here, i is the monthly interest rate (your annual rate divided by 12) and n is the total number of monthly payments. Once you have the monthly payment, the total interest is simple arithmetic:
Total Interest = (Monthly Payment × Number of Payments) − Original Loan Amount
For example, take a $250,000 mortgage at 6.5% annual interest over 30 years. The monthly rate is 0.065 / 12 = 0.005417. The number of payments is 360. Running the formula gives a monthly payment of about $1,580. Multiply that by 360 payments and you get $568,800 in total payments. Subtract the $250,000 you borrowed, and you’ll pay roughly $318,800 in total interest over the life of the loan.
This is why even small rate differences matter on large loans. That same $250,000 mortgage at 5.5% instead of 6.5% would save you more than $60,000 in total interest.
Total Interest on a Credit Card
Credit cards calculate interest differently from loans because your balance changes throughout the month as you make purchases and payments. Most issuers use the average daily balance method.
The process works in three steps:
- Find your daily periodic rate: Divide your card’s APR by 365. A 20% APR gives a daily rate of about 0.0548% (0.20 / 365 = 0.000548).
- Calculate your average daily balance: Add up your balance for each day of the billing cycle, then divide by the number of days. If you carried $1,000 for the first 10 days and $1,100 for the remaining 20 days of a 30-day cycle, that’s ($1,000 × 10 + $1,100 × 20) / 30 = $1,066.67.
- Multiply: Average daily balance × daily periodic rate × number of days in the billing cycle. Using the numbers above: $1,066.67 × 0.000548 × 30 = $17.54 in interest for that month.
To estimate total interest on credit card debt over many months, you’d repeat this calculation for each billing cycle. But because your balance fluctuates with payments and new charges, there’s no single clean formula that covers the entire payoff period the way there is for a fixed loan. Online credit card payoff calculators handle this iteration for you.
Using a Spreadsheet to Calculate Total Interest
If you want a precise answer without doing the math by hand, spreadsheet programs like Excel and Google Sheets have a built-in function called CUMIPMT that calculates cumulative interest paid between any two payment periods on a fixed-rate loan.
The syntax is:
=CUMIPMT(rate, nper, pv, start_period, end_period, type)
For a $250,000 mortgage at 6.5% over 30 years, you’d enter:
=CUMIPMT(0.065/12, 360, 250000, 1, 360, 0)
That tells the function to use a monthly rate of 6.5%/12, over 360 payments, on a $250,000 loan, from payment 1 through payment 360, with payments at the end of each period (which is standard). The result is the total interest paid over the full loan. The function returns a negative number because it represents money going out; just ignore the minus sign.
The real power of CUMIPMT is flexibility. Want to know how much interest you’ll pay in just the first 5 years? Change the end_period to 60. Curious how much interest remains if you’re already 10 years in? Set start_period to 121 and end_period to 360. This makes it easy to see how extra payments or refinancing at different points would affect your total cost.
Make sure your rate and payment periods use the same units. For monthly payments, divide the annual rate by 12 and express the loan term in months. Mixing annual rates with monthly periods will produce wildly wrong results.
Choosing the Right Method
The method you use depends entirely on the type of interest you’re dealing with. For a one-time loan between individuals or a short-term note, simple interest works. For savings accounts, CDs, and investment growth projections, use the compound interest formula and match the compounding frequency to what your bank actually uses (most savings accounts compound daily). For mortgages, car loans, and student loans with fixed monthly payments, the amortized loan approach or CUMIPMT function will give you an accurate total. For credit cards, the average daily balance method reflects how issuers actually charge you each month.
In every case, the core idea is the same: total interest equals what you pay (or receive) minus the original principal. The formulas just account for the different ways interest gets applied over time.