Methodology
No black boxes. Here is how every Casvia calculator works, the formula it uses, and the official source for each UK figure.
Compound interest (Compound Interest Calculator)
The lump sum uses the classic formula A = P × (1 + r/f)f·t, where P is the initial amount, r the annual rate (as a decimal), f the compounding frequency per year and t the years. Monthly contributions are added as a monthly-compounded annuity. Interest earned = future value − total paid in.
Investment return (Investment Return Calculator)
Future value is modelled with monthly compounding: FV = P × (1 + r/12)12t + M × [(1 + r/12)12t − 1] / (r/12), where P is the initial investment, M the monthly investment, r the annual return and t the years. Growth = FV − total invested. In target mode the monthly amount is solved numerically (bisection) so that FV equals the target. No fees, inflation or tax are modelled.
Core growth model
The ISA and Savings calculators share the same growth model. Interest or returns are compounded monthly, and regular contributions are assumed to be paid at the end of each month. The future value is:
FV = P(1 + r/12)12t + M × [ (1 + r/12)12t − 1 ] / (r/12)
- P = initial amount (already in the ISA, or starting balance)
- M = monthly contribution
- r = assumed annual return (as a decimal, e.g. 0.05 for 5%)
- t = number of years
Each result is split into contributions (money you put in) and growth (everything earned on top), so you can see how much of the pot came from your own saving.
Lifetime ISA bonus (ISA Calculator)
For a Lifetime ISA, each year's contribution is capped at £4,000 and the government adds 25% (max £1,000 per year). The bonus is modelled as credited at the end of each year before the next year compounds. The £4,000 counts within your overall £20,000 ISA allowance. GOV.UK · 2026/27
Inflation adjustment (Savings Calculator)
When enabled, the nominal future value is discounted back to today's purchasing power: Real value = FV / (1 + i)t, where i is the annual inflation rate. Each year's contributions are discounted separately to today's pounds.
Income Tax (Income Tax & Salary calculators)
Income Tax is charged on taxable income = gross income − Personal Allowance. The standard Personal Allowance is £12,570; above £100,000 it reduces by £1 for every £2 until it reaches zero at £125,140. Taxable income is then taxed at 20% up to £37,700 of taxable income, 40% from £37,701 to £112,570, and 45% above that (England, Wales & NI). GOV.UK · 2026/27
National Insurance (Salary Calculator)
Class 1 employee National Insurance is charged on earnings above the Primary Threshold (£12,570) at 8% up to the Upper Earnings Limit (£50,270), then 2% on earnings above that. It is separate from Income Tax and is calculated on gross earnings, not after the Personal Allowance. GOV.UK · 2026/27
Mortgage (Mortgage Calculator)
For a repayment mortgage the monthly payment uses the annuity formula: M = P × r(1 + r)n / [ (1 + r)n − 1 ], where P is the loan amount (property price − deposit), r is the monthly interest rate (annual ÷ 12) and n is the number of monthly payments. Total interest = M × n − P. For an interest-only mortgage, the monthly payment is P × r. LTV = loan ÷ property price.
Mortgage overpayment (Mortgage Overpayment Calculator)
The calculator simulates the loan month by month. Each month: interest = balance × (annual rate ÷ 12), then the balance is reduced by (monthly repayment + overpayment). Interest saved = total interest without overpayments − total interest with overpayments; time saved = original months − months to clear. Assumes a constant rate, no fees and monthly overpayments from day one.
Stamp Duty (Stamp Duty Calculator)
Property purchase tax is progressive: each band rate applies only to the slice of the price inside that band, so tax = Σ slice × band rate. Bands differ by nation — SDLT (England & NI), LBTT (Scotland) and LTT (Wales) — with separate first-time buyer reliefs. Additional-property surcharges are not modelled. GOV.UK · gov.scot · gov.wales · 2026/27
Pension (Pension Calculator)
The pot is projected with monthly compounding using the same annuity model as savings. After-tax cost = monthly contribution × (1 − tax relief rate). The 25% tax-free lump sum is capped at the lump sum allowance of £268,275 (2026/27). The annual allowance is £60,000 for 2026/27. GOV.UK · 2026/27
Retirement (Retirement Calculator)
Shortfall = desired annual income − guaranteed income (e.g. full new State Pension £12,547.60/year for 2026/27). Pot needed = shortfall ÷ 4% (the 4% withdrawal rule of thumb). Monthly saving is solved so that the pot reaches the target by retirement. GOV.UK · 2026/27
Capital Gains Tax (Capital Gains Tax Calculator)
Chargeable gain = proceeds − allowable costs. Taxable gain = chargeable gain − annual exempt amount (£3,000, 2026/27). Gains within the unused basic-rate band are taxed at 18%, the remainder at 24%. Business Asset Disposal Relief gains are taxed at a flat 18%. GOV.UK · 2026/27
Dividend Tax (Dividend Tax Calculator)
The first £500 of dividend income is tax-free. The rest is taxed at the rate for the band your dividends fall into on top of other income: 10.75% basic, 35.75% higher, 39.35% additional (2026/27). GOV.UK · 2026/27
Inflation (UK Inflation Calculator)
Annual compounding: future nominal = A × (1 + r)^n; today's value of a future amount = A ÷ (1 + r)^n. Latest CPI is 2.9% (July 2026, ONS); the Bank of England targets 2%. ONS · Jul 2026
Key assumptions
- A constant annual return with monthly compounding — real returns vary year to year and are not guaranteed.
- No fees, charges or platform costs.
- No withdrawals during the period.
- ISA growth is modelled tax-free (correct for ISAs in the UK).
Official data sources & years
Every UK figure on this site comes from an official source and is labelled with the tax year it applies to. Current figures (2026/27 tax year, 6 April 2026 – 5 April 2027):
| Figure | Value | Tax year | Source |
|---|---|---|---|
| Adult ISA annual allowance | £20,000 | 2026/27 | GOV.UK — ISAs |
| Lifetime ISA annual limit | £4,000 (25% bonus) | 2026/27 | GOV.UK — Lifetime ISA |
| Junior ISA annual allowance | £9,000 | 2026/27 | HMRC — Junior ISAs |
| Personal Savings Allowance (interest) | £1,000 / £500 / £0 | 2026/27 | GOV.UK — Tax on savings interest |
| Cash ISA limit (under 65) — future | £12,000 | 2027/28 (from 6 Apr 2027) | GOV.UK — Autumn Budget 2025 |
| Personal Allowance (Income Tax) | £12,570 | 2026/27 | GOV.UK — Income Tax rates |
| Income Tax bands (rUK) | 20% / 40% / 45% | 2026/27 | GOV.UK — Income Tax rates |
| NI Primary Threshold / UEL (Class 1) | £12,570 / £50,270 (8% / 2%) | 2026/27 | GOV.UK — NI rates & allowances |
Rules and thresholds can change. Figures above were checked against official sources on the date of publication. Always verify against the latest official guidance before making financial decisions.
How we check our calculators
Every calculator on Casvia goes through the same quality process before it is published, and the underlying rates are reviewed when a new tax year begins:
- Formula review. Each formula is checked against standard financial mathematics — the annuity formula for repayments, progressive bands for Stamp Duty, the Personal Allowance taper for Income Tax, and so on.
- Boundary testing. Tax thresholds and allowance boundaries are tested separately, including the £100,000–£125,140 Personal Allowance taper, the £4,000 LISA cap, the ISA allowance and every Stamp Duty band ceiling.
- Source verification. UK rates and allowances are checked against HMRC, GOV.UK, the Scottish and Welsh governments and other official sources, and are labelled with the tax year they apply to.
- Browser calculation. Calculations run locally in your browser. No inputs are sent to a server and no data is stored.
- Update policy. Tax and allowance data is reviewed when a new tax year begins. Where a future change is already announced (such as the 2027/28 Cash ISA limit), it is noted on the relevant page.