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Pensions & RetirementPension2026/27 Tax Year

SIPP Growth Calculator

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Disclaimer: Illustrative projection based on the figures and assumptions entered. Pension pot growth is not guaranteed, investment values can fall as well as rise, and statutory retirement rules or tax relief rates may change. For personal retirement planning, consult an FCA-regulated financial adviser or contact Pension Wise.

How SIPP tax relief actually reaches your pension

Under relief at source your provider adds basic rate relief automatically, turning £400 into £500. A higher rate taxpayer must claim the rest separately — and many never do.

Figures stated for the 2026/27 UK tax year. Content last checked against official sources on .

What this calculator does

  • Shows what a net personal contribution becomes once basic rate relief is added.
  • Identifies the additional relief a higher or additional rate taxpayer can claim.
  • Projects the resulting pot forward, net of platform charges.
  • Flags where contributions approach the annual allowance.

How the calculation works

A personal pension such as a SIPP normally operates relief at source. You pay from money that has already been taxed, and the provider reclaims basic rate relief from HMRC and adds it to your pot. Because relief is calculated as a proportion of the gross contribution rather than added on top of the net one, £400 of your money becomes £500 in the pension: the £100 added is 20% of the £500 gross, not 20% of the £400 you paid. If you pay tax above the basic rate, you are entitled to further relief at your marginal rate — but that part does not arrive automatically. It has to be claimed, through Self Assessment or by contacting HMRC, and it is paid to you rather than into the pension. That is the single most commonly missed piece of UK pension tax relief. The projection then compounds the gross contributions forward net of the platform charge.

The rule

Gross contribution = net contribution ÷ 0.8. Provider relief = gross − net. Further relief claimable = gross × (marginal rate − 20%).

Step by step

  1. Take the net monthly amount you actually pay.
  2. Divide by 0.8 to give the gross contribution the pension receives.
  3. The difference is the basic rate relief the provider reclaims for you.
  4. Multiply the gross contribution by your marginal rate less 20% to give the relief you must claim yourself.
  5. Compound the gross contributions and starting pot forward, net of the platform charge.

Worked example

A higher rate taxpayer with a £20,000 SIPP paying £400 net a month for twenty years, assuming 5% growth and a 0.5% platform charge.

What was entered

Inputs used in the worked example
Current SIPP pot value£20,000
Your net monthly contribution£400
Marginal tax relief rate40%
Expected annual return5%
Annual platform fee0.5%
Years until retirement20

The arithmetic

  1. £400 net divided by 0.8 gives a gross contribution of £500 a month.
  2. The provider reclaims £100 a month of basic rate relief and adds it to the pot automatically.
  3. As a 40% taxpayer, a further £100 a month of relief is claimable — but only if actually claimed.
  4. Unclaimed, that is £1,200 a year, or £24,000 over the twenty years, simply left with HMRC.
  5. Compounding £500 a month plus the starting £20,000 at 5% less charges gives a projected pot of about £239,561.

What the calculator returns

Results produced by the worked example
Gross monthly contribution£500.00
Relief added by your provider£100.00
Further relief you can claim£100.00
Projected SIPP value£239,560.57

Key assumptions

  • The pension operates relief at source, which is standard for a personal pension or SIPP.
  • Your marginal rate stays the same throughout, and you have enough income taxed at that rate to claim the relief.
  • Growth is steady at the rate entered and the platform charge is applied annually.

Limitations

  • The extra relief above basic rate is paid to you, not into the pension, so it only compounds if you choose to reinvest it.
  • Tax relief is limited to the higher of your relevant UK earnings and £3,600 gross a year, so you cannot get relief on more than you earn.
  • Scottish taxpayers have different marginal rates, so the claimable amount differs.
  • The annual allowance, its taper for high earners, and the much lower money purchase allowance after flexible access are not modelled here.
  • This is a projection, not advice, and investment returns are not certain.

Common questions

Why does £400 become £500 rather than £480?
Because relief is 20% of the gross contribution, not 20% added to the net one. £500 gross taxed at 20% leaves £400, so the £100 added is exactly the tax that would have been paid on £500.
How do I claim the higher rate relief?
Through your Self Assessment return, or by contacting HMRC directly if you do not file one. It is not automatic, and it is not added to your pension — it comes back to you, usually as a tax code adjustment or a repayment.
Can I contribute more than I earn?
You can pay in more, but relief is capped at the higher of your relevant UK earnings and £3,600 gross a year. Contributions above that get no relief, which removes most of the point.
Is a SIPP better than my workplace pension?
Not usually, if your employer contributes to the workplace scheme — that contribution is money a SIPP will not give you. A SIPP is generally a complement rather than a replacement, and this is a genuine planning question rather than an arithmetic one.

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Official sources

Every figure in this guide was checked against the sources below. Where a source could not confirm a figure, it is marked as requiring verification rather than presented as settled.