A rate that is directly comparable
Annual equivalent rate (AER) represents the annual effect of the rate including compounding. For an unchanged full-year rate with interest reinvested, a £10,000 deposit at 4.50% AER earns an estimated £450.
Do not compound the AER twice
An effective monthly rate can be derived as (1 + AER ÷ 100) raised to 1/12, minus 1. Dividing AER by 12 and compounding those monthly percentages would overstate the advertised annual return.
How bonuses affect the ranking
The published AER may already include an introductory interest bonus. We use that published rate in the estimate, and adjust it after a bonus ends only when its rate and end date or duration are recorded. Without those details, the estimate assumes the published rate continues, subject to any other recorded rate changes. This can overstate returns if an unrecorded bonus expires during the year. Comparison results rank by estimated 12-month interest; the All ISA accounts rates table sorts by advertised AER.
Variable rates and longer fixed terms
A variable rate can rise or fall; the estimate assumes it stays the same except for known changes. A two-year fixed account still locks money for two years even though we display the first 12 months of estimated interest.
Cash ISA Finder · Calculation methodology
Checked 1 October 2026. General information, not personalised financial advice.