Make the difference visible
Savings interest comparison calculator
Compare two AERs and see the extra interest over the period you choose, before tax and fees.
The useful answer
Enter a balance, a current AER, an alternative AER and a period in months. The calculator estimates the interest under each rate and the difference between them. The example rates are illustrative and do not represent current offers.
Try your numbers
No sign-up. Calculations stay in this page.
Extra interest before tax
£150.00Current: £50.00. Alternative: £200.00. Over 12 months under the assumptions below.
Rates are illustrative, not live offers. Assumes unchanged AER, compounding, no deposits, withdrawals, fees or tax. Actual payment schedules and variable rates can change the result.
What to know
- The extra interest is alternative interest minus current interest.
- AER is treated as a yearly effective rate, with compounding.
- Rates, access rules, fees and tax can change the real outcome.
The formula used here.
Estimated interest = balance × ((1 + AER ÷ 100) raised to the power of months ÷ 12, minus 1). Using the annual effective rate this way models compounding across the entered period.
We calculate the interest for each rate, round each to pennies, and compare the two. This is a simplified model; an account’s actual daily calculation and payment schedule can produce a different result.
Compare the improvement, not just the headline.
If your current account already pays interest, the entire interest amount at an alternative rate is not a new saving. Subtract what the current account would produce under the same assumptions.
What must stay the same for the estimate.
The model assumes the entered balance stays invested, rates do not change and interest compounds. It excludes extra deposits, withdrawals, fees and tax. A promotional rate that ends halfway through the period needs a different calculation.
No product eligibility or minimum-balance check is performed. A higher entered rate can be mathematically attractive while belonging to an account that does not fit your circumstances.
Read the conditions before moving money.
Check withdrawal access, notice periods, deposit limits, bonus conditions and any required linked account. Consider when the money will be needed and whether the rate is fixed or variable.
This tool compares assumptions you enter. It does not show live rates, recommend a particular account or open one. Use the provider’s current information and an appropriate adviser when your decision needs personalised advice.
Sources & further reading
Examples are illustrative unless stated otherwise.
- Lodo: your money manager in WhatsApp · Checked 7 September 2026
Something changed? Tell us so we can check it.