Percentages in Finance Explained
Personal finance is built on percentages. Interest rates, mortgage deals, ISA returns, pension contributions, inflation, and tax all use percentage figures that directly affect your money. This guide explains each one in plain English, with worked examples to show how the maths works in practice.
Interest rates and APR
When a lender advertises a loan or credit card, they will typically quote two different rates: the interest rate and the Annual Percentage Rate (APR). These are not the same thing, and the difference matters when you are comparing products.
The stated interest rate (sometimes called the nominal rate) is simply the rate applied to the outstanding balance to calculate interest charges. The APR is a broader figure: it includes the interest rate plus any mandatory fees and charges expressed as an annual percentage of the total amount borrowed. By law, lenders in the UK must quote APR, which means you can use it to compare products on a like-for-like basis.
Worked example: A personal loan is advertised at 6.9% APR over 3 years on a £5,000 borrowing.
At 6.9% APR, the approximate monthly repayment on a £5,000 loan over 36 months is around £154.
Total repaid: approximately £5,544
Total interest cost: approximately £544
When comparing loans, always use the APR rather than the headline interest rate. A loan with a 5.9% rate but substantial arrangement fees may cost more overall than one with a 6.5% rate and no fees. The APR captures this difference.
Mortgage rates as percentages
Even a small difference in mortgage rate has a significant cash impact because of the large sums involved and the long repayment terms. This is one area where understanding percentages directly affects household finances.
Worked example: A £200,000 repayment mortgage over 25 years.
At 4.5% annual interest rate: approximately £1,111 per month
At 5.5% annual interest rate: approximately £1,228 per month
The 1% rate difference costs around £117 per month, or over £1,400 per year.
Fixed-rate mortgages lock in your interest rate percentage for a set period, typically two or five years, giving certainty over monthly payments. Variable-rate mortgages (including trackers and standard variable rates) move with the Bank of England base rate or the lender's own rate, so your monthly payment can go up or down.
Your loan-to-value ratio (LTV) is the mortgage amount expressed as a percentage of the property value. It is one of the key factors that determines what interest rate you are offered. A lower LTV means less risk for the lender, which typically means a better rate for you.
LTV calculation
(Mortgage amount ÷ Property value) × 100
Example: £160,000 mortgage on a £200,000 property = 80% LTV. Putting down a larger deposit to move from 90% LTV to 75% LTV can unlock a noticeably cheaper rate.
ISA returns and compound interest
When interest is compounded, you earn interest not just on your original deposit but on all the interest that has already accumulated. Over time, this creates a snowball effect that significantly increases returns compared to simple interest.
Worked example: £10,000 in a Cash ISA at 4.5% interest per year, compounded annually, for 5 years.
Year 1: £10,000 × 1.045 = £10,450
Year 2: £10,450 × 1.045 = £10,920.25
Year 3: £10,920.25 × 1.045 = £11,411.66
Year 4: £11,411.66 × 1.045 = £11,925.19
Year 5: £11,925.19 × 1.045 = £12,462.02
Total growth: £2,462. Simple interest at 4.5% for 5 years would give only £2,250.
The rule of 72 is a useful mental shortcut for estimating how long it takes money to double at a given interest rate. Divide 72 by the annual interest rate percentage, and the result is the approximate number of years to double your money.
Rule of 72
At 4.5% interest: 72 ÷ 4.5 = 16 years to double your money
At 6% interest: 72 ÷ 6 = 12 years to double your money
At 9% interest: 72 ÷ 9 = 8 years to double your money
See how compound interest builds over longer periods with the compound interest worksheet.
Inflation and what it means for your money
Inflation measures how much the general price level is rising, expressed as an annual percentage. In the UK, inflation is most commonly measured by the Consumer Prices Index (CPI) and the Retail Prices Index (RPI). CPI is the official target measure used by the Bank of England; RPI tends to run slightly higher and is still used in some wage agreements and index-linked products.
Worked example: Inflation is 3% and your savings account pays 2% interest.
Your savings grow in nominal terms (the number goes up), but in real terms you are losing purchasing power.
Real return = Interest rate - Inflation rate
Real return = 2% - 3% = -1%
In other words, your money buys 1% less each year even though the balance is growing.
To calculate a real return more precisely, use the Fisher equation: divide (1 + nominal rate) by (1 + inflation rate) and subtract 1. For most everyday purposes, subtracting inflation from the nominal rate gives a close enough estimate.
The UK experienced a sharp inflation spike following the pandemic and the 2022 energy crisis. CPI peaked at over 11% in October 2022, a level not seen since the early 1980s. The Bank of England's target rate is 2%. Inflation returned closer to target by 2024, but the period highlighted how quickly high inflation can erode the real value of cash savings.
Pension contribution percentages
Workplace pension contributions are expressed as percentages of qualifying earnings. Under auto-enrolment rules, the minimum contribution is 8% of qualifying earnings in total: at least 3% from the employer and at least 5% from the employee (including tax relief). Qualifying earnings are your earnings between a lower and upper threshold set by the government each tax year.
Worked example: Starting salary of £28,000, total contribution of 8%, invested for 35 years with 5% annual growth.
Annual contribution: £28,000 × 8% = £2,240 per year
Monthly contribution: approximately £187
Using a compound growth formula at 5% per year for 35 years, this grows to approximately £190,000, assuming contributions remain constant in cash terms.
If salary increases over time and contributions grow with it, the eventual pot would be substantially larger.
The earlier you start and the higher your contribution percentage, the greater the benefit from compounding. Increasing from 8% to 10% total contributions early in a career can make a meaningful difference to the eventual pension pot, because those extra contributions have the longest time to grow.
Many employers will match additional voluntary contributions up to a set percentage, which is essentially free additional pension saving. If your employer offers matched contributions, contributing enough to maximise that match is generally the most efficient use of pension contributions before considering other options.
National Insurance and income tax
Both income tax and National Insurance (NI) in the UK use a banded, marginal rate system. This means higher rates only apply to the portion of income that falls within each band, not to all of your income.
Income tax rates (England, Wales and Northern Ireland)
A common misunderstanding is that moving into the 40% tax band means all income is taxed at 40%. It does not. Only the income above the higher rate threshold is taxed at 40%. The income below the threshold continues to be taxed at the lower rates.
National Insurance for employed earners uses a similar banded structure, with different thresholds and rates. Class 1 NI contributions are payable on earnings above the primary threshold.
For current income tax and NI rates, see the HMRC income tax rates page. Rates and thresholds change each tax year and may differ for Scottish taxpayers.