Financial Calculators UK: Compound Interest, Mortgage and Betting

Three tools, one design rule: each shows the working rather than only the answer. A calculator that returns a single number teaches you nothing about which input moved it, and the input that moved it is usually the decision.

• The tools

• Compound interest calculator

  Monthly contributions and a rate, resolved into a balance for every year, with your own money separated from the interest.

• Mortgage overpayment calculator

  What a regular overpayment takes off the interest and off the term, with the contractual payment derived rather than assumed.

• Matched betting calculator

  Lay stake, exchange liability and the qualifying loss, with separate handling for a free bet whose stake is not returned.

Why these three and not thirty

Most calculator collections are padding. A currency converter, a tip splitter and six variations on a percentage are not tools, they are page count. The three here survive because each answers a question where the arithmetic is genuinely hard to hold in your head and the answer changes a real decision.

Compound interest is unintuitive because the curve bends late, so people either give up too early or overestimate what a short run does. Mortgage overpayment is unintuitive because the saving depends on when in the term you do it, not just how much. And a back-and-lay calculation is arithmetic nobody does by hand correctly under time pressure, which is exactly when it gets done.

Everything runs in your browser

No figure you type leaves the page. There is no form submission, nothing is stored between visits, and the calculators work with the page open and the network gone. That is a deliberate choice rather than a technical limitation: a mortgage balance and an income are the two numbers people are least willing to hand over, and there is no reason a tool like this needs a server.

It also means the arithmetic is inspectable. If a result looks wrong, the working is on the page and the formula is standard. You can check it against your own spreadsheet and the two should agree to the penny.

What these calculators deliberately do not do

None of them fetch a live rate. No interest rate, mortgage rate, betting price or exchange commission is supplied by this site, because a static page cannot keep a live figure honest and a stale rate is worse than an empty field. Every rate input is yours to fill from the account, offer or market you are actually looking at.

None of them model tax, and none of them model inflation. Both are real and both depend on circumstances a calculator cannot see. Where the omission changes the interpretation of a result: a nominal balance over twenty-five years, a gross rate entered where an AER belongs: the individual page says so rather than leaving you to find out.

And none of them give advice. A calculator tells you what the arithmetic does. Whether overpaying a mortgage beats saving, or whether an offer is worth the afternoon, depends on facts about you that are not fields on a form.

Reading a result properly

Change one input at a time and watch which figure moves. That is the whole method, and it is more informative than any single run. If doubling the monthly contribution moves the total but not the shape of the curve, the rate is doing the work; if a small rate change swings the total hard, the term is doing the work.

Then check the result against something. For compound interest, a bank's own projection. For a mortgage, the statement. For a matched bet, the two account balances after settlement. A tool that agrees with reality is useful; a tool nobody ever reconciles is decoration.

Where a calculator is the wrong tool

Three situations, and recognising them saves time. The first is anything with a decision point partway through: a rate that changes at year five, a mortgage you intend to move in three years, a savings plan you will stop contributing to when a child starts school. A single-run calculator cannot represent a branch, and stacking two runs together only works if you keep track of what you carried forward.

The second is anything where the tax treatment is the whole question. Whether a pension contribution beats an ISA contribution, or whether a rental property is worth holding through a company, turns almost entirely on allowances, thresholds and your other income. The arithmetic is the easy part and it is not the part that decides.

The third is anything with a person on the other side of it. Retention pricing, a lender's willingness to reduce a term rather than a payment, whether a bookmaker restricts an account. None of these are computable, and treating them as though they were is how a confident plan meets an uncooperative counterparty. Where that applies, the individual page says what to ask for rather than pretending a number settles it.

Frequently asked questions