Calculator models
Compounding uses a nominal annual rate divided by twelve, monthly contributions at month-end, and a fixed rate. The savings solver uses the same model to find a nonnegative monthly contribution. The debt tool accrues APR divided by twelve before each payment, includes a final partial payment, and rejects payments that cannot reduce the balance.
Taxes, inflation, changing rates, fees, and new borrowing are excluded. Actual credit cards may use daily balances. Inputs are bounded to avoid meaningless or impractical illustrations. Reference-value and edge-case tests check the arithmetic.
The car total cost tool uses the standard installment-loan payment formula (or an even split at 0% APR), counts only payments made while you own the car, adds the remaining loan balance at sale, adds constant yearly running costs, and subtracts a resale value you estimate. Sales tax is applied to the full price. Default values are example assumptions, not market data.
Borrowing, business and cost calculators
These eight tools share one rule: you enter the terms, in one currency you choose. No provider data, live rates or offers are built in, and currency selection never converts amounts. Reference-value tests check each formula against independently computed results.
- Loan cost comparison: installment payment P × r / (1 − (1 + r)^−n) plus monthly fees. A deducted fee lowers the payout; an added fee raises the amount borrowed. Total cost = total repaid − money received. The monthly rate i is the internal rate of return (bisection) of payout and payments; nominal APR = 12 × i, effective annual rate = (1 + i)^12 − 1.
- Debt consolidation check: each current debt accrues APR ÷ 12 monthly and is repaid with its payment until zero. The consolidation loan is an installment loan on the combined balance (plus a financed fee). Saving = current interest − new interest − fee; negative savings are shown as an extra cost with a warning.
- Factor rate to APR: payback = amount × factor, split into equal payments (5 per week for business-day payments). The periodic IRR of the money received after fees and the payments gives nominal APR (× 260 or × 52) and the effective annual rate. Invoice factoring: fee = invoice × fee % × started 30-day periods; annualised cost = (1 + fee ÷ advance)^(365 ÷ days) − 1.
- Ad-spend card return: net = min(spend × reward rate, cap) − foreign-currency spend × FX fee − annual fee. Break-even spend = annual fee ÷ (reward rate − foreign share × FX fee), unless the cap is reached first.
- Crypto purchase cost: payment fee on the amount paid, trading fee on the order, spread as a markup over the market price (value = filled ÷ (1 + spread)), flat withdrawal fee. Recurring mode multiplies per-buy costs by buys per year and adds withdrawals.
- Broker cost: 12 × plan fee + orders × (flat + order size × %) + traded amount × foreign share × FX fee + custody (% and flat) + fund ongoing charges on the entered portfolio value, without growth.
- Lease or buy: monthly cash flows over the lease term, discounted at (1 + annual rate)^(1/12) − 1. Buying credits the estimated residual value and charges any loan balance at the end. The optional tax effect is simplified (lease payments deductible; straight-line depreciation plus loan interest when buying); tax rules vary by country.
- Balance transfer planner: the fee is added to the balance; promotional rate ÷ 12 applies during the promotion, the follow-up rate afterwards. Payment needed = installment formula over the promotional months. Deferred-interest promotions are not modelled.
All eight are illustrations, not advice or offers. They assume month-end payments and constant terms; real contracts can use daily interest, different dates and additional charges.
Comparison criteria
Budget tools are compared by documented workflow, platform, and practical questions. We list selected examples, not the whole market. We have not tested support quality, bank-feed reliability, or overall usability. Current prices are not maintained in this edition.
Rankings and commercial relationships
No numeric scores, “best overall” winners, user reviews, or active affiliate offers appear in the preview. Any future ranking needs a documented scoring model, evidence, coverage limits, and disclosures.