Every formula, assumption, and primary data source behind all five free calculators — explained clearly so you can trust the numbers before you rely on them.
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.
This calculator tells you your exact payoff date and total interest cost given your balance, APR, and monthly payment. It uses standard credit card amortization — the same method your card issuer uses to apply interest each billing cycle.
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.
This calculator shows both payoff strategies side by side — total interest paid, total time to debt-free, and your first payoff date under each method. It uses the same amortization formula as the credit card calculator, applied across multiple debts simultaneously.
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.
This calculator shows how much time and total interest you save by adding an extra amount to your regular loan payment each month. It applies standard loan amortization and compares your original payoff schedule against the accelerated schedule.
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.
This calculator compares your current debt payoff trajectory against a proposed consolidation loan — showing total interest paid, monthly payment, and payoff date under each scenario. The goal is to surface the full-cost comparison, not just the rate comparison.
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).
This calculator models federal student loan repayment under different payment scenarios — showing payoff date, total interest, and the impact of extra payments. It does not model income-driven repayment forgiveness (which depends on income, family size, and plan type) but accurately models principal-and-interest payoff math for any fixed monthly payment.
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.
Every statistic cited across Debt Clarity Tools articles and calculator context panels comes from primary government or regulatory sources. We do not cite affiliate-linked financial publications or estimated industry data.
| Source | What We Use It For | Update Frequency |
|---|---|---|
|
Federal Reserve G.19 Consumer Credit Report federalreserve.gov/releases/g19 |
Average credit card interest rates (APR) for all US cardholders | Monthly |
|
Federal Reserve Bank of New York — Consumer Credit Panel newyorkfed.org/microeconomics/hhdc |
Total US credit card debt outstanding ($1.21T+); household debt trends | 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 interest rates by year; repayment plan terms | Annual |
|
Federal Reserve Bank of New York — Student Loan Data newyorkfed.org |
Total US student loan debt outstanding ($1.77T+); 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 |
Dr. James Frederick Smiling holds a PhD in STEM-Mathematics Education from NC State University and teaches statistics and quantitative literacy at the college level. 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.
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.