What Is a Fixed Rate Cash ISA
What Is a Fixed Rate Cash ISA
If you have a lump sum you won't need for a while and want to shelter the interest from tax, a fixed rate Cash ISA could be the right choice. The interest rate is agreed when you open the account and stays the same for the whole term, so you know from the outset exactly what your savings will earn.
This guide walks through what a fixed rate Cash ISA is, how it works in practice, how it differs from a variable rate Cash ISA, and what to weigh up before opening one.
What Is a Fixed Rate Cash ISA?
A fixed rate Cash ISA is a tax-free savings account that pays a guaranteed rate of interest for a set period. The period is called the fixed term, and it typically runs anywhere from one to five years.
The rate is agreed upfront. It doesn't shift during the term, regardless of what happens to rates elsewhere in the market. The trade-off is that access to your money is limited while the account is running. Most providers either restrict withdrawals or apply a charge if you take money out before the term ends.
Like every Cash ISA, the interest is exempt from income tax, which can make a meaningful difference over a longer term.
These accounts tend to appeal to savers who want predictable returns on a lump sum they can afford to leave alone.
How Does a Fixed Rate Cash ISA Work?
Fixed rate Cash ISAs are typically designed for lump sum savings. You deposit the money when you open the account, agree to leave it in place for the term, and receive interest at the rate set at the start.
The Fixed Term
The fixed term is how long your money is locked in. A shorter term, such as one year, ties up your savings for less time and gives you access to your money sooner. Longer terms of up to five years keep the money out of reach for longer, which may suit savers planning further ahead.
Interest on a Fixed Rate Cash ISA
Interest is worked out on the balance in the account at the rate agreed at the start. Some accounts pay interest annually, some monthly, and some at the end of the term.
You'll see AER (Annual Equivalent Rate) quoted alongside each product, which lets you compare accounts on the same footing.
Because the rate doesn't change, working out what you'll earn over the term is straightforward, provided the money stays in the account.
Fixed vs Variable Cash ISAs
Cash ISAs generally fall into one of two categories: fixed rate or variable rate.
With a fixed rate Cash ISA, the rate is locked in for the full term. If market rates fall during that period, your rate stays the same. If market rates rise, you will not benefit from the increase until the term ends.
A variable rate Cash ISA works differently. The rate can move up or down while you hold the account, and providers usually give notice of changes.
Variable rate products often come with easier access to the money, which can make them a sensible choice for an emergency fund or savings you may need at short notice.
What to Consider Before Opening a Fixed Rate Cash ISA
Access to Your Money
Fixed rate Cash ISAs are designed to be left alone. Taking money out early usually means closing the account or paying an early access charge, often calculated as a set number of days' interest. Always check the specific terms of the product before applying, particularly if there's any chance you will need the money sooner than planned.
Deposit Windows
Most fixed rate Cash ISAs only let you pay money in during a short window when the account is first opened. Once that window closes, no further deposits are allowed until the account matures. If you're planning to add to your savings month by month, this isn't the right account type for that.
Your ISA Allowance
The annual ISA allowance for the 2026/27 tax year is £20,000. This figure covers all contributions across every ISA you hold, not just one account. Since the rule change in April 2024, savers can pay into more than one Cash ISA in the same tax year, as long as the total does not exceed the allowance.
The government has also announced plans to cap Cash ISA contributions at £12,000 a year for savers under 65 from April 2027. The overall £20,000 allowance would still apply across all ISA types, and those aged 65 and over would keep the full £20,000 Cash ISA allowance.
Fixed Rate Cash ISAs at Monmouthshire Building Society
At Monmouthshire Building Society, we offer a range of fixed rate Cash ISAs to suit different savings goals.
Our 1 Year Fixed Rate Cash ISA is a shorter-term option for savers who want a guaranteed rate without tying their money up for several years. For customers who prefer to manage their account digitally, our App Exclusive 1 Year Fixed Rate Cash ISA offers the same principle, opened and managed through the MonBS app. Savers looking for a longer commitment can consider our 2 Year Fixed Rate ISA, which locks in a rate for a two-year term.
Each account has its own summary box and terms and conditions, which set out the current rate, minimum deposit, access rules, and any charges for early withdrawal. It is worth reading these carefully before applying so you know exactly what you are agreeing to.
Summary
A fixed rate Cash ISA is a tax-free savings account that pays a guaranteed rate of interest for a set term, usually one to five years. In exchange for a predictable return, access to the money is limited while the account is running. It is an option worth considering for savers with a lump sum they can put aside for a defined period, and who value knowing exactly what their savings will earn.
To find out more about the fixed rate Cash ISAs available at Monmouthshire Building Society, visit our ISA page or explore our full range of savings accounts. You can also learn more about us and how we work as a mutual building society.