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How our calculators work

This page explains how each SmartCashLab calculator does its math, what it assumes, and what it leaves out, so you can judge how closely a result fits your own loan or card. It also explains how we write our guides and handle corrections.

What all our calculators have in common

  • Whole cents. Money is tracked in whole cents. Each period’s interest is rounded to the nearest cent, with halves rounded away from zero.
  • A clean final payment. The last payment is reduced to exactly the remaining balance plus that period’s interest, so the balance ends at zero and never goes negative.
  • Fixed rates and on-time payments. Rates stay the same for the period they apply to, and every payment is made on time and in full.
  • In your browser. The math runs on your device. The worked examples in our guides are produced by the same calculator code, using hypothetical inputs.

Biweekly mortgage calculator

  • Monthly schedule. A standard fixed-rate amortization. The monthly rate is the annual rate divided by 12, and the level payment comes from the standard amortization formula, rounded to the nearest cent. At a 0% rate, the payment is the loan divided by the number of months, rounded up to the cent.
  • True biweekly (the default). The biweekly payment is half of the monthly principal and interest payment, rounded to the cent, paid every 14 days, for 26 payments a year. Interest is charged each period at the annual rate divided by 26, and each payment is applied as soon as it is received. A real year has slightly more than 26 two-week periods; the calculator uses exactly 26, a common convention.
  • One extra payment a year (optional mode). The loan stays on the monthly schedule, and one additional full monthly payment goes to principal with every 12th payment.
  • Time saved. Counted as whole calendar months between the monthly and biweekly payoff dates.
  • Scope. Principal and interest only. It does not include property taxes, homeowners insurance, mortgage insurance, escrow, or any program or enrollment fee.

Car loan payoff calculator with extra payments

  • Loan type. A fixed-rate, simple-interest, fully amortizing loan with monthly payments.
  • Interest. Each month, interest is the balance times the APR divided by 12, rounded to the cent.
  • Regular payment. The standard amortized payment, rounded to the nearest cent. At 0% APR, the loan divided by the number of months, rounded up to the cent.
  • Extra payment. Added to every monthly payment, starting with the first, and applied entirely to principal.
  • Months saved. The number of payments without the extra minus the number with it.
  • Scope. No fees, prepayment penalties, late fees, add-on products, or taxes. It does not model precomputed-interest loans or daily interest accrual.

Balance transfer calculator

  • Staying on your current card. Each month, interest of the balance times your current APR divided by 12, rounded to the cent, is added, and then your monthly payment is applied.
  • Transferring. The transfer fee, the balance times the fee percentage rounded to the cent, is added to the balance up front. No interest is charged during the promo months. After that, interest is the remaining balance times the post-promo APR divided by 12.
  • Same payment in both. The same fixed monthly payment is used in both scenarios, made at the end of each month, with no minimum-payment rules.
  • Net savings. Interest on the current card minus interest on the new card minus the fee.
  • Breakeven month. The first month when the fee plus interest on the new card, to date, is no more than the interest you would have paid on the current card, to date. If the running total falls behind again later, the calculator says so.
  • Verdict. Net savings under one dollar either way are shown as about even.
  • Assumptions. No new purchases, no late or missed payments, no annual fee, no rate changes, and the transfer completes immediately. It models a 0% intro APR, not deferred interest.

Debt snowball vs avalanche calculator

  • Interest. Each month, every open debt is charged its balance times its APR divided by 12, rounded to the cent, before payments are applied.
  • One budget. The same total monthly budget is paid every month. It must cover every minimum payment; the calculator will not run a plan below that.
  • Order. Snowball pays the smallest balance first (ties: higher APR, then the order you entered). Avalanche pays the highest APR first (ties: smaller balance, then the order you entered).
  • Rolling payments forward. Each month, every minimum is paid, and the rest of the budget goes to the current target. When a debt is paid off, its payment rolls into the next target, and money left over in a payoff month goes to the next debt in the same month.
  • Lump sum. An optional one-time extra payment is added to the first month.
  • Minimums only. The comparison baseline pays each debt’s own minimum until it is paid off, without reusing freed-up payments.
  • Assumptions. Fixed rates and minimums, no new charges, no fees, and on-time payments. Plans are capped at 50 years.

What the calculators do not cover

The calculators do not include taxes, escrow, insurance, or fees other than the balance transfer fee. They do not model variable or adjustable rates, rate changes, late or missed payments, penalty rates, or a lender’s own rules for applying payments. Lenders, servicers, and card issuers may calculate interest differently, for example daily instead of monthly, so your actual figures can differ. Results are estimates, not a quote or an offer, and they say nothing about whether you would be approved for any product.

How we write our guides

  • Examples come from the calculators. Every worked example uses hypothetical inputs and is computed with the same code as the calculator it links to. Before a guide is published, its numbers are checked against that code.
  • No rate claims. We do not publish live rate data, market averages, or statistics. Rates in our examples are labeled as hypothetical.
  • No promises. We do not promise savings, approval, or any outcome. Guides explain how the math works so you can make your own decision.
  • Not advice. Our guides are educational. They are not financial, tax, or legal advice.

Affiliate independence

Some SmartCashLab pages may include affiliate links, and if you use one we may earn a commission at no extra cost to you. Compensation never changes the math: the calculators give the same result whether or not a product pays us, and no affiliate partner reviews or approves our calculators or guides. Read our affiliate disclosure for details.

Corrections

If you think a calculation, example, or explanation is wrong, please contact us. Tell us the page, the numbers you entered, and what you expected. We will review it and correct the page if needed.