Every Debt Clarity Tools calculator runs standard amortization in your own browser, with no sign-up. A $10,000 credit card balance at 23.79% APR accrues $198.25 of interest in its first month alone.
That single number is why balances feel frozen. Below is every formula, every assumption, and every primary source behind all eight calculators, written out so you can check the arithmetic yourself.
Last updated · Reviewed by Dr. James Frederick Smiling, PhD in Mathematics Education
"Most debt advice fails because it skips the arithmetic. People are told to feel differently about money when what they actually need is to see the number. A payoff date is not a motivational idea. It is the solution to an equation, and anyone can be shown how it is solved."
Amortization is the schedule that decides how much of each payment kills interest and how much kills the balance. Everything on this site is built on it.
Four rules, applied to all eight calculators: run locally in your browser, use standard financial math, state every assumption out loud, and have the formulas reviewed by a mathematics education professor.
All calculations happen locally. Your balance, rate, and payment data never leave your device or touch our servers.
Every formula matches industry-standard amortization and compound interest methods used by banks and the CFPB.
We document every assumption explicitly, so you know where the calculator ends and where your judgment begins.
All formulas reviewed by Dr. James Frederick Smiling, PhD in STEM-Mathematics Education from NC State University.
It applies interest to your balance first, subtracts the rest of your payment from principal, then repeats that cycle month by month until the balance hits zero.
You give it three inputs: balance, APR, and the fixed monthly payment you can actually make. It returns your payoff month and the total interest you will pay to get there. This is the same method your issuer uses to build your statement.
The calculator iterates this cycle month by month until the balance reaches zero. Total interest is the sum of all monthly interest charges across the full payoff period.
On $6,000 at 23.79% APR, the minimum payment takes 252 months and costs $10,787 in interest. A flat $300 a month takes 26 months and costs $1,718.
Here is the same balance run through the credit card calculator at five payment levels. The minimum row uses the common US formula of accrued interest plus 1% of the balance, with a $25 floor. Every other row is a fixed payment held constant until payoff.
| Monthly payment | Months to zero | Total interest | Total paid |
|---|---|---|---|
| Minimum only | 252 (21 years) | $10,787 | $16,787 |
| $150 | 81 (6 yr 9 mo) | $6,035 | $12,035 |
| $200 | 47 (3 yr 11 mo) | $3,202 | $9,202 |
| $250 | 33 (2 yr 9 mo) | $2,225 | $8,225 |
| $300 | 26 (2 yr 2 mo) | $1,718 | $7,718 |
Computed with the amortization loop described above. Round to the nearest dollar; your issuer may differ by a few dollars because most cards accrue interest daily rather than monthly.
The difference in interest between paying the minimum on that $6,000 card and paying a flat $300 a month. Same balance. Same rate. The only variable that changed was the number you type into the payment box.
This is the part that catches careful people. Somebody who pays on time every single month for eleven straight years can still be looking at a five-figure interest bill, because on-time and paid-down are two different things. The minimum payment is engineered to keep the account current, not to end it.
Run your own balance in the free Credit Card Payoff calculator
It runs your full debt list twice, once sorted by smallest balance and once by highest APR, then reports total interest and payoff date for each ordering.
Both runs use the same amortization loop as the credit card calculator, applied across every account at once, so the only thing that changes between them is which debt receives your extra payment.
Snowball: Debts are sorted by current balance, smallest first. Minimum payments are applied to all accounts each month. Any extra payment (beyond combined minimums) is directed entirely to the smallest balance until it's paid off, then rolled to the next smallest.
Avalanche: Debts are sorted by APR, highest first. The same rollover mechanic applies, extra payment above combined minimums targets the highest-rate balance until eliminated, then cascades down.
It builds two amortization schedules for the same loan, one at your required payment and one with your extra amount added, then subtracts the totals.
The difference between those two schedules is your interest saved and your months saved. Nothing is estimated or rounded up front; both schedules are run to the last penny.
The extra payment reduces principal immediately, which reduces the interest accrued in every future month. The compounding effect of early principal reduction is why early extra payments save significantly more than the same dollars paid later.
It prices your current accounts to their payoff dates, prices the proposed loan to its payoff date, and shows both total costs side by side.
That full-cost view matters because a consolidation loan can carry a lower rate and still cost more overall if the term is long enough. Rate alone does not answer the question.
Your current trajectory uses the same multi-debt amortization as the Snowball/Avalanche calculator, each balance continues at its current rate and minimum payment. The consolidation scenario models a single new loan using the standard amortization formula (same as the Extra Payment calculator).
Federal loans accrue daily, so the planner multiplies your principal by the annual rate over 365, then scales that to a 30.44 day average month.
It models payoff date, total interest, and the effect of extra payments for any fixed monthly amount. It does not model income-driven forgiveness, which depends on your income, family size, and plan type, so those outcomes are outside what arithmetic alone can tell you.
The capitalization model reflects how federal loan servicers treat interest during deferment, forbearance, and income-driven plans where payments don't cover accruing interest. On a standard repayment plan with adequate payments, capitalization does not occur.
It is a hybrid of the snowball and the avalanche. Debts are first grouped into balance bands (the default band is $2,500). The calculator attacks the lowest band first, and inside each band it pays the highest APR first. If two debts tie on both, the smaller balance goes first.
Every month it charges interest on each debt, pays every minimum, and sends your extra amount plus any freed-up minimums from paid-off debts to the current target. It runs the snowball and the avalanche on the same debts at the same time, so you see all three payoff dates and interest totals side by side.
It computes your scheduled principal and interest payment with the standard amortization formula, then runs two month-by-month schedules: one at the scheduled payment and one with your extra amount added. The difference is your interest saved and months cut.
On a $300,000 mortgage at 6.8% over 30 years, the scheduled payment is $1,955.78 and month one is $1,700.00 of interest. Adding $200 a month pays the loan off in 275 months instead of 360 and cuts total interest from $404,079 to $292,827, a saving of about $111,252.
Most car loans are simple-interest loans, so the calculator charges interest on the remaining balance each month and applies the rest of the payment to principal. It uses the same amortization engine as the mortgage calculator, run on auto-loan terms.
On a $30,000 loan at 9% APR for 60 months, the payment is $622.75 and month one is $225.00 of interest. Paying $100 extra a month finishes in 50 months instead of 60 and saves about $1,289 in interest.
You enter one balance, its APR, and your minimum payment. The tool shows how much of that payment goes to interest this month, how much reaches the balance, and how long payoff takes if you keep paying exactly that amount. It then reruns the same balance with an extra amount added so you can see the interest saved.
Example: $5,000 at 23.79% APR accrues $99.12 of interest in month one. Holding the payment at a fixed $150 clears it in 56 months with about $3,262 in interest.
Primary government and regulatory releases only: Federal Reserve G.19, the New York Fed Consumer Credit Panel, the CFPB, and Federal Student Aid. No affiliate-linked publications.
Total US credit card balances as of the second quarter of 2026, per the Federal Reserve Bank of New York Household Debt and Credit Report released August 11, 2026. Student loan balances stood at $1.65 trillion and total household debt at $18.8 trillion in the same release.
Two rate figures matter and they are not the same number. The Federal Reserve's G.19 release put the average rate on credit card accounts actually assessed interest at 22.15% for the second quarter of 2026, up from 21.52% in the first quarter, while the average across all accounts was 20.94%. Our worked examples use 23.79% APR, which tracks the higher advertised rate on new card offers, because that is closer to what somebody opening a card today is quoted. Whatever your own statement says, type that in instead.
| Source | What We Use It For | Update Frequency |
|---|---|---|
|
Federal Reserve G.19 Consumer Credit Report federalreserve.gov/releases/g19 |
Average credit card APR. Q2 2026: 22.15% on accounts assessed interest, 20.94% across all accounts. | Monthly |
|
Federal Reserve Bank of New York, Consumer Credit Panel newyorkfed.org/microeconomics/hhdc |
Total US credit card debt outstanding ($1.26 trillion, Q2 2026); household debt trends ($18.8 trillion) | Quarterly |
|
Consumer Financial Protection Bureau (CFPB), Consumer Credit Card Market Report consumerfinance.gov |
Minimum payment behavior, interest cost to consumers, issuer profit data | Biennial |
|
Federal Student Aid, StudentAid.gov studentaid.gov |
Federal student loan rates by year. Undergraduate Direct Loans first disbursed July 1, 2026 to June 30, 2027: 6.52% fixed. | Annual |
|
Federal Reserve Bank of New York, Student Loan Data newyorkfed.org |
Total US student loan debt outstanding ($1.65 trillion, Q2 2026); borrower counts | Quarterly |
|
IRS Publication 970, Tax Benefits for Education irs.gov/publications/p970 |
Student loan interest deduction rules referenced in student loan planner context | Annual |
|
Freddie Mac Primary Mortgage Market Survey (PMMS) freddiemac.com/pmms |
Typical mortgage interest rate ranges used in extra payment examples | Weekly |
Standard amortization. The monthly payment on a fixed-rate loan is P × i / (1 - (1 + i)-n), where P is the principal, i is the annual rate divided by 12, and n is the number of monthly payments. Credit card payoff runs the same arithmetic month by month, applying interest first and the remainder to principal.
No. Every calculation runs locally in your browser using JavaScript. Your balances, rates, and payment amounts are never transmitted to Debt Clarity Tools or to any third party, and nothing is stored after you close the tab.
No. All eight calculators are free and open with no account, no email, and no paywall. There is no gated result screen.
Credit card examples use 23.79% APR, which tracks the average advertised rate on new card offers. The Federal Reserve G.19 release for the second quarter of 2026 puts the average on accounts actually assessed interest at 22.15% and the all-account average at 20.94%. Student loan examples use the fixed rate Federal Student Aid publishes for the year the loan was disbursed, currently 6.52% for undergraduate Direct Loans first disbursed between July 1, 2026 and June 30, 2027. Your own rate should always be entered directly.
They are exact for the inputs you give, and they assume every payment lands on schedule at a fixed rate. Our calculators accrue interest monthly at APR divided by 12; most card issuers accrue daily, which typically shifts total interest by a few dollars on a $6,000 balance. Promotional rates that expire, fees, and missed payments will move the date further. Treat the output as the arithmetic ceiling on your progress, not a guarantee.
Primary government and regulatory sources only: the Federal Reserve G.19 Consumer Credit Report, the Federal Reserve Bank of New York Consumer Credit Panel, the Consumer Financial Protection Bureau, and Federal Student Aid. The New York Fed put US credit card balances at $1.26 trillion and student loan balances at $1.65 trillion in its second quarter 2026 report. The full table is in the sources section above. We do not cite affiliate-linked financial publications.
Dr. James Frederick Smiling, who holds a PhD in Mathematics Education and teaches statistics and financial literacy at the college level, wrote and reviews every formula on this page. He is an educator, not a CPA, CFP, attorney, or licensed credit counselor, and nothing here is personalized financial advice.
Dr. James Frederick Smiling, who holds a PhD in STEM-Mathematics Education from NC State University and teaches college statistics and financial literacy, wrote and reviews every formula here.
He built Debt Clarity Tools because most free debt calculators either lack transparency about their math or are embedded in sites with financial products to sell. He is an educator, not a CPA, CFP, attorney, or licensed credit counselor.
The methodology described on this page reflects standard financial mathematics as taught in undergraduate finance and applied mathematics curricula. All formulas have been verified against amortization methods published by the CFPB and the Federal Reserve's consumer finance division.