Nobody's name is on this site. That is a deliberate choice, and it costs something: on a page about money, a named author you can look up is one of the cheapest reasons to trust a number.
So the arithmetic has to carry it instead. Every calculator here states its formula, names the authority it follows, and prints the assumptions it made next to the figure they produced. This page collects all of that in one place and then works a full example through by hand, so you can check us rather than take our word for it.
The rules every calculator here follows
1. Cents are rounded as they are posted
Interest is rounded to whole cents at the moment it is applied, and any payment derived from a balance is rounded the same way. That matches what a bank or an issuer actually posts to an account, and it is why the rows in a schedule add up to the total shown above them. Carrying full precision and only rounding for display produces a table whose column does not sum to its own total, which is a page contradicting itself in front of anyone checking.
2. Interest is applied before the payment
On every borrowing calculator, each period adds interest to the balance and then subtracts the payment. That is the order every US issuer and servicer uses, and it is why paying a balance in full still incurs one final period of interest.
3. Contributions land at the start, withdrawals at the end
On savings calculators, money you add is treated as arriving at the start of the period, so it earns that period's interest. Money you withdraw is treated as leaving at the end, after interest. Both are the conservative reading of the reader's position, and most calculators do not say which convention they use.
4. A calculator that cannot answer says so
Where the inputs do not produce a real answer, the page states the reason instead of returning a number. A payment that does not cover the interest means the debt never clears, and saying "never" is more use than a very large number of years. A pot whose interest covers the withdrawal never runs out, and that is a different answer from a long one.
5. Nothing here is advice
No page on this site recommends an option, ranks two choices, or tells you what you should do. Where there are two reasonable courses, both are computed and both are shown. What suits you depends on things a calculator cannot see.
A worked example you can check
This is the credit card payoff calculator, run by hand. Take a balance of $8,400 at 22.9% APR, paying $250 a month.
The monthly rate is the APR divided by twelve:
22.9 / 100 / 12 = 0.0190833…, or 1.90833% a month.
Month one:
| Step | Working | Result |
|---|---|---|
| Opening balance | $8,400.00 | |
| Interest | 8,400.00 × 0.0190833 | $160.30 |
| Balance with interest | 8,400.00 + 160.30 | $8,560.30 |
| Payment | $250.00 | |
| To principal | 250.00 − 160.30 | $89.70 |
| Closing balance | 8,560.30 − 250.00 | $8,310.30 |
Note that of a $250 payment, only $89.70 reduced the debt. The rest was the cost of carrying it for a month.
Repeating that until the balance reaches zero gives:
- 55 payments, the last one partial and smaller than $250.
- $5,155.94 in total interest.
- $13,555.94 paid altogether.
Those are the figures the calculator shows for those inputs. If you work it through and get something different, we would rather know: tell us.
Where each formula comes from
| Area | Authority followed |
|---|---|
| Credit card interest and minimum payments | Consumer Financial Protection Bureau guidance; Regulation Z, 12 CFR 1026; the minimum payment disclosure required by the Credit CARD Act of 2009 |
| Mortgage amortisation, recast, biweekly, ARM caps | The standard amortisation formula; ARM disclosure rules at 12 CFR 1026.20 |
| Private mortgage insurance thresholds | Homeowners Protection Act of 1998, 12 USC 4901 and following |
| HELOC draw and repayment structure | Regulation Z, 12 CFR 1026.40 |
| Reverse mortgage principal limits | HUD Single-Family Housing Policy Handbook 4000.1, and the HECM rules at 24 CFR Part 206 |
| APY, CDs, money market accounts | Truth in Savings Act; Regulation DD, 12 CFR 1030, Appendix A |
| 529 plans | Section 529 of the Internal Revenue Code |
What we deliberately leave out
Every mortgage figure on this site is principal and interest only. Property tax, hazard insurance, HOA dues and escrow are real money and they are not principal and interest. Folding them in silently produces a "payment" that matches nothing on any document you hold, so we keep them out and say so on each page.
We also do not model tax on savings interest, inflation, or investment returns that vary year to year. Each page lists its own omissions in a section headed "what this does not account for", and that section is written to be read rather than to be a disclaimer.
When we get something wrong
We will. The formulas here are tested against worked examples computed independently before the code was written, and the test suite runs on every build, but tests only catch what someone thought to check.
If a figure here disagrees with your lender, your bank, or your own arithmetic, say so. A specific disagreement with the numbers is the most useful thing anyone can send us, and it is worth more than any credential we could have put at the top of this page.