Cash ISA Calculator – How Much Will You Actually Earn?

"How much interest will I earn on a Cash ISA?" has a two-second answer and a ten-minute answer.

The two-second one: multiply your balance by the rate. £20,000 at 4.64% is £928.

The ten-minute one is the useful one, because it accounts for monthly payments, compounding, bonus rates that vanish after a year, and the question most people skip entirely: whether you'd keep more in a normal savings account that isn't tax-free but pays more.

The DepositScout ISA calculator does the ten-minute version in about thirty seconds. This guide walks through how to use it, what the outputs mean, and the worked examples I'd want to see before committing my own allowance.

How to use the Cash ISA calculator

The calculator has four inputs. They're all sliders, so you can drag them around and watch the chart move.

1. ISA deposit. The lump sum you're putting in now, from £0 to the £20,000 allowance. It defaults to £5,000. The bar underneath shows what percentage of your 2026/27 allowance you've used.

2. Monthly contribution. What you'll add each month, £0 to £1,000. The calculator warns you if the deposit plus 12 months of contributions would breach £20,000 in a single tax year.

3. Interest rate. Either type a rate, or pick a live Cash ISA product from the list and the rate fills in automatically from the database. Pick a real account; it keeps you honest about what's available rather than what you'd like.

4. Term. One to 20 years.

It gives you back the final balance, total interest earned, and a year-by-year chart that splits the pot into your deposit, your contributions and the interest. The maths is monthly compounding of the rate you entered, which is the standard assumption and is close enough to how most providers calculate it.

Worked examples at October 2026 rates

The best easy access Cash ISA rate this week is 4.64% (Trading 212 and Chip, both including a 12-month bonus). The best one-year fixed is 4.95%, and the best five-year fixed is 5.25%. All examples use the calculator's monthly compounding.

Two things jump out of that table. First, dripping £833 a month in earns about £215, while the same £10,000 paid in on day one earns about £475, because money you haven't deposited yet can't earn anything. If you've got the lump sum, pay it in early in the tax year. Second, the five-year fixed at 5.25% only beats five years of easy access at 4.64% by about £780, which is the price of being able to get at your money. Whether that's worth it depends entirely on whether you'll need it.

Why the "actually" matters: four things the headline rate hides

1. Bonus rates that fall off a cliff

Nearly every top easy access ISA right now is a 12-month bonus on top of a much lower underlying rate. Trading 212's 4.64% includes a 1.04% bonus, Chip's includes 1.10%, Moneybox's includes 1.18%, and Tesco Bank's 4.20% drops to 1.05% in month 13. The calculator assumes the rate holds for the whole term, because it can't know when you'll move. So if you're modelling three years at 4.64%, you're really modelling one year at 4.64% and two years of you remembering to transfer. Put the anniversary in your calendar or set a rate alert.

2. AER versus gross

AER is the rate including compounding, so it's the one to compare on. Gross is the rate before compounding. On accounts that pay interest monthly, gross is slightly lower than AER (a 4.64% AER account might pay 4.54% gross monthly). The calculator uses the AER you give it, which is the right call for comparing products.

3. Variable means it will change

The Bank of England held the base rate at 3.75% in September, but three of the nine committee members voted for a rise, and the next decision is on 5 November. Easy access ISA rates follow the base rate with a lag of weeks. A five-year projection at a variable rate is a guess with a chart. If you want the number to be real, fix it.

4. Withdrawals on flexible versus non-flexible ISAs

Take £5,000 out of a flexible ISA and you can put it back in the same tax year without touching your allowance. Do it in a non-flexible ISA and that £5,000 of allowance is gone. The calculator doesn't model withdrawals, so if you expect to dip in, check the flexible ISA guide before you pick a product.

Do you even need a Cash ISA? Run the tax calculator first

This is the question I'd answer before the ISA one. Cash ISAs are tax-free, but the best non-ISA easy access accounts are paying 5.00% against the best ISA's 4.64%, and most people have a personal savings allowance that makes some or all of their interest tax-free anyway:

• Basic-rate taxpayer: £1,000 of interest a year tax-free. At 5% that covers £20,000 of savings.

• Higher-rate taxpayer: £500 tax-free. At 5% that's £10,000.

• Additional-rate taxpayer: nothing.

Worked out on £20,000 for a year:

• Basic rate: 5% savings account earns £1,000, fully covered by the allowance. ISA earns £928. Savings account wins by £72.

• Higher rate: 5% earns £1,000, of which £500 is taxed at 40%, leaving £800. ISA earns £928. ISA wins by £128.

• Additional rate: 5% earns £1,000, taxed at 45%, leaving £550. ISA wins by £378.

And on £50,000 at 4.6%, where the allowance runs out for everyone: a basic-rate taxpayer loses £260 to tax, a higher-rate taxpayer £720 and an additional-rate taxpayer £1,035. That's the point at which the ISA stops being a nice-to-have. The savings tax calculator takes your income, your balance and a real product and tells you what you'd keep after tax, which is the number to compare with the ISA calculator's output.

The honest summary: if you're a basic-rate taxpayer with under £20,000 in cash, you don't need a Cash ISA this year on the numbers alone. The reason to open one anyway is the next section.

The April 2027 change: use the full £20,000 while you can

From 6 April 2027, anyone under 65 will only be able to put £12,000 a year into a Cash ISA. The overall £20,000 allowance stays, but the remaining £8,000 will have to go into a stocks and shares ISA or not be used. Over-65s keep the full £20,000 in cash. Existing balances aren't affected; it only applies to new money from the 2027/28 tax year.

Which changes the calculation for the current tax year. Money inside a Cash ISA stays tax-free for as long as it's there, and it's allowed to grow well past the annual limit. If you expect to have more than £12,000 a year to shelter in cash from 2027, filling the full £20,000 this year and next is the last chance to do it. Run the calculator at £20,000 and £12,000 and the gap compounds: at 4.64% over five years it's about £2,000 of extra tax-free interest from the larger deposit.

Frequently asked questions

How much interest does £20,000 earn in a Cash ISA?

At the current best easy access rate of 4.64%, about £928 over a year (£948 with monthly compounding). At the best five-year fixed rate of 5.25%, about £5,990 over five years. Rates checked 6 October 2026.

Is a Cash ISA worth it if I'm a basic-rate taxpayer?

On this year's numbers, only if you have more than about £20,000 in savings, because your £1,000 personal savings allowance covers the interest below that and non-ISA rates are higher. It becomes worth it if you'll exceed the allowance in future, or want to lock in tax-free status ahead of the April 2027 £12,000 cash cap.

Does the ISA calculator account for tax?

No, because there's no tax inside an ISA. Use the savings tax calculator to see what the same money would earn after tax outside one.

What is the Cash ISA allowance for 2026/27?

£20,000 across all ISA types. From 6 April 2027 the cash portion is capped at £12,000 for under-65s.

Is interest in a Cash ISA paid monthly or yearly?

It depends on the provider. Most easy access ISAs pay annually; some pay monthly. The calculator assumes monthly compounding, which makes a difference of a few pounds on £20,000.

Can I lose money in a Cash ISA?

Not in nominal terms. The balance can only go up. In real terms, if the rate is below inflation (3.1% CPI in August 2026) you lose purchasing power, which is why the rate matters more than the wrapper.