Cumulative Interest Calculator
Total the interest on a loan between any two payments (for example the first year of a mortgage), or the interest a savings account earns with monthly deposits and daily, monthly or yearly compounding. Download the full month-by-month schedule as a spreadsheet.
Cumulative interest calculator
How cumulative interest is calculated
On a loan, each month's interest is the balance × (APR ÷ 12). The fixed payment covers that interest first and the rest reduces the balance, so interest is heaviest at the start. Cumulative interest is the sum of those monthly interest amounts over the payments you pick, the same result as Excel's CUMIPMT function. A $300,000, 30-year mortgage at 6.5% costs $1,896.20 a month; about $19,400 of the first year's $22,754 in payments is interest.
On savings, interest is added at the compounding frequency and then earns interest itself. The calculator converts the APR to an equivalent monthly rate, adds deposits at the end of each month, and keeps a running total of the interest credited.
How often interest compounds
More frequent compounding earns slightly more at the same APR. At 4.25%, monthly compounding gives an APY of 4.334%; daily gives 4.341%; yearly gives 4.25%. Banks must show the APY on savings accounts and CDs under the Truth in Savings Act, so compare APYs, not APRs.
The compounding formula
Future value of a lump sum: FV = P × (1 + r/k)k×t, where r is the APR, k the compoundings per year and t the years. With monthly deposits D the deposits add D × ((1 + i)n − 1) ÷ i for monthly rate i over n months. Cumulative interest = ending balance − everything you deposited.
Saving with interest: what moves the total most
- Time: $300 a month at 4.25% earns about $8,760 of interest in 10 years but about $69,900 in 25 years.
- Rate: the rate matters more the longer you save; on short horizons, deposit size matters more.
- Taxes: savings interest is taxable each year unless it's inside an IRA, 401(k) or HSA.
Common questions
What is cumulative interest?
The total interest paid (on a loan) or earned (on savings) over a period, added up month by month.
How do I calculate cumulative interest on a loan?
Add the interest part of each payment in the range. Each month's interest is the remaining balance × APR ÷ 12. Excel's CUMIPMT does the same.
How much interest will I pay in the first year of a mortgage?
On $300,000 at 6.5% for 30 years, about $19,400 of the first 12 payments ($22,754) is interest.
How much interest does $10,000 earn in 10 years?
At 4.25% compounded monthly with no deposits, about $5,280, for a balance of about $15,280.
Is daily compounding better than monthly?
Slightly. At 4.25% the APY is 4.341% daily vs 4.334% monthly, a few dollars a year on $10,000.
Sources
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General information, not financial, tax or legal advice. Check important numbers with the official source before acting on them.