FI Calculator Guide

The FI Calculator is one of IndepAI’s most powerful tools. It calculates your path to Financial Independence based on your income, expenses, savings, and expected returns.

How It Works

The FI Calculator uses the following formula:

FI Number = Annual Expenses × FI Multiplier

Where the FI Multiplier is typically 25 (based on the 4% safe withdrawal rate):

FI Multiplier = 100 / Withdrawal Rate = 100 / 4 = 25

To calculate years to FI, we use compound growth:

Years to FI = ln((FI Number × r + Savings) / (Current × r + Savings)) / ln(1 + r)

Where r is the real return rate (nominal return - inflation).

Basic Usage

curl -X POST https://indepai.app/api/v1/calculator \
  -H "Content-Type: application/json" \
  -d '{
    "currentAge": 30,
    "annualIncome": 80000,
    "annualExpenses": 40000,
    "currentSavings": 100000
  }'

Input Parameters

Parameter	Type	Required	Default	Description
`currentAge`	number	Yes	-	Your current age (18-100)
`annualIncome`	number	Yes	-	Gross annual income
`annualExpenses`	number	Yes	-	Total annual expenses
`currentSavings`	number	Yes	-	Current invested assets
`targetWithdrawalRate`	number	No	4	Safe withdrawal rate (%)
`expectedReturn`	number	No	7	Expected annual return (%)
`inflationRate`	number	No	2	Expected inflation rate (%)
`email`	string	No	-	Email to associate with calculation
`saveResult`	boolean	No	true	Save for analytics

Response Fields

Field	Description
`fiNumber`	Target amount needed for FI
`yearsToFI`	Years until FI is reached
`fiAge`	Age when you’ll reach FI
`fiDate`	Projected FI date (ISO 8601)
`savingsRate`	Current savings rate (%)
`monthlySavings`	Monthly savings amount
`annualSavings`	Annual savings amount
`currentProgress`	Current progress toward FI (%)
`yearlyProjections`	Year-by-year wealth projections

Advanced Examples

Different Withdrawal Rates

For early retirement (more conservative 3% rate):

{
  "currentAge": 28,
  "annualIncome": 100000,
  "annualExpenses": 35000,
  "currentSavings": 200000,
  "targetWithdrawalRate": 3
}

This increases your FI Number to 33.3× expenses instead of 25×.

Aggressive Returns

If you expect higher returns (e.g., 100% stocks):

{
  "currentAge": 25,
  "annualIncome": 60000,
  "annualExpenses": 30000,
  "currentSavings": 50000,
  "expectedReturn": 10,
  "inflationRate": 2
}

Conservative Scenario

Planning for lower returns and higher inflation:

{
  "currentAge": 35,
  "annualIncome": 90000,
  "annualExpenses": 45000,
  "currentSavings": 150000,
  "expectedReturn": 5,
  "inflationRate": 3
}

Understanding the Projections

The yearlyProjections array shows your wealth trajectory:

{
  "yearlyProjections": [
    {
      "year": 0,
      "age": 30,
      "netWorth": 100000,
      "totalContributions": 100000,
      "totalGains": 0,
      "progressPercent": 10.0,
      "fiReached": false
    },
    {
      "year": 1,
      "age": 31,
      "netWorth": 145000,
      "totalContributions": 140000,
      "totalGains": 5000,
      "progressPercent": 14.5,
      "fiReached": false
    }
    // ... continues until FI + buffer
  ]
}

Use Cases

async function calculateFI(userInput) {
  const response = await fetch("https://indepai.app/api/v1/calculator", {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify(userInput),
  });

  const result = await response.json();

  if (result.success) {
    return {
      targetAmount: result.data.fiNumber,
      yearsRemaining: result.data.yearsToFI,
      retirementAge: result.data.fiAge,
      savingsRate: result.data.savingsRate,
    };
  }

  throw new Error(result.error);
}

2. Scenario Comparison

Compare different scenarios:

const scenarios = [
  { name: "Current", expenses: 40000 },
  { name: "Frugal", expenses: 30000 },
  { name: "Comfortable", expenses: 50000 },
];

const results = await Promise.all(
  scenarios.map((s) =>
    fetch("https://indepai.app/api/v1/calculator", {
      method: "POST",
      headers: { "Content-Type": "application/json" },
      body: JSON.stringify({
        currentAge: 30,
        annualIncome: 80000,
        annualExpenses: s.expenses,
        currentSavings: 100000,
      }),
    }).then((r) => r.json())
  )
);

// Display comparison table
results.forEach((r, i) => {
  console.log(`${scenarios[i].name}: FI in ${r.data.yearsToFI} years`);
});

3. Charting Progress

Use the yearlyProjections to create a growth chart:

const { data } = await result.json();

// For Chart.js or similar
const chartData = {
  labels: data.yearlyProjections.map((p) => p.age),
  datasets: [
    {
      label: "Net Worth",
      data: data.yearlyProjections.map((p) => p.netWorth),
    },
    {
      label: "Contributions",
      data: data.yearlyProjections.map((p) => p.totalContributions),
    },
    {
      label: "FI Target",
      data: data.yearlyProjections.map(() => data.fiNumber),
      borderDash: [5, 5],
    },
  ],
};

Error Handling

Common validation errors:

{
  "success": false,
  "error": "Validation error",
  "code": "VALIDATION_ERROR",
  "details": [
    { "path": "currentAge", "message": "Must be between 18 and 100" },
    { "path": "annualExpenses", "message": "Must be greater than 0" }
  ]
}

Handle them gracefully:

const result = await response.json();

if (!result.success) {
  if (result.code === "VALIDATION_ERROR") {
    result.details.forEach((err) => {
      showFieldError(err.path, err.message);
    });
  } else {
    showGeneralError(result.error);
  }
  return;
}

Rate Limits

The calculator endpoint has these limits:

Tier	Requests/Day
Starter	1,000
Pro	10,000

Check rate limit headers:

X-RateLimit-Limit: 100
X-RateLimit-Remaining: 95
X-RateLimit-Reset: 1705312800

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