Building Society Savings Calculator: Interest & Growth Guide

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Free UK Savings Growth Calculator

Building Society Savings Calculator: Interest & Growth Guide

Use this building society savings calculator to estimate how an opening lump sum, regular monthly deposits and an advertised AER could grow over time. It separates the money you put in from the interest earned, so a high headline rate is easier to judge realistically.

The calculator is especially useful when comparing a fixed-rate bond with a regular saver, testing different AERs or checking what happens when you add £50, £100, £250 or another amount each month.

Use the Savings Calculator See Worked Examples Understand AER
Estimate, not a provider quote: actual building society interest can differ because providers may calculate interest daily, change variable rates, restrict monthly deposits, round interest differently or pay interest away instead of adding it to the account.

Calculator and editorial guide reviewed 22 September 2026. Enter the current AER from the exact account Summary Box you are considering rather than relying on an older search result.

Savings Interest Calculator: Quick Answers

Opening balance The lump sum already available when the calculation starts.
Monthly saving The amount you plan to add each month. Set it to £0 when comparing a lump-sum bond.
AER Annual Equivalent Rate. It helps compare savings accounts on a consistent annual basis.
Total contributions Your opening balance plus all monthly deposits. This is your own money, not interest.
Estimated interest Projected growth above the amount you contributed, assuming the entered rate stays unchanged.
Variable-rate warning A calculator cannot predict future changes to a building society’s variable savings rate.

Building Society Savings Calculator

Enter the opening balance, monthly amount, AER and number of years. For a fixed bond, set the monthly contribution to £0 if the account does not accept further deposits.

Calculate Your Estimated Savings Growth

This model converts AER to an equivalent monthly growth rate and assumes interest remains in the account so it can compound.

£
Use £0 if you are starting from scratch.
£
Check the account’s monthly deposit limit first.
%
Enter the current advertised AER for the account.
A fixed account may have a much shorter actual term.
Earlier deposits normally earn slightly more interest.
Provider statements may use different rounding.
Estimated final balance £0
Total money contributed £0
Estimated interest earned £0
Enter your figures and select “Calculate growth”.
Year Contributed Interest earned Estimated balance
Not suitable for every account: the calculator assumes one unchanged AER and no withdrawals. A variable saver, tiered-rate account, bonus account, account with monthly limits or a product where interest is paid away can produce a different result.

How the Savings Calculator Works

AER is an annualised comparison rate. For the calculator, the annual rate is converted into an equivalent monthly rate so both the opening balance and each monthly deposit can grow for the amount of time they are actually in the account.

Lump-sum savings

With no further deposits, the basic compound-growth model is:

Future balance = starting balance × (1 + AER)years

This assumes interest remains in the account and the annual rate does not change.

Regular monthly savings

Each deposit earns interest for a different length of time. January’s deposit can earn for much longer than December’s deposit.

This is why multiplying the final annual contribution total by the headline AER usually overstates regular-saver interest.

Provider calculations can be more precise. Many building societies calculate interest daily using the exact balance and deposit date. This calculator is designed for useful comparison, not to reproduce every provider’s statement to the penny.

What Does AER Mean on a Building Society Savings Account?

AER means Annual Equivalent Rate. It is designed to show what the interest rate would amount to over a year when interest is added to the savings and allowed to compound.

AER is useful for comparison It gives savers a common annual measure when providers pay interest at different intervals.
Gross rate can differ An account paying monthly interest may display a monthly gross annual rate slightly below its AER because AER allows for compounding.
Fixed AER A fixed savings rate normally stays unchanged for the stated fixed term, subject to the product terms.
Variable AER A building society can change a variable rate in line with the account terms, so a multi-year projection is only an illustration.

Independent UK guidanceMoneyHelper: Interest Rates and Compound Interest

After opening this link:

  1. Read the savings-interest explanation rather than the borrowing APR section.
  2. Find the compound-interest example.
  3. Compare how the same starting balance grows at different annual rates.
  4. Return to the calculator above and enter the AER from the building society account you are considering.

Worked Example: £10,000 Left to Compound for 5 Years

The table below holds the starting balance and time period constant so you can see the effect of rate alone. It assumes no withdrawals, no extra deposits and the same AER throughout.

AER
Balance after 5 years
Interest earned
2.00%
About £11,041
About £1,041
4.00%
About £12,167
About £2,167
6.00%
About £13,382
About £3,382
The difference becomes larger over time because interest can itself earn interest. That is the compounding effect.

How to Calculate a Building Society Regular Saver

Regular savers often display some of the highest building society headline rates, but the full annual contribution is not sitting in the account from day one.

Incorrect shortcut

You save £200 a month, so total deposits are £2,400. It is tempting to calculate 6.5% of £2,400 and expect £156 interest.

That overstates the result because most of the £2,400 was not in the account for the full year.

Better calculation

Treat each £200 monthly deposit separately. The first deposit earns for nearly the whole term while the final contribution earns interest for only a short period.

Exact provider illustrations can still differ slightly because interest may be calculated daily.

Calculator setup for a regular saver: set opening balance to £0, enter the monthly limit, enter the current AER and use 1 year if the product runs for 12 months.

How to Calculate a Fixed-Rate Bond

For a fixed bond that accepts one opening deposit and no further additions, set the monthly contribution to £0 and enter the starting lump sum, AER and term.

Use the fixed AER from the exact issue.
Enter only money accepted during the funding window.
Set monthly contributions to £0 if additions are not allowed.
Match the calculator years to the actual term as closely as possible.
Check whether interest compounds or is paid away.
Check whether withdrawals are prohibited before maturity.
If monthly or annual interest is paid to another bank account rather than retained in the bond, the calculator’s compound result will be higher than the amount remaining in the savings account itself.

Monthly Interest vs Annual Interest: Why Results Can Differ

Some building societies let you choose monthly or annual interest. When monthly interest is added back to the savings account, it can compound during the year. When it is paid away to a current account, that interest no longer grows inside the savings account.

Interest choice
What happens
Calculator treatment
Interest retained
Interest remains with the savings and can earn further interest.
This calculator models retained, compounded interest.
Interest paid away
The interest is transferred elsewhere for spending or income.
Use the result as a growth comparison, not the exact future account balance.
Variable rate
The provider can alter future interest under its terms.
Recalculate whenever the AER changes.

Should the Calculator Deduct Tax From Savings Interest?

The calculator deliberately shows gross estimated interest. Your actual tax position depends on your other income, your Personal Allowance, the starting rate for savings, your Personal Savings Allowance and whether the money is held inside a tax-free ISA.

£1,000 2026/27 Personal Savings Allowance for eligible basic-rate taxpayers.
£500 2026/27 Personal Savings Allowance for eligible higher-rate taxpayers.
£0 Personal Savings Allowance for additional-rate taxpayers.
Up to £5,000 Starting rate for savings can apply where other taxable income is low enough.
Do not simply deduct 20% from every savings projection. Many people have some or all of their interest covered by allowances, while Cash ISA interest is normally outside the ordinary savings-interest tax calculation.

Official tax guidanceGOV.UK: How Much Savings Interest Is Tax-Free?

After opening this link:

  1. Check whether any of your Personal Allowance remains available.
  2. Read the starting-rate-for-savings section if your other taxable income is relatively low.
  3. Check the Personal Savings Allowance for your tax band.
  4. Add together taxable savings interest from relevant accounts rather than looking at only one building society.
  5. Use the official guidance if your calculator projection suggests interest may exceed your available allowances.

Using the Calculator for a Cash ISA

You can use exactly the same interest calculator for a Cash ISA: enter the balance, monthly savings, AER and term. The difference is tax treatment rather than the mathematics of compounding.

£20,000 Current overall adult ISA subscription allowance for the 2026/27 tax year.
Tax-free Qualifying Cash ISA interest does not normally count towards ordinary taxable savings interest.
If comparing a Cash ISA with an ordinary savings account, do not compare AER alone. Compare the interest you expect to keep after considering your personal tax position and access requirements.

Official ISA rulesGOV.UK Individual Savings Accounts

After opening this link:

  1. Check the allowance for the tax year in which you are making new subscriptions.
  2. Subtract any ISA subscriptions you have already made.
  3. Use the remaining allowance as a limit when modelling additional Cash ISA deposits.
  4. Use the receiving provider’s formal transfer process for existing ISA money rather than treating an ISA transfer as a fresh cash deposit.

FSCS Protection When the Calculator Shows a Large Balance

Compounding and regular deposits can eventually push a large savings balance above the standard deposit-protection limit. That is worth checking before allowing a substantial balance to build with one authorised firm.

£120,000 Current standard FSCS deposit-protection limit per eligible person, per authorised firm.
Not per account Balances under the same banking authorisation can need to be added together.
£1.4 million Potential temporary high-balance protection for qualifying life events, subject to FSCS rules.

Official protection checkerFSCS Bank & Savings Protection Checker

After opening this link:

  1. Enter the building society or bank holding your savings.
  2. Check the authorised firm behind the brand.
  3. Look for other brands using the same authorisation.
  4. Add together eligible deposits held under the same protection limit.
  5. If your calculator projection approaches the standard limit, review how your savings are distributed.

Current Building Society Rate Examples to Test in the Calculator

These examples were checked in September 2026 and are included to demonstrate how the calculator can be used. They are not recommendations and may change or close after this page is published.

Nationwide Flex Regular Saver — 6.50% AER variable Current qualifying current-account customers can save up to £200 a month. Use £0 opening balance, £200 monthly and 1 year to model the structure.
Coventry Sunny Day Saver (3) — 5.00% AER variable Current product terms allow up to £150 each calendar month. The Society’s exact illustration uses daily interest calculations.
Coventry Fixed Rate Saver (28) — 4.20% AER fixed Designed for a lump sum and currently fixed to 30 November 2029. Set monthly contributions to £0 when modelling a single deposit.

Official Nationwide exampleNationwide Flex Regular Saver

After opening this link:

  1. Confirm the current AER.
  2. Check the monthly deposit limit.
  3. Read the withdrawal rule because the higher rate can depend on how the account is used.
  4. Look at Nationwide’s own estimated-interest illustration.
  5. Use the calculator on this page for alternative monthly amounts or comparison rates.

Official Coventry regular saverCoventry Sunny Day Saver (3)

After opening this link:

  1. Confirm the current variable AER.
  2. Check the maximum amount that can be deposited each calendar month.
  3. Read Coventry’s own estimated-balance example.
  4. Check what happens after the 12-month saver period.
  5. Recalculate whenever Coventry changes the variable rate.

Official Coventry fixed saverCoventry Fixed Rate Saver (28)

After opening this link:

  1. Confirm the current fixed AER and maturity date.
  2. Check the funding window and maximum balance.
  3. Read the access section before depositing emergency money.
  4. Choose annual or monthly interest information relevant to how you want the account operated.
  5. Enter the applicable AER and planned lump sum into the calculator above.

Common Savings Calculator Mistakes

Using the whole year’s regular-saver deposits from day one Monthly deposits enter gradually, so later deposits earn interest for less time.
Assuming a variable rate lasts for years Use a long-term variable-rate projection as a scenario, not a promise.
Ignoring product funding limits A 6% regular saver may accept only a few hundred pounds per month rather than a £20,000 lump sum.
Ignoring withdrawal charges A calculated return is less useful if accessing the money triggers lost interest or a lower rate.
Mixing AER and gross monthly rates Use AER consistently when comparing different savings products.
Deducting tax automatically Your actual taxable interest depends on allowances, other income and whether the savings are inside an ISA.
Forgetting interest paid away If monthly interest is transferred out for spending, it is not compounding inside the savings balance.
Ignoring FSCS limits Long-term growth can eventually take a large balance above standard deposit protection with one authorised firm.

Related Building-Society.org Savings Guides

Use these published rate guides to find a current AER and product structure, then bring the relevant figures back to the calculator.

Nationwide Building Society Savings Rates 2026: Accounts & ISAs Coventry Building Society Savings Rates 2026: Accounts & ISAs Yorkshire Building Society Savings Rates 2026: Accounts & ISAs Darlington Building Society: Savings, Mortgages & Online Services Stafford Building Society: Savings, Mortgages & Account Guide

Official and Independent Resources Used in This Guide

MoneyHelper: Interest Rates Explained GOV.UK: Tax-Free Savings Interest GOV.UK: Individual Savings Accounts FSCS Savings Protection Checker Nationwide Flex Regular Saver Coventry Sunny Day Saver (3) Coventry Fixed Rate Saver (28)

How This Calculator Should Be Used

Use the exact account’s current AER.
Set monthly deposits to £0 for a one-off lump-sum bond.
Respect the provider’s monthly contribution limit.
Recalculate when a variable interest rate changes.
Check whether interest remains in the account.
Read withdrawal restrictions separately.
Consider tax only after calculating gross interest.
Check FSCS protection for large projected balances.

Last meaningful editorial and calculator review: 22 September 2026.

Independent Calculator Notice

Building-society.org is an independent informational website. It is not a bank, building society, HM Revenue & Customs, MoneyHelper or the Financial Services Compensation Scheme.

Calculator results are illustrations and are not personalised financial advice, a savings quote or a guarantee of future returns. Actual interest depends on the provider’s calculation method, deposit dates, withdrawals, rate changes and account terms.

Before depositing money: read the building society’s current Summary Box, check whether the rate is fixed or variable, confirm access restrictions and verify FSCS protection.

Building Society Savings Calculator FAQs

1. How do I calculate interest on a building society savings account?

Enter your opening balance, regular monthly contribution, AER and saving period in the calculator. It estimates the final balance, total contributions and compound interest earned if the entered rate remains unchanged.

2. What does AER mean in the savings calculator?

AER means Annual Equivalent Rate. It provides an annualised way to compare savings interest while accounting for compounding. Use the current AER shown in the exact account’s Summary Box.

3. Does the calculator use compound interest?

Yes. The calculator assumes interest stays in the savings account and can itself earn interest. It converts the entered AER into an equivalent monthly rate for the projection.

4. Why is regular-saver interest lower than the headline rate multiplied by my total annual deposits?

Because you do not deposit the full annual amount on day one. Money added later in the year earns interest for fewer months, so the headline AER cannot simply be multiplied by the final total deposited.

5. Can I use the calculator for a fixed-rate bond?

Yes. Enter the lump sum as the opening balance, set monthly contributions to £0 and enter the fixed AER. Match the period to the bond term as closely as possible and remember that the account may not permit withdrawals.

6. Can I use the calculator for a Cash ISA?

Yes. Interest growth is calculated in the same way. The main difference is tax treatment: qualifying Cash ISA interest is tax-free under ISA rules. Check your remaining annual ISA allowance before making new subscriptions.

7. Does the calculator include tax on savings interest?

No. It shows gross estimated interest because tax depends on your other taxable income, Personal Allowance, starting rate for savings, Personal Savings Allowance and whether the money is held inside an ISA.

8. Why might the building society’s interest figure differ from this calculator?

A provider may calculate interest daily, use exact transaction dates, round differently, change a variable rate, apply balance tiers or pay interest out of the account. Those details can produce a slightly different result.

9. What happens if the savings rate changes?

If the account has a variable rate, recalculate using the new AER. A long-term projection using today’s variable rate is only a scenario and should not be treated as a guaranteed future balance.

10. How much building society savings money is protected by FSCS?

The current standard FSCS deposit-protection limit is £120,000 per eligible person, per authorised firm. Balances under brands that share one banking authorisation may need to be added together when checking the limit.

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