SIPP Calculator: Project Your Pot and Tax Relief

Use the SIPP calculator

Enter your current pot, your monthly or annual contribution, your age, your target retirement age and an assumed annual growth rate. The tool grosses up your payments with basic-rate tax relief, compounds the balance each year, and returns a projected pot value. Treat the output as a planning estimate, not a guarantee.

What a SIPP actually is

A self-invested personal pension is a pension wrapper you control yourself. Instead of a provider or employer choosing where your money goes, you pick the investments inside it: funds, shares, investment trusts, ETFs, sometimes commercial property. It carries the same tax treatment as any other personal pension. Money goes in with tax relief, grows free of UK income tax and capital gains tax inside the wrapper, and from age 55 (rising to 57 from April 2028) you can normally take 25% as a tax-free lump sum, with the rest taxed as income when you draw it.

The trade-off is responsibility. A SIPP gives you flexibility and usually lower charges than an old-style personal pension, but no one is managing the asset choices for you. The calculator above does not pick investments. It models what a given contribution and growth rate could produce, so you can see whether your plan is on track.

How SIPP tax relief works

This is the part that makes a SIPP worth understanding properly, and where the SIPP calculator earns its keep. When you pay into a SIPP from your own pocket, your provider claims basic-rate tax relief from HMRC and adds it to your pot automatically. This is called relief at source.

The relief is 20%, but it is added on the grossed-up figure, not your net payment. Put another way, you pay in 80% and HMRC tops it up to 100%. So a £80 contribution becomes £100 in the pension. The formula is simple:

• Gross contribution = net contribution ÷ 0.80
• Tax relief added = gross contribution − net contribution

If you are a higher-rate or additional-rate taxpayer, the 20% added at source is not the whole story. You can claim further relief through your Self Assessment tax return, effectively reclaiming the difference between basic rate and your highest rate. A higher-rate taxpayer (40%) claims back another 20% of the gross contribution; an additional-rate taxpayer (45%) claims back another 25%. That extra relief comes to you as a tax reduction or refund, not into the pension, unless you choose to pay it back in.

Scotland is different. Scottish income tax has more bands, with rates from 19% up to 48%. Your provider still adds 20% relief at source regardless of where you live in the UK, but a Scottish taxpayer in, say, the intermediate (21%), higher (42%) or advanced (45%) band claims the remainder against their own band rates through Self Assessment. The mechanics are the same; only the percentages you reclaim change. If you are a starter-rate (19%) Scottish taxpayer you still receive the full 20% at source and HMRC does not claw back the 1% difference.

Contribution limits and the annual allowance

Tax relief is generous but not unlimited. Two rules cap how much you can usefully pay in.

First, you can normally only get tax relief on contributions up to 100% of your relevant UK earnings each tax year (or up to £3,600 gross if you have little or no earnings). Pay in more than you earn and the excess gets no relief.

Second, the annual allowance limits total pension contributions across all your pensions, from you and any employer, before a tax charge applies. For most people the annual allowance is £60,000 for 2026/27. High earners can see this tapered down, and anyone who has flexibly accessed a defined contribution pension may be limited by the much lower Money Purchase Annual Allowance. If you have unused allowance from the previous three tax years, carry forward may let you contribute more in one go. These limits get technical quickly, so we cover them on a dedicated page rather than cramming them here.

How the SIPP calculator works

The calculator does two jobs in sequence. It grosses up each contribution for basic-rate relief, then projects the pot forward with compound growth.

The gross-up step uses the relief-at-source formula above: your net payment is divided by 0.80 to find the gross amount that lands in the pension. The projection step then compounds the balance:

• End-of-year pot = (starting pot + gross contributions for the year) × (1 + growth rate)

It repeats that for every year until your chosen retirement age. Growth is an assumption you set, not a promise. Real returns vary year to year and charges eat into them, so it is sensible to model a cautious figure as well as an optimistic one and see the range.

Worked example: Priya, paying in £400 a month

Priya is 35, lives in England and is a higher-rate taxpayer. She pays £400 a month from her bank account into her SIPP and wants to retire at 65. Here is how the numbers break down.

Step 1 — gross up the contribution. Her £400 net becomes £400 ÷ 0.80 = £500 gross in the pension. HMRC has added £100 of basic-rate relief to every monthly payment. Over a year that is £4,800 of her own money plus £1,200 of relief at source, so £6,000 going in.

Step 2 — claim the higher-rate relief. Because Priya pays 40% tax, she can reclaim a further 20% of the gross through Self Assessment: 20% of £6,000 = £1,200 a year back in her pocket (or off her tax bill). That does not go into the pot automatically, but if she reinvests it, her real cost of building a £6,000-a-year pension is closer to £3,600.

Step 3 — project the pot. Assume 5% average annual growth and that she increases nothing. Paying £6,000 gross a year (£500 a month) for 30 years, compounding at 5%, grows to roughly £398,000 by 65. Her own contributions over that time were £144,000 net; tax relief at source added £36,000; investment growth did the rest. The higher-rate relief she reclaimed each year is on top of that.

The lesson the SIPP calculator makes obvious: the tax relief and the compounding are both doing heavy lifting, and starting early matters far more than the size of any single payment.

A second example: a basic-rate saver

Tom is 40, earns £32,000 and pays in a one-off £2,000 from a bonus. Grossed up, that is £2,000 ÷ 0.80 = £2,500 in his SIPP, with £500 added by HMRC. As a basic-rate taxpayer he has no further relief to claim; the 20% at source is the lot. Left to grow at 5% for 25 years, that single £2,500 could become around £8,470 by his mid-sixties without him adding another penny.

How to get more from your SIPP

• Reinvest your higher-rate relief. Many higher and additional-rate taxpayers forget to claim the extra relief at all, or spend it. Putting it back into the SIPP compounds the benefit.
• Mind the order with a workplace pension. If your employer matches contributions, capturing that match usually beats funnelling the same money into a SIPP, because the employer money is free. A SIPP is often best for what you save beyond the match.
• Use carry forward before it lapses. Unused annual allowance only reaches back three tax years. If you have had a strong earnings year, a larger one-off contribution may mop up old allowance you would otherwise lose.
• Watch the £100,000 cliff edge. Once income passes £100,000, your Personal Allowance is cut by £1 for every £2 above it, creating an effective 60% tax band up to £125,140. A SIPP contribution that brings your adjusted income back below £100,000 can reclaim that allowance, which is one of the most tax-efficient moves available to people in that range.

Common mistakes to avoid

• Confusing net and gross. If you mean to put £500 of actual money in, you pay £500 net and the pot receives £625 gross. People who type their gross target as the payment end up over-contributing. Decide whether your figure is what leaves your bank or what lands in the pension.
• Assuming the pension grabs all your relief. Only the basic-rate 20% arrives automatically. Higher and additional-rate relief must be claimed through Self Assessment or by contacting HMRC. Miss it and you have left money on the table.
• Paying in more than you earn. Relief is capped at 100% of your relevant earnings (or £3,600 gross if you earn little). A non-earner cannot gross up £40,000 and expect £10,000 of free relief.
• Ignoring the annual allowance and the MPAA. Exceed your allowance and a tax charge can wipe out the relief. If you have already flexibly drawn from a pension, your future allowance may be far lower than £60,000.
• Treating the projection as a promise. Growth assumptions are not returns. Inflation, charges and poor years all bite. Model more than one growth rate.

SIPP vs a workplace pension

A workplace pension is arranged by your employer, often with their contributions and a default investment fund, and basic-rate relief is sometimes given through net pay rather than relief at source. A SIPP is yours alone, with wider investment choice and usually relief at source. Neither is universally better. Many people keep both: the workplace scheme for the employer match, a SIPP for control over the rest. The tax relief on contributions works the same way for both; the difference is who runs the money.

Sources and further reading

For the official rules on pension tax relief and how it is applied, see gov.uk: Tax on your private pension contributions. For an independent plain-English overview of how SIPPs work, the government-backed MoneyHelper guide to SIPPs is a good starting point. Figures used here reflect the 2026/27 tax year.

These results are estimates for guidance only and are not personal tax or financial advice. Pension and investment decisions carry risk; consider regulated advice before acting.

Related calculators

To plan the wider picture, try our pension calculator for a full retirement projection, or work out exactly what HMRC adds to your payments with the pension tax relief calculator. If you are close to your contribution cap, check the limits with the pension annual allowance calculator, and see what your savings could be worth with the pension pot calculator.