Savings interest calculator: what will your savings earn?

The calculator is above. This page explains exactly how it works, so you can check any figure it gives you, and where its limits are.

How the calculation works

Every rate in the calculator is an AER, the annual equivalent rate. It already accounts for interest earning interest over a year, so it is the fairest way to compare accounts that pay monthly with those that pay yearly. Our guide to what AER means explains it in more detail.

To work out a part-year or a period longer than a year, the calculator turns each AER into a monthly rate:

monthly rate = (1 + AER) to the power of 1/12, minus 1

It then grows your deposit by that monthly rate once a month, for the number of months you choose. Twelve months at the monthly rate gives back exactly the AER, which is the point. The interest is the final balance minus what you put in. It assumes one deposit at the start and no withdrawals.

Real accounts work slightly differently. Chip and most apps calculate interest daily and pay it monthly, and rates are variable, so the actual figure can move up or down. Treat every result as an estimate.

A worked example

Take £5,000 in the Chip Smart Cash ISA, which pays 4.68% AER (variable tracker) for 12 months, then 3.50% AER.

• The monthly rate during the boost is about 0.382%.
• After 12 months, the interest is £234.00, which matches 4.68% of £5,000.
• After 18 months, it is £324.81, because the last six months earn 3.50%.
• If the boost had lasted all 18 months, it would have been £355.08. The gap is what a calculator that ignores the boost ending would overstate.

Boosts that end

Chip’s headline rates include a new-customer boost. Instant Access pays 3.51% AER (variable tracker) for 12 months, then 3.00% AER. The Easy Access Saver pays 3.81% AER (variable tracker) for 12 months, then 3.50% AER. The full list, with the tracker margins under Bank Rate, is on our Chip interest rates page.

The calculator applies each boosted rate for its boost length and then switches to the standard rate. It does the same for other apps where the bonus length and the rate afterwards are published, such as the Plum and Moneybox Cash ISAs and the Chase Saver boost. Our guide to introductory bonus rates covers what to do when a boost ends.

The Easy Access Saver row assumes you make 3 or fewer withdrawals in 12 months. From the next one, the rate drops by 2.10% AER for the rest of the period, which the calculator does not model. If you expect to dip in often, Instant Access is the closer comparison; see Instant Access vs Easy Access.

Reading the Chip rows

The three Chip rows answer different needs, so the highest number is not automatically the right one. The Smart Cash ISA usually comes out on top while its boost lasts, and its interest is tax-free, but it only takes up to £20,000 a tax year across all your ISAs. If you have already used your allowance elsewhere, the ISA row does not apply to you.

Instant Access has unlimited withdrawals with no penalty, so it suits money you may need at short notice. The Easy Access Saver pays more on paper, but it limits penalty-free withdrawals and does not count for the referral bonus. With the bonus ticked, Instant Access can come out ahead of Easy Access on smaller amounts, because the £50 is a bigger share of the total.

Try a few lengths. At 12 months the boost does all the work. At 24 months, half the period is at the standard rate, and the gap between Chip and an app with a higher standard rate narrows or reverses.

The £50 referral bonus

Tick the box and the calculator adds £50 to the Chip accounts that count for the referral: the Smart Cash ISA, the Instant Access Account or the Prize Savings Account. The Easy Access Saver does not count, so its row never shows the bonus. The box only works for £1,000 or more over three months or longer, because the terms need £1,000 kept for 90 days in a row, and if your balance drops below £1,000 at any point, the 90-day countdown starts again from zero.

The bonus is a one-off and the bonus itself does not earn interest, so it is added after the interest rather than compounded. It is only for people who have never held a Chip account before, and it is paid at Chip's discretion. If you are not sure you qualify, the bonus checker takes five questions.

The other apps’ rows

Other apps are shown at their published headline rates, read from each provider’s own page. Several of those rates come with strings, so each row carries a short note:

• Plum: the top Easy Access rate is for Plum Max, a paid plan (Basic free; Plus £3.99, Boost £7.99, Max £14.99 a month). The Cash ISA pays 4.73% AER variable for 12 months, then 2.54%, including a bonus with conditions.
• Moneybox: the Simple Saver rate shown is the reward rate, which needs a balance in another qualifying Moneybox account. The Cash ISA pays 4.72% AER variable for 12 months, then 3.45%.
• Trading 212: interest on cash is paid on uninvested money in an investment account, not a savings account, and part may be held in money market funds without deposit protection.
• Monzo: the top Instant Access Pot rate needs Monzo Max (Free; Extra £3, Perks £9, Max from £19 a month), and you need a Monzo current account.
• Chase: the Chase Saver pays 2.25% AER variable; 4.5% for new customers with the boost. The boost is 2.25% AER boost for 12 months; open within your first 31 days, and you need a Chase current account.

Subscription fees are not deducted from any row. On small balances, a paid plan can cost more than the extra interest it earns. For the full picture of which Cash ISA suits whom, see the best Cash ISA apps comparison.

What is left out

Premium Bonds and Chip’s own Prize Savings Account are not in the table, because they pay prizes, not interest. What you get depends on luck in each draw, and most holders get less than the published prize fund rate. Our Chip Prize Savings vs Premium Bonds comparison covers how they work.

The calculator also ignores regular top-ups, withdrawals, rate changes and any boost you have already used up. If you have held a Chip account before, your rate may already be the standard one.

Tax on savings interest

The calculator shows interest before tax. Interest in a Cash ISA is not taxed. You can pay up to £20,000 into ISAs this tax year; our guide to the Cash ISA allowance explains how it works.

Interest outside an ISA counts towards your Personal Savings Allowance. According to GOV.UK, basic-rate taxpayers can earn up to £1,000 of interest a year without tax, higher-rate taxpayers £500, and additional-rate taxpayers nothing. Above that, interest may be taxable at your usual rate. A flexible ISA like the Smart Cash ISA also lets you take money out and put it back in the same tax year without using more allowance.