Flat Rate to APR Converter

What is a flat rate?

A flat rate charges interest on the full amount you borrowed for the whole term, even as you pay the balance down. That makes it look cheaper than it is — the real cost is the APR.

With a reducing-balance rate, interest is charged only on what you still owe, so it falls as you repay. A flat rate ignores that and charges the same interest every month, which is why the APR ends up far higher than the flat rate quoted.

Flat rates are easy to work out — that is their appeal. Multiply the amount by the rate by the years and you have the total interest. But that simplicity hides the true cost, because the calculation pretends you still owe every pound you borrowed right up to the final month. In reality, the balance you owe falls steadily from the first payment.

The APR reverses that pretence. It expresses the deal as an annual percentage of the reducing balance, the way a real loan behaves, and it folds in any compulsory fees. That is why the APR is the only number UK law lets lenders use to advertise the cost, and the only number you should use to compare.

Flat rate vs reducing balance — the core difference

The core difference is what the interest is charged on: a flat rate always charges on the original amount, while reducing balance charges only on what you still owe. Same deal, two very different numbers.

Picture a £15,000 loan over four years. Under a reducing-balance deal, the balance you owe falls every month as you repay, so the interest charged each month falls with it. By the final year you owe only a few thousand pounds, so the interest on those last payments is small. The annual cost — the APR — reflects that falling curve.

A flat rate behaves as if none of that repayment happens. It charges the same interest in month 48 as in month 1, because it keeps applying the rate to the full original £15,000. You are paying interest on money you have already handed back to the lender. The flat percentage looks tidy, but the true annual cost is nearly double it on a typical term.

Why a flat rate looks cheaper than it is

A flat rate looks cheaper because the number is smaller, but you pay interest on money you have already repaid. A 5% flat rate works out close to a 9.6% APR.

The trap is psychological. A dealer quoting a tidy 5% sounds far cheaper than a rival quoting 9.6% APR, yet both can be the identical deal. The first figure is simply the flat rate dressed up to look small. Until you convert it, you are comparing a smaller number with a more honest one and reaching the wrong conclusion.

This is exactly why UK consumer-credit law insists on the APR. The flat rate is allowed in conversation, but the figure that has to appear in any advertisement, quote or agreement is the representative APR. Treat the flat rate as decoration; treat the APR as the price.

Flat rate vs APR compared

The flat rate and the APR describe the same deal, but only the APR shows the true cost. A flat rate is roughly half the APR for a typical term.

The total interest is the same — what differs is which number you compare on. By law, UK lenders must quote a representative APR, so always compare deals on that. Run any monthly quote through the APR calculator to check.

Notice the row that matters most: what each rate is charged on. That single difference — the original amount versus the falling balance — is the entire reason the APR runs nearly twice the flat rate. Once you see it, the flat rate stops fooling you.

The rule of thumb: flat rate to APR

As a rule of thumb, the APR is roughly double the flat rate on a three-to-four-year term — multiply the flat rate by about 1.9 to 2.1. Use it for a quick sanity check, then convert properly.

The multiplier is not exactly 2, and it shifts with the term. A longer term pushes the APR a touch higher relative to the flat rate, because the balance falls more slowly and you carry it for longer. A shorter term pulls them slightly closer. The table above assumes a 48-month term, the most common for car finance.

Treat the rule of thumb as a smoke alarm, not a measurement. If a dealer quotes a flat rate that converts to an APR wildly different from rivals, something is off. Run the real numbers through the APR calculator before you trust any figure.

What is a representative APR?

A representative APR is the annual cost at least 51% of accepted applicants actually receive — and it includes compulsory fees, under FCA rules. It is the honest, comparable price of a finance deal.

The 51% rule matters. A representative 9.6% APR means the lender expects most accepted customers to get that rate or a cheaper one, but up to 49% may be offered a higher rate based on their credit. The rate you personally get depends on your credit file, income and outgoings — the representative figure is the floor for comparison, not a promise to you.

Because the representative APR folds in compulsory fees — arrangement fees, document fees and the like — it is the only number that captures the true cost in one figure. A flat rate never does, and neither does the bare monthly payment. That is why comparing on APR is not a preference; it is the only fair way to line two deals up.

Is it legal to quote a flat rate?

Yes, a dealer can mention a flat rate, but the figure that must appear in any advertisement, written quote or agreement is the representative APR. The flat rate alone never satisfies the legal requirement.

The Consumer Credit Act 1974 and the FCA's Consumer Credit sourcebook (CONC) set the rules. Advertisements for regulated consumer credit must display a representative APR prominently, and pre-contract information must show the APR alongside the rate of interest. A verbal flat rate at the desk is not a substitute for the APR on the paperwork.

In practice, some salespeople still lead with the flat rate because it sounds lower and closes deals. There is nothing illegal about saying it, but you should never compare on it. Always ask for the APR in the quote, confirm it on the pre-contract document, and only then decide. If a dealer resists putting the APR in writing, that is a warning sign.

A worked example, month by month

Month by month, the balance you owe falls — but a flat rate keeps charging interest on the original amount, which is why the APR climbs to nearly double. The gap between the two is the true cost the flat rate hides.

Take the same £15,000 over 48 months at a 5% flat rate. The total flat interest is fixed at £3,000, spread as £62.50 a month regardless of the balance. In month 1, when you still owe close to £15,000, that £62.50 is reasonable. But by month 36 you might owe only around £4,000, and you are still being charged £62.50 — as if you still owed the full £15,000.

A reducing-balance deal at the equivalent 9.6% APR charges interest only on the remaining balance. The charge shrinks every month alongside the debt, so by the final year you pay only a few pounds of interest a month. Both deals cost £3,000 in total over the term — but the APR honestly annualises that cost against the money you actually had outstanding, and it comes out near 9.6%.

This is the whole point of the converter above. Put in the flat rate and it rebuilds the monthly payment, then solves for the APR that produces that same monthly on a reducing balance. What looks like a tidy 5% turns out to be a true 9.6% annual cost — and that is the number you weigh against rival quotes.

How to compare two finance deals fairly

To compare two deals fairly, convert any flat rate to APR, check the total amount payable, add in the fees, and line them up over the same term. Skip any step and the comparison breaks.

Once two deals sit side by side on APR, total payable and term, the cheaper one is obvious. The same logic protects you against a 0% deal that quietly swaps the discount for a higher list price — see the 0% finance deals page for the test. For the mechanics of turning a monthly payment back into an APR, use the APR calculator.

1. Convert any flat rate to APR first, using the converter above or the rule of thumb. Never compare a flat rate with an APR directly.
2. Check the total amount payable, not just the monthly — a longer term lowers the monthly but raises the total interest.
3. Add in compulsory fees, which the representative APR already includes; do not let a low rate with a hidden fee slip past.
4. Compare over the same term, so you are weighing like for like — a 9.6% APR over 36 months is not the same deal as 9.6% over 60 months.
5. Confirm your personal APR, since up to 49% of accepted applicants may pay more than the representative rate.

Always compare car finance on APR

Compare every car finance deal on its representative APR, never the flat rate or the monthly. It is the only number that lets you weigh deals fairly.

• A low flat rate can hide a high APR — always convert it first.
• A low monthly can mean a long term and more total interest — check the total amount payable.
• The representative APR includes compulsory fees, so it captures the true cost in one figure.