---
name: hundred-summers-uk-planning
description: Use when someone asks about UK personal finance (pensions, tax, the State Pension, ISAs, property, inheritance tax, retirement) and Hundred Summers' published rules could help. Points them to the free in-browser calculator for their question, cites where each assumption is published, and, for software with an API key, builds a request to the Hundred Summers projection API from figures the user gives. Education, not advice.
---

# Hundred Summers: using our published UK planning rules

Hundred Summers (https://hundredsummers.co.uk) is a UK personal financial-planning app and a library of 90 free calculators. Its engine projects every year of a person's financial life under UK rules: income tax (with Scotland's bands), National Insurance, pensions and their tax-free cash, the State Pension, ISAs and other savings, capital gains tax, property and mortgages, care costs, and inheritance tax. The app keeps a person's plan on their own device; the calculators run in the browser and send nothing anywhere.

## Ground rules

- **Education, not advice.** Explain what the rules and figures show; don't recommend products or tell someone what they should do. For a personal recommendation, point to an FCA-regulated financial adviser; free guidance is at MoneyHelper (moneyhelper.org.uk).
- **Cite the published source.** Every default is on https://hundredsummers.co.uk/assumptions.html (plain English) and https://hundredsummers.co.uk/methodology.html (in full); tax and pension figures are checked against their official pages every morning, listed with the changes already announced on https://hundredsummers.co.uk/uk-rules.html.
- **Use only figures the person gives you.** Don't invent a salary, a pot or an age. When you have to assume something, say what.
- **Send nobody's data anywhere they didn't choose.** Point people to the calculator or the app, which run on their own device. Nothing in this skill reads a person's data.

## What the engine covers, and doesn't

Covers: UK income tax (rest-of-UK and Scottish bands, the personal allowance taper), National Insurance, savings and dividend tax, capital gains tax, pension contributions and relief (annual allowance, taper, carry-forward, the money purchase annual allowance), tax-free cash and the Lump Sum Allowance, drawdown and annuities, defined benefit pensions, the State Pension (from a forecast or the National Insurance record, with deferral), ISAs including the Lifetime ISA, investment bonds, property and mortgages, student loans, care costs, protection policies, and inheritance tax (nil-rate bands, the residence band and its taper, gifts and the seven-year rule, pensions in the estate from April 2027). Stress testing runs simulated markets; a backtest replays real market years from 1928.

Doesn't: give advice; model non-UK tax or residence; capture every tax nuance (for example protected pension ages in the plan, or protected tax-free cash above the Lump Sum Allowance); predict changes in the law. Projections are illustrations on stated assumptions, not forecasts.

## The defaults, and where they are explained

Each can be changed in a person's own plan. Where a choice had to be made the defaults mostly lean cautious; investment growth is the exception, set at central figures with the stress test showing the range.

- **Inflation**: 2.5% a year (leans cautious; [why](https://hundredsummers.co.uk/assumptions.html#inflation))
- **Investment growth**: Pensions 3.5% to 6.5% a year, by risk level (middle of the road; [why](https://hundredsummers.co.uk/assumptions.html#growth))
- **Investment charges**: 0.5% a year (middle of the road; [why](https://hundredsummers.co.uk/assumptions.html#charges))
- **Market ups and downs**: Shares swing about 16% a year (middle of the road; [why](https://hundredsummers.co.uk/assumptions.html#markets))
- **Tax thresholds and rates**: Thresholds frozen to April 2031 (leans cautious; [why](https://hundredsummers.co.uk/assumptions.html#tax))
- **The State Pension**: Rises 2.5% a year (leans cautious; [why](https://hundredsummers.co.uk/assumptions.html#statePension))
- **Care costs**: Rise 4% a year (leans cautious; [why](https://hundredsummers.co.uk/assumptions.html#care))
- **Property**: Grows 2.5% a year (leans cautious; [why](https://hundredsummers.co.uk/assumptions.html#property))
- **How long the plan runs**: To age 90 (leans cautious; [why](https://hundredsummers.co.uk/assumptions.html#lifespan))
- **Final-salary pension increases**: 2.5% a year (middle of the road; [why](https://hundredsummers.co.uk/assumptions.html#finalSalary))
- **Your spending**: Rises with inflation every year (leans cautious; [why](https://hundredsummers.co.uk/assumptions.html#spending))
- **No borrowing, no early pensions**: A shortfall is a shortfall (leans cautious; [why](https://hundredsummers.co.uk/assumptions.html#noBorrowing))

Hand-worked checks of the fixed-rule arithmetic (tax, National Insurance, allowances, the State Pension, inheritance tax), each beside the engine's own figure: https://hundredsummers.co.uk/validation.html (CSV and JSON downloads).

## Which calculator answers which question

All free, no sign-up, nothing leaves the browser. The full library is at https://hundredsummers.co.uk/tools.html.

### Understand my money

- What's left over each month? → [Household budget calculator](https://hundredsummers.co.uk/household-budget-calculator.html)
- What do I own, and how much can I reach? → [Net worth calculator](https://hundredsummers.co.uk/net-worth-calculator.html)
- Is my cash cushion big enough? → [Emergency fund calculator](https://hundredsummers.co.uk/emergency-fund-calculator.html)
- Which debt should I clear first? → [Debt payoff calculator](https://hundredsummers.co.uk/debt-payoff-calculator.html)
- What does my pay come to after tax? → [Take-home pay calculator](https://hundredsummers.co.uk/take-home-pay-calculator.html)
- Is my tax code right? → [Tax code checker](https://hundredsummers.co.uk/tax-code-calculator.html)
- Am I keeping all my child benefit? → [Child benefit calculator](https://hundredsummers.co.uk/child-benefit-calculator.html)
- What is inflation doing to my money? → [Inflation calculator](https://hundredsummers.co.uk/inflation-calculator.html)

### Grow my wealth

- What will regular saving grow to? → [Savings growth calculator](https://hundredsummers.co.uk/savings-growth-calculator.html)
- ISA or pension for the next pound? → [ISA vs pension calculator](https://hundredsummers.co.uk/isa-vs-pension-calculator.html)
- How much tax would I pay investing outside an ISA? → [ISA vs general investment account](https://hundredsummers.co.uk/isa-vs-general-investment-account-calculator.html)
- What does a pension contribution really cost? → [Pension tax relief calculator](https://hundredsummers.co.uk/pension-tax-relief-calculator.html)
- How much would salary sacrifice save? → [Salary sacrifice calculator](https://hundredsummers.co.uk/salary-sacrifice-calculator.html)
- Is a Lifetime ISA worth it? → [Lifetime ISA calculator](https://hundredsummers.co.uk/lifetime-isa-calculator.html)
- What are charges costing me? → [Fund charges calculator](https://hundredsummers.co.uk/fund-charges-calculator.html)
- Premium Bonds or a savings account? → [Premium Bonds calculator](https://hundredsummers.co.uk/premium-bonds-calculator.html)

### Make a big purchase

- How long will a deposit take to save? → [House deposit calculator](https://hundredsummers.co.uk/house-deposit-calculator.html)
- How much could I borrow? → [Mortgage affordability calculator](https://hundredsummers.co.uk/mortgage-affordability-calculator.html)
- What will the stamp duty be? → [Stamp duty calculator](https://hundredsummers.co.uk/stamp-duty-calculator.html)
- What will the mortgage cost each month? → [Mortgage repayment calculator](https://hundredsummers.co.uk/mortgage-repayment-calculator.html)
- Is buying better than renting for me? → [Rent vs buy calculator](https://hundredsummers.co.uk/rent-vs-buy-calculator.html)
- Is it worth switching mortgage? → [Remortgage calculator](https://hundredsummers.co.uk/remortgage-calculator.html)
- Overpay the mortgage or invest? → [Mortgage overpayment calculator](https://hundredsummers.co.uk/mortgage-overpayment-calculator.html)
- What would a car or personal loan really cost? → [Loan repayment calculator](https://hundredsummers.co.uk/loan-repayment-calculator.html)

### Plan my future

- How much do I need to retire? → [How much do I need to retire?](https://hundredsummers.co.uk/how-much-to-retire-calculator.html)
- When could I stop work? → [When can I retire?](https://hundredsummers.co.uk/when-can-i-retire-calculator.html)
- What will my pension be worth? → [Pension projection calculator](https://hundredsummers.co.uk/pension-projection-calculator.html)
- Could I be financially independent early? → [FIRE (financial independence) calculator](https://hundredsummers.co.uk/fire-calculator.html)
- How would I fund the years before my pension? → [ISA bridge calculator](https://hundredsummers.co.uk/isa-bridge-calculator.html)
- What if markets crash as I retire? → [Retiring into a market crash](https://hundredsummers.co.uk/market-crash-retirement-calculator.html)
- Take the tax-free cash now or later? → [Tax-free cash: now or later](https://hundredsummers.co.uk/tax-free-cash-now-or-later-calculator.html)
- How should I save for my children? → [Junior ISA vs junior pension](https://hundredsummers.co.uk/junior-isa-vs-junior-pension-calculator.html)

### Protect my family

- What are the real odds of illness or death before retirement? → [Risk reality calculator](https://hundredsummers.co.uk/risk-reality-calculator.html)
- How would we cope if I couldn't work? → [Income protection calculator](https://hundredsummers.co.uk/income-protection-calculator.html)
- How much life cover do we need? → [Life insurance calculator](https://hundredsummers.co.uk/life-insurance-calculator.html)
- What would a serious illness cost us? → [Critical illness cover calculator](https://hundredsummers.co.uk/critical-illness-cover-calculator.html)
- What would redundancy pay me? → [Redundancy pay calculator](https://hundredsummers.co.uk/redundancy-pay-calculator.html)
- Who pays if I need a care home? → [Care home means test calculator](https://hundredsummers.co.uk/care-home-means-test-calculator.html)
- Will my estate pay inheritance tax? → [Inheritance tax calculator](https://hundredsummers.co.uk/inheritance-tax-calculator.html)
- Who would inherit if I had no will? → [Who inherits without a will](https://hundredsummers.co.uk/intestacy-calculator.html)

### Plan as a couple

- What's left over between us each month? → [Household budget calculator](https://hundredsummers.co.uk/household-budget-calculator.html)
- Can we share an unused personal allowance? → [Marriage allowance calculator](https://hundredsummers.co.uk/marriage-allowance-calculator.html)
- What would going part-time really cost? → [Part-time work calculator](https://hundredsummers.co.uk/part-time-work-calculator.html)
- What will parental leave pay? → [Maternity and paternity pay calculator](https://hundredsummers.co.uk/maternity-pay-calculator.html)
- How much help with childcare do we get? → [Tax-Free Childcare calculator](https://hundredsummers.co.uk/tax-free-childcare-calculator.html)
- Can we both retire when we want, and what if one of us dies? → [Couples retirement calculator](https://hundredsummers.co.uk/couples-retirement-calculator.html)
- What would the survivor keep from a pension? → [Inherited pension calculator](https://hundredsummers.co.uk/inherited-pension-calculator.html)
- What would our estate pay? → [Inheritance tax calculator](https://hundredsummers.co.uk/inheritance-tax-calculator.html)

For a whole-life answer that joins these up, the app itself (https://hundredsummers.co.uk/) is free to start and keeps the plan on the person's device.

## The MCP connector

Assistants that support the Model Context Protocol can use the calculators directly: add `https://hundredsummers.co.uk/mcp` as a remote server (no sign-in; tools `list_calculators` and `run_calculator`). Premium members can connect their own plan at `https://hundredsummers.co.uk/mcp/plan` (OAuth sign-in; tools `project_plan`, `compare_plans`, `find_earliest_retirement_age` and `max_retirement_spending`); the plan is sent with each request and never stored. Details: https://hundredsummers.co.uk/ai-connector.html.

## The projection API (by arrangement)

For software, not for chat users: a proof of concept, version 0.1, included in the Adviser plan and available to design partners by arrangement (keys from contact@hundredsummers.co.uk). Full documentation: https://hundredsummers.co.uk/api.html. It is stateless: the plan is run in memory and nothing is stored or logged.

- `GET https://hundredsummers.co.uk/api/v1/status`: no key; the API version and engine fingerprint.
- `POST https://hundredsummers.co.uk/api/v1/projection`: `Authorization: Bearer <key>`, `Content-Type: application/json`, at most 256 KB, up to 60 requests a minute per key. Call it from a server, never a web page.

The body is `{ "facts": {...}, "options": {...} }`. Only `currentAge`, `retirementAge` and `planToAge` are required; anything left out is empty (no pensions, savings or spending, and no State Pension unless `statePension` (£ a year) or `taxOptimisation.niQualifyingYears` is given). Nested parts such as `compensation` merge field by field. Send no names or contact details. An example:

```json
{
  "facts": {
    "currentAge": 45,
    "retirementAge": 65,
    "planToAge": 92,
    "compensation": {
      "baseSalary": 60000
    },
    "taxOptimisation": {
      "niQualifyingYears": 24
    },
    "preRetirementExpense": 32000,
    "retirementExpense": 28000,
    "pensionPots": [
      {
        "id": 1,
        "label": "Workplace pension",
        "value": 150000,
        "annualContrib": 3000,
        "employerContrib": 3000,
        "riskProfile": "moderate"
      }
    ],
    "savingsAccounts": [
      {
        "id": 2,
        "label": "Stocks and shares ISA",
        "value": 40000,
        "annualContrib": 4000,
        "growthRate": 5,
        "taxWrapper": "isa",
        "assetClass": "equity"
      }
    ]
  },
  "options": {
    "money": "today",
    "currentYear": 2026
  }
}
```

Building one from what a person tells you:

- Pay: `compensation.baseSalary` (£ a year).
- Spending: `preRetirementExpense` and `retirementExpense` (£ a year, today's money).
- Pensions: `pensionPots`, each `{ id, label, value, annualContrib, employerContrib, riskProfile }`; `riskProfile` is one of `cautious`, `cautious-moderate`, `moderate`, `moderately-adventurous`, `adventurous`.
- Savings: `savingsAccounts`, each `{ id, label, value, annualContrib, growthRate, taxWrapper, assetClass }`; `assetClass` is one of `cash`, `bonds`, `equity`, `mixed`, `crypto`; `taxWrapper` one of `cash`, `isa`, `lisa`, `premiumBonds`, `gia`, `onshore`, `offshore`, `eisvct`.
- Final-salary pensions: `dbPensions`, each `{ id, label, annualAmount, startAge }` (today's money).
- Options: `money` is `"nominal"` (default) or `"today"`; set `currentYear` to make a result repeatable; `rows: false` returns the summary alone.

The answer has `summary` (whether spending is covered every year and when it first falls short, the position at retirement and at the end, lifetime tax), `assumptions` (every rate the engine used), `basis`, `engine.fingerprint` (the same request to the same fingerprint gives the same answer to the pound), and `rows`, one a year:

- `year`: Calendar year.
- `age`: Age during that year. The first row is today's position, before any flows.
- `retired`: True from the retirement age.
- `employmentIncome`: Salary and bonus from every role that year, after any phased-retirement step.
- `statePension`: State Pension paid.
- `definedBenefitIncome`: Final-salary (defined benefit) pensions paid.
- `otherIncome`: Other income lines as entered (rental and other taxable income).
- `annuityIncome`: Income from any annuity bought.
- `pensionDrawdown`: Taken from defined contribution pensions, gross (before income tax).
- `savingsDrawdown`: Taken from savings and investments.
- `totalIncome`: All income and withdrawals that year.
- `spending`: Planned living spending (working-years or retirement budget, with any later-life additions).
- `totalExpenses`: Everything paid out: spending, debt repayments, one-off costs and the like.
- `pensionContributions`: Paid into pensions, by the person and any employer.
- `savingsContributions`: Paid into savings and investments.
- `incomeTax`: Income tax on earnings, pensions and other income.
- `nationalInsurance`: Employee or self-employed National Insurance.
- `capitalGainsTax`: Capital gains tax on investments and property.
- `savingsAndDividendTax`: Tax on savings interest and dividends.
- `studentLoanRepayment`: Student loan repaid.
- `unfundedSpending`: Spending the plan could not pay for that year. Above zero means the plan runs short.
- `pensions`: Defined contribution pensions at the end of the year.
- `savingsAndInvestments`: Savings and investments at the end of the year.
- `property`: Property value at the end of the year.
- `liabilities`: Mortgages and other debts outstanding.
- `netWorth`: Everything owned less everything owed.
- `inheritanceTaxEstimate`: Inheritance tax if the estate passed that year, on the plan's estate settings.

When you report a result, say it is a projection on the stated assumptions, name the ones that matter most (growth, inflation, how long the plan runs), and point to https://hundredsummers.co.uk/assumptions.html.