Understanding how to calculate compound interest in the UK is one of the most powerful steps you can take toward building long-term wealth. Whether you’re saving for a house deposit, growing an ISA, planning for retirement, or investing monthly, compound interest can dramatically increase your money over time. In this guide, we’ll explain what compound interest is , how it’s calculated in the UK , and how you can easily work it out using a free UK compound interest calculator . What Is Compound Interest? Compound interest is interest calculated not only on your original savings (the principal) but also on the interest that accumulates over time. In simple terms: You earn interest on your interest. This is what makes compound interest so powerful — the longer your money is invested, the faster it can grow. For example: Simple interest earns interest only on the original amount. Compound interest earns interest on the original amount plus previous interest. How Is C...