Simple Interest Calculator UK 2025/26
Calculate simple interest on loans, savings and investments instantly. Enter your principal amount, interest rate and time period to see exactly how much interest you will earn or pay. Compare simple interest with compound interest side by side.
Calculation Breakdown
Simple vs Compound Interest Comparison
Simple Interest Quick Reference
| Principal | Rate | Time | Interest | Total |
|---|---|---|---|---|
| £1,000 | 3% | 1 year | £30.00 | £1,030.00 |
| £5,000 | 4% | 2 years | £400.00 | £5,400.00 |
| £10,000 | 5% | 3 years | £1,500.00 | £11,500.00 |
| £20,000 | 3.9% | 4 years | £3,120.00 | £23,120.00 |
| £50,000 | 4.5% | 5 years | £11,250.00 | £61,250.00 |
| £100,000 | 5% | 10 years | £50,000.00 | £150,000.00 |
Simple vs Compound Interest: £10,000 at 5%
| Years | Simple Interest Total | Compound Interest Total | Difference |
|---|---|---|---|
| 1 | £10,500 | £10,500 | £0 |
| 2 | £11,000 | £11,025 | £25 |
| 3 | £11,500 | £11,576 | £76 |
| 5 | £12,500 | £12,763 | £263 |
| 10 | £15,000 | £16,289 | £1,289 |
| 20 | £20,000 | £26,533 | £6,533 |
| 30 | £25,000 | £43,219 | £18,219 |
Simple Interest Formula
The simple interest formula is one of the most fundamental equations in finance:
I = P × R × T
Where:
- I = Interest earned or charged (in pounds)
- P = Principal — the original amount borrowed, saved or invested
- R = Annual interest rate expressed as a decimal (divide the percentage by 100, so 5% becomes 0.05)
- T = Time period expressed in years (if you have months, divide by 12; if days, divide by 365)
To calculate the total amount including both the principal and the interest, use:
A = P + I = P(1 + R × T)
For example, if you invest £10,000 at 5% for 3 years:
- I = £10,000 × 0.05 × 3 = £1,500
- A = £10,000 + £1,500 = £11,500
The beauty of simple interest is its predictability. The interest amount is exactly the same every year — in this case, £500 per year for each of the three years. There is no compounding effect, so each year's interest is calculated solely on the original £10,000.
Simple vs Compound Interest
Understanding the difference between simple and compound interest is crucial for making informed financial decisions. Here is a clear comparison of the two methods:
Simple interest calculates interest only on the original principal. The interest earned each period stays constant. If you deposit £10,000 at 5% simple interest, you earn exactly £500 every year regardless of how many years the money remains invested.
Compound interest calculates interest on both the principal and all previously earned interest. Each period, the interest base grows because last period's interest is added to it. The same £10,000 at 5% compound interest earns £500 in year one, but £525 in year two (5% of £10,500), £551.25 in year three (5% of £11,025), and so on.
Over 10 years, the difference becomes substantial:
| Metric | Simple Interest | Compound Interest |
|---|---|---|
| Starting amount | £10,000 | £10,000 |
| Interest rate | 5% per year | 5% per year |
| Time period | 10 years | 10 years |
| Total interest earned | £5,000 | £6,289 |
| Final amount | £15,000 | £16,289 |
| Difference | £1,289 more with compound interest | |
The key takeaway: for short-term periods of a year or less, simple and compound interest produce very similar results. Over longer periods, compound interest generates significantly more returns for savers — or significantly more cost for borrowers.
When Is Simple Interest Used?
While compound interest is more common in modern finance, simple interest is still used in several important contexts:
Products That Use Simple Interest
- Personal loans (some): Certain personal loans, particularly those from credit unions and community lenders, use simple interest. Monthly payments reduce the principal first, so the interest charge decreases over time.
- Car finance (flat rate): Many UK car finance agreements quote a "flat rate" of interest, which is essentially simple interest calculated on the full original loan amount for the entire term. A 3.9% flat rate on £20,000 over 4 years means £20,000 × 0.039 × 4 = £3,120 total interest. Note that the equivalent APR is roughly double the flat rate because you are repaying principal monthly.
- Treasury bills and government bonds: UK Treasury bills (T-bills) and certain government bonds pay simple interest. The interest is calculated on the face value and does not compound during the holding period.
- Short-term loans: Bridging loans, payday loans (when calculated daily), and trade finance often use simple interest because the loan terms are short enough that compounding makes little practical difference.
- Overdrafts (some): Certain bank overdrafts calculate interest on the outstanding balance each day using a simple interest method, though most now use an equivalent annual rate (EAR).
Products That Do NOT Use Simple Interest
- Mortgages: UK mortgages use compound interest (typically calculated daily or monthly). The interest compounds on the outstanding balance.
- Savings accounts: Almost all UK savings accounts pay compound interest, usually calculated daily and paid monthly or annually.
- Credit cards: Credit card interest compounds daily on the outstanding balance, which is why credit card debt can grow so quickly.
- ISAs and investment accounts: Returns in ISAs and investment accounts compound automatically as interest, dividends and capital gains are reinvested.
Step-by-Step Calculation
Follow these clear steps to calculate simple interest for any scenario. We will work through three complete examples.
Example 1: £5,000 Loan at 5% for 3 Years
Step 1: Identify the values: P = £5,000, R = 5% = 0.05, T = 3 years
Step 2: Apply the formula: I = P × R × T
Step 3: Calculate: I = £5,000 × 0.05 × 3 = £750
Step 4: Total repayment: A = £5,000 + £750 = £5,750
Monthly interest: £750 ÷ 36 months = £20.83 per month
Daily interest: £750 ÷ 1,095 days = £0.68 per day
Example 2: £10,000 Savings Bond at 4.5% for 2 Years
Step 1: P = £10,000, R = 4.5% = 0.045, T = 2 years
Step 2: I = £10,000 × 0.045 × 2 = £900
Step 3: Total at maturity: A = £10,000 + £900 = £10,900
Monthly interest: £900 ÷ 24 months = £37.50 per month
Comparison: With compound interest (annually), you would earn £920.25 — just £20.25 more over 2 years.
Example 3: £20,000 Car Finance at 3.9% Flat Rate for 4 Years
Step 1: P = £20,000, R = 3.9% = 0.039, T = 4 years
Step 2: I = £20,000 × 0.039 × 4 = £3,120
Step 3: Total repayment: A = £20,000 + £3,120 = £23,120
Step 4: Monthly payment: £23,120 ÷ 48 months = £481.67
Important: The flat rate of 3.9% translates to an APR of approximately 7.5% because you are repaying the loan monthly but interest is charged on the original full amount for the entire term. Always compare the APR, not the flat rate, when shopping for car finance.
Converting Between APR and Flat Rate
When dealing with car finance and some personal loans, you will encounter two different ways interest rates are quoted: the flat rate and the APR (Annual Percentage Rate). Understanding the difference is essential to avoid overpaying.
What is a Flat Rate?
A flat rate calculates interest on the full original loan amount for the entire term, even though you are making monthly repayments that reduce the outstanding balance. This is simple interest applied to the initial principal. The flat rate is always lower than the equivalent APR because it does not account for the reducing balance.
What is APR?
APR (Annual Percentage Rate) is the true cost of borrowing. It accounts for the fact that you are repaying principal each month, so the amount you actually owe decreases over time. Because interest on a flat-rate loan is charged on money you have already repaid, the APR is roughly 1.8 to 2 times the flat rate.
Approximate Conversion
A commonly used approximation is:
APR ≈ Flat Rate × 1.85 to 2.0
For more precise conversion, the factor depends on the loan term:
| Loan Term | Flat Rate | Approx. APR | Multiplier |
|---|---|---|---|
| 1 year | 3% | 5.4% | ×1.80 |
| 2 years | 3% | 5.6% | ×1.87 |
| 3 years | 3% | 5.7% | ×1.90 |
| 4 years | 3.9% | 7.5% | ×1.92 |
| 5 years | 3.9% | 7.6% | ×1.95 |
Example: A car dealer offers 2.9% flat rate over 3 years. The approximate APR is 2.9% × 1.9 = 5.5% APR. Always compare the APR between different finance offers, as a lower flat rate from one lender can actually cost more than a higher flat rate from another if the terms differ.
How to Use This Calculator
Our Simple Interest Calculator gives you instant, accurate results for any combination of principal, rate and time. Here is how to get the most from it:
Step 1: Choose Your Calculation Type
Select what you want to calculate from the dropdown. Interest Earned / Total Amount is the default and most common — it tells you how much interest accrues on a given principal. Rate Required calculates the interest rate needed to earn a specific amount of interest. Time Required calculates how long it takes to earn a target amount of interest.
Step 2: Enter Your Values
Type in the principal amount in pounds sterling, the annual interest rate as a percentage (for example, 5 for 5%), and the time period. You can select years, months or days from the dropdown — the calculator automatically converts everything to years for the formula.
Step 3: View Results and Comparison
Results update automatically as you type. The top row shows interest earned, total amount, daily interest and monthly interest. Below that, a detailed breakdown shows each step of the calculation. The comparison section shows what the same principal, rate and time would produce with compound interest, so you can see the difference at a glance. If the time period exceeds one year, a year-by-year table appears showing how the balance grows each year.
Frequently Asked Questions
Simple interest is a method of calculating interest where the charge is based only on the original principal amount. Unlike compound interest, which charges interest on both the principal and accumulated interest, simple interest remains constant each period. If you invest £10,000 at 5% simple interest, you earn exactly £500 every year — the base never changes. The formula is I = P × R × T, where P is the principal, R is the annual rate as a decimal, and T is the time in years.
The simple interest formula is I = P × R × T. I is the interest amount in pounds, P is the principal (the original sum), R is the annual interest rate expressed as a decimal (divide the percentage by 100), and T is the time in years. To find the total amount including principal and interest, use A = P + I, or equivalently A = P(1 + RT). For example, £8,000 at 4% for 5 years: I = 8,000 × 0.04 × 5 = £1,600, and the total amount is £9,600.
Simple interest is calculated only on the original principal and stays the same each period. Compound interest is calculated on the principal plus all previously accumulated interest, so the interest amount grows each period. Over 10 years, £10,000 at 5% earns £5,000 with simple interest but £6,289 with compound interest — a difference of £1,289. The longer the time period, the bigger the gap. For a single year, they produce identical results.
Simple interest is used for certain personal loans (especially from credit unions), car finance quoted at a "flat rate", Treasury bills and government bonds, short-term bridging loans, and some overdraft facilities. It is NOT used for mortgages, standard savings accounts, credit cards, or ISAs — all of which use compound interest. Car finance flat rates are the most common everyday example of simple interest in the UK.
Simple interest is better for borrowers and worse for savers. Borrowers pay less total interest with simple interest because the interest does not compound on itself — you only ever pay interest on the original amount borrowed. Savers, on the other hand, earn more with compound interest because their interest earns further interest. This is why savings accounts use compound interest and some loans use simple interest. A £10,000 loan at 5% for 10 years costs £5,000 in simple interest versus £6,289 with compound — saving the borrower £1,289.
A flat rate (simple interest) on car finance is always lower than the true APR because it does not account for your monthly repayments reducing the balance. As a rough guide, multiply the flat rate by 1.85 to 2.0 to get the approximate APR. For example, a 3.9% flat rate over 4 years is approximately 7.5% APR. The exact conversion depends on the loan term and repayment frequency. UK regulations require all lenders to display the APR, so always compare APR figures rather than flat rates when choosing between finance offers.
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