Free Online Finance Calculators
Run the numbers on loans, mortgages and pay — privately.
Money calculations are the last thing you want sitting on a third party's server. SwiftTooly's finance tools compute amortization schedules, compound growth, take-home pay and sales tax locally in your browser. You can plug in salaries, debts and interest rates with confidence that the numbers go no further than your own machine.
5 Finance
Why use SwiftTooly's Finance
Sensitive by default
Salary, debt and interest figures never leave your browser. No account, no profile, no leak.
Standard formulas
Amortization, compound interest and tax math follow the same equations your bank uses.
Transparent breakdowns
See the principal vs interest split, monthly vs total cost, and effective rates.
Globally usable
Work in any currency — formatting follows the number; we never assume a country.
Finance in your browser vs uploaded to a server
SwiftTooly (browser)
Loan and interest math runs as a small JavaScript loop in the page. Your inputs and the schedule never touch a server.
Typical upload-to-server tools
Many “free” finance calculators are lead-gen funnels — your salary, debt, and email feed into a marketing database before you ever see a result.
Which formulas do these finance tools use?
Almost everything in this group reduces to one of two pieces of arithmetic. Amortisation governs the loan and mortgage calculators: a fixed payment is derived that clears a balance over a set number of periods while interest accrues each period on whatever balance remains. That structure is what produces the familiar pattern where early payments are mostly interest and the principal portion grows over time. Compound growth governs the compound interest calculator: a balance multiplied repeatedly by one plus the periodic rate, so returns are earned on previous returns. The two are mirror images — one is compounding working against you, the other compounding working for you — and both share the property that time in the exponent matters far more than people expect. That single fact explains why extending a loan term raises total interest disproportionately, and why starting to invest early beats investing more later.
Comparing options on the right number
The most consequential habit in personal finance arithmetic is choosing which number to compare. Loans compared on monthly payment alone will always favour the longest term, because a longer term always lowers the payment — and always increases the total interest. A £20,000 loan at 7 percent over ten years costs more than twice the interest of the same loan over five, while the monthly payment falls by only around a third. Total interest over the life of the loan is the figure that says what borrowing actually costs. The same principle applies elsewhere. Job offers should be compared as annual gross-equivalent including employer pension contributions, insurance and leave entitlement, not on headline salary. Investment projections should use a real after-fee rate rather than a nominal one, because inflation and a one percent annual charge both compound against you over decades. Discounts should be compared as effective percentages, since stacked offers and multi-buys are structured to look larger than they are.
What is deliberately not here
These are arithmetic tools and they are not financial advice, which is a distinction worth stating plainly rather than burying. They model the terms you enter and nothing else. Real borrowing carries arrangement fees, early repayment charges, insurance and variable rates that a fixed-rate model cannot represent, which is why the annual percentage rate exists as a comparison figure and why a lender's quote will differ from any calculation here. Nothing on these pages accounts for tax, which varies by jurisdiction and circumstance and materially changes both borrowing costs and investment outcomes. Nothing here knows your situation, your risk tolerance, or what else is happening in your finances. For a decision that matters — a mortgage, a pension, a significant investment — arithmetic is the starting point and a qualified adviser is the next step. As with every tool on the site, the figures you enter stay in your browser and are never transmitted. That last point matters more here than almost anywhere else on the site. Income, debt, savings balances and property values are among the most sensitive categories of personal data there are, and they are exactly what a finance calculator asks for. Running the arithmetic locally means there is no record of what you modelled, no profile being assembled, and nothing that could be breached or sold.
Frequently asked questions
Are these calculators a substitute for financial advice?
- No. They produce mathematically correct results for the formulas they use, but real-world loans, taxes and salaries depend on local rules. Use the numbers as a starting point, not as professional advice.
Which compounding frequencies are supported?
- The Compound Interest Calculator supports annual, semi-annual, quarterly, monthly and daily compounding, plus optional recurring contributions.
Does the mortgage calculator include taxes and insurance?
- It focuses on principal and interest. Property tax, insurance and PMI vary by jurisdiction — add them as separate line items if you need a full PITI estimate.
Will my salary or loan numbers be logged?
- Never. The calculation happens in your browser and we don't attach the input to any analytics event.