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Compare the same monthly surplus as mortgage overpayment or investing under your chosen return assumption.
£0–£10,000,000; whole pounds
0%–25%
1–40 whole years
£0–£100,000; whole pounds; £0 shows the no-overpayment baseline
−100% to 100%
+£74,178
Above £0 means the investment value is higher in this model; below £0 means the overpay-path position is higher. Neither result establishes a suitable choice.
Changing an input adds the assumptions to the page URL. URLs are not encrypted, so anyone with a copied link can see those values.
Investment value compared with mortgage benefit plus any released-payment investment
| Year | Overpay path incremental position | Investment account value | Standard mortgage interest | Interest with overpayment |
|---|---|---|---|---|
| 1 | £6,125.31 | £6,190.15 | £8,909.36 | £8,784.05 |
| 2 | £12,532.02 | £12,813.61 | £17,615.18 | £17,083.16 |
| 3 | £19,233.04 | £19,900.71 | £26,108.10 | £24,875.06 |
| 4 | £26,241.92 | £27,483.91 | £34,378.35 | £32,136.44 |
| 5 | £33,572.78 | £35,597.93 | £42,415.70 | £38,842.93 |
| 6 | £41,240.42 | £44,279.93 | £50,209.45 | £44,969.04 |
| 7 | £49,260.30 | £53,569.68 | £57,748.41 | £50,488.11 |
| 8 | £57,648.62 | £63,509.70 | £65,020.86 | £55,372.24 |
| 9 | £66,422.29 | £74,145.53 | £72,014.58 | £59,592.28 |
| 10 | £75,599.04 | £85,525.87 | £78,716.75 | £63,117.71 |
| 11 | £85,197.36 | £97,702.82 | £85,113.98 | £65,916.63 |
| 12 | £95,236.62 | £110,732.17 | £91,192.26 | £67,955.65 |
| 13 | £105,737.08 | £124,673.57 | £96,936.94 | £69,199.86 |
| 14 | £116,719.93 | £139,590.87 | £102,332.70 | £69,612.76 |
| 15 | £128,443.93 | £155,552.38 | £107,363.49 | £69,612.76 |
| 16 | £141,188.53 | £172,631.19 | £112,012.57 | £69,612.76 |
| 17 | £155,034.34 | £190,905.53 | £116,262.38 | £69,612.76 |
| 18 | £170,068.08 | £210,459.06 | £120,094.60 | £69,612.76 |
| 19 | £186,382.94 | £231,381.35 | £123,490.03 | £69,612.76 |
| 20 | £204,079.11 | £253,768.19 | £126,428.61 | £69,612.76 |
| 21 | £223,264.27 | £277,722.11 | £128,889.35 | £69,612.76 |
| 22 | £244,054.14 | £303,352.81 | £130,850.30 | £69,612.76 |
| 23 | £266,573.09 | £330,777.65 | £132,288.50 | £69,612.76 |
| 24 | £290,954.73 | £360,122.24 | £133,179.94 | £69,612.76 |
| 25 | £317,342.59 | £391,520.94 | £133,499.49 | £69,612.76 |
Cumulative interest, rounded to the nearest penny
| Year | Standard mortgage interest | Interest with overpayment | Interest saved to date |
|---|---|---|---|
| 1 | £8,909.36 | £8,784.05 | £125.31 |
| 2 | £17,615.18 | £17,083.16 | £532.02 |
| 3 | £26,108.10 | £24,875.06 | £1,233.04 |
| 4 | £34,378.35 | £32,136.44 | £2,241.91 |
| 5 | £42,415.70 | £38,842.93 | £3,572.77 |
| 6 | £50,209.45 | £44,969.04 | £5,240.41 |
| 7 | £57,748.41 | £50,488.11 | £7,260.30 |
| 8 | £65,020.86 | £55,372.24 | £9,648.62 |
| 9 | £72,014.58 | £59,592.28 | £12,422.30 |
| 10 | £78,716.75 | £63,117.71 | £15,599.04 |
| 11 | £85,113.98 | £65,916.63 | £19,197.35 |
| 12 | £91,192.26 | £67,955.65 | £23,236.61 |
| 13 | £96,936.94 | £69,199.86 | £27,737.08 |
| 14 | £102,332.70 | £69,612.76 | £32,719.94 |
| 15 | £107,363.49 | £69,612.76 | £37,750.73 |
| 16 | £112,012.57 | £69,612.76 | £42,399.81 |
| 17 | £116,262.38 | £69,612.76 | £46,649.62 |
| 18 | £120,094.60 | £69,612.76 | £50,481.84 |
| 19 | £123,490.03 | £69,612.76 | £53,877.27 |
| 20 | £126,428.61 | £69,612.76 | £56,815.85 |
| 21 | £128,889.35 | £69,612.76 | £59,276.59 |
| 22 | £130,850.30 | £69,612.76 | £61,237.54 |
| 23 | £132,288.50 | £69,612.76 | £62,675.74 |
| 24 | £133,179.94 | £69,612.76 | £63,567.18 |
| 25 | £133,499.49 | £69,612.76 | £63,886.73 |
Green: investment value is higher. Housing blue: overpay-path position is higher. Grey: equal. Neither outcome establishes a suitable choice.
| Effective annual investment return | Investment value | Overpay-path position | Investment minus overpay path |
|---|---|---|---|
| -2.00% effective annual return | £117,866.13 | £191,838.19 | -£73,972.06 |
| 0.00% effective annual return | £150,000.00 | £213,886.72 | -£63,886.72 |
| 2.00% effective annual return | £193,937.17 | £238,918.39 | -£44,981.22 |
| 4.00% effective annual return | £254,424.06 | £267,339.38 | -£12,915.32 |
| 6.00% effective annual return | £338,144.48 | £299,607.40 | £38,537.08 |
| 7.00% effective annual return | £391,520.94 | £317,342.59 | £74,178.35 |
| 8.00% effective annual return | £454,495.48 | £336,237.51 | £118,257.97 |
| 10.00% effective annual return | £616,662.45 | £377,808.45 | £238,854.00 |
The model applies your same monthly surplus to either extra mortgage principal or an investment account. It holds the mortgage rate and effective annual investment return constant, converting the investment return to an equivalent monthly rate; if the overpayment path clears the mortgage early, it invests the released payment budget for the remaining modelled months. A real lender may restrict overpayments or charge an early repayment fee; a real investment return varies and may be negative.
Before deciding, keep accessible emergency savings, compare expensive debts, check the mortgage offer for allowance and charges, and consider liquidity, investment fees and tax-wrapper rules. This page does not model ISA allowance, tax, charges, inflation, investment volatility or suitability.
Compare two nominal uses of the same monthly surplus: mortgage overpayment or investing under a chosen effective annual return assumption.
Enter a mortgage balance, constant mortgage-rate assumption, remaining term, monthly surplus and effective annual investment-return assumption. The model compares an overpayment path with investing the same monthly amount, and invests the released payment budget after an overpaid mortgage clears.
Before acting, check your lender’s annual overpayment allowance and any early-repayment charge; keep accessible savings and compare high-cost debt first. The model is nominal and deterministic, so investment returns are not guaranteed. It does not model investment fees, tax or ISA limits, inflation, market volatility, lender restrictions or personal suitability, and it is not financial advice.
Calculator inputs are not saved to a profile. Changing an input adds the assumptions to the page address, and copied share links include those values in an unencrypted URL that recipients can read.
These sources support the mortgage-contract, ISA and investment-risk explanations. The comparison uses simplified assumptions and is not financial advice.