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Compare tax-sheltered and taxable investing using the same opening portfolio and annual cash contribution.
· UK tax assumptions use 2026/27.
Investment assumptions
£0–£20,000. Applied equally to each scenario.
-100–30 %
0–15 %
1–50 years
Opening portfolio
£0–£100,000,000. Applied to ISA, GIA and SIPP.
£0–£50,000. Original purchase amount; capped at opening value.
Dividend and CGT rate assumption
Modelled outcome over 20 years
+£65,352
Modelled difference between the two account values after 20 years.
The share link includes your portfolio and tax assumptions in an unencrypted URL.
The model uses a £3,000 Capital Gains Tax annual exempt amount and a £500 dividend allowance. It assumes no other gains, losses or dividends use these allowances.
The selected basic-rate assumption applies 10.75% dividend tax and 18% CGT to modelled income and gains. Actual tax bands can span more than one rate. Bed-and-ISA sells GIA holdings up to the unused portion of the £20,000 annual ISA limit; annual cash contributions use the rest and disposals may realise CGT. The SIPP comparison uses the same opening value and grosses up new annual contributions by a 20% basic-rate top-up before investment; withdrawal tax and higher-rate relief claims are not modelled.
GIA value is net of modelled tax at actual disposals; SIPP is before withdrawal tax
| Year | ISA | GIA after sale CGT | SIPP before withdrawal tax |
|---|---|---|---|
| Year 1 | £63,850 | £63,786 | £66,438 |
| Year 2 | £78,670 | £78,506 | £84,026 |
| Year 3 | £94,526 | £94,226 | £102,845 |
| Year 4 | £111,493 | £111,012 | £122,982 |
| Year 5 | £129,648 | £128,937 | £144,528 |
| Year 6 | £149,073 | £148,079 | £167,582 |
| Year 7 | £169,858 | £168,519 | £192,250 |
| Year 8 | £192,098 | £190,346 | £218,646 |
| Year 9 | £215,895 | £213,654 | £246,888 |
| Year 10 | £241,358 | £238,543 | £277,108 |
| Year 11 | £268,603 | £265,122 | £309,443 |
| Year 12 | £297,755 | £293,503 | £344,041 |
| Year 13 | £328,948 | £323,810 | £381,062 |
| Year 14 | £362,324 | £356,174 | £420,674 |
| Year 15 | £398,037 | £390,733 | £463,058 |
| Year 16 | £436,250 | £427,637 | £508,410 |
| Year 17 | £477,137 | £467,046 | £556,936 |
| Year 18 | £520,887 | £509,128 | £608,859 |
| Year 19 | £567,699 | £554,065 | £664,417 |
| Year 20 | £617,788Higher projected account value: ISA | £552,436 | £723,863 |
Dividend tax each year plus CGT only when gains are modelled as realised
| Year | Tax this year | Cumulative tax |
|---|---|---|
| Year 1 | £65 | £65 |
| Year 2 | £94 | £159 |
| Year 3 | £126 | £285 |
| Year 4 | £160 | £445 |
| Year 5 | £196 | £641 |
| Year 6 | £234 | £875 |
| Year 7 | £275 | £1,150 |
| Year 8 | £319 | £1,469 |
| Year 9 | £366 | £1,835 |
| Year 10 | £416 | £2,251 |
| Year 11 | £470 | £2,721 |
| Year 12 | £527 | £3,248 |
| Year 13 | £588 | £3,836 |
| Year 14 | £653 | £4,489 |
| Year 15 | £723 | £5,212 |
| Year 16 | £797 | £6,009 |
| Year 17 | £876 | £6,885 |
| Year 18 | £961 | £7,846 |
| Year 19 | £1,052 | £8,898 |
| Year 20 | £50,764 | £59,662 |
Estimated tax by source over 20 years; assumptions use the selected basic-rate band
| Scenario | Capital Gains Tax | Dividend tax | Total tax |
|---|---|---|---|
| GIA only | £49,615 | £10,047 | £59,662 |
Compare tax-free ISA growth with a taxable General Investment Account (GIA) using matching opening values and annual contributions. See estimated dividend tax, CGT on realised disposals, an optional Bed-and-ISA scenario and a simplified SIPP illustration.
Enter an annual contribution, an opening portfolio value, its GIA cost basis, expected capital growth and dividend yield. The opening value is shared across the ISA, GIA and SIPP. Only GIA holdings use a taxable cost basis, which cannot exceed the opening value. Annual contributions are treated as arriving halfway through each year. Dividends are assumed reinvested; GIA dividends are taxed annually using the selected tax-band assumption.
CGT is modelled only when a GIA disposal occurs: at the end of the projection for the GIA-only scenario, and on annual Bed-and-ISA transfers plus final sale when that strategy is enabled. The optional SIPP illustration adds a 20% basic-rate top-up to new contributions, but does not model tax on pension withdrawals.
Current inputs are included in the page URL so you can share or bookmark a comparison. The URL is not encrypted. This calculator does not save portfolio inputs in browser storage; remove the input values from the URL before sharing if you do not want to disclose them.
These GOV.UK and HMRC sources explain the ISA, dividend, Capital Gains Tax and pension rules referenced by this simplified projection.