# FIRE and Coast FIRE calculator documentation

Inputs, formulas, assumptions and API/MCP examples for the fire and coast fire calculator. Reproduce a calculation and cite its sources.

Canonical calculator: https://klarfort.com/tools/fire-calculator/
Content updated: 2026-09-07
Assumptions reviewed: 2026-09-07

## Overview
FIRE means financial independence, retire early. This calculator turns the annual spending not covered by entered income into an investment target using your chosen withdrawal rate.

Estimate the months needed to reach that target with regular saving, or calculate the balance needed today for Coast FIRE without future contributions. All spending and saving amounts are expressed in today's money.

## How to use
1. Choose Standard, Lean, Fat, Barista, Coast or Expat mode and enter your target monthly spending.
2. Enter the invested balance, withdrawal rate, return and inflation assumptions.
3. Enter saving as a monthly amount or a share of take-home income. Add pension or part-time income where the selected mode uses it.
4. Set the starting month and, for Coast, months until retirement. Calculate and review the target, timing and real return.

## Understanding the result
The FIRE target divides annual spending left after modeled income offsets by the withdrawal rate. Months to freedom is the first whole month the investment balance reaches that target, within 600 months.

Coast target is the balance needed now to grow to the retirement target without more contributions. Reaching it does not fund current living costs or guarantee future returns or withdrawals.

## Questions
### How do Lean and Fat FIRE differ in this calculator?
Both use the monthly spending you enter, with no hidden multiplier. Choose a spending target that represents the scenario you want to explore; changing the mode alone does not change that target.

### How are Barista income and pensions treated?
Barista mode subtracts entered part-time income from spending as well as the entered pension. Expat mode excludes pension income. Other modes use the entered pension as an ongoing income offset in today's money.

### Does the withdrawal rate predict how long money lasts?
No. It is an editable planning assumption used to calculate a target. The model uses a constant real return and does not determine whether withdrawals can continue indefinitely or establish pension eligibility.

## Default worked example
Months to target: 500 months. Months to your entered spending target under a constant real-return assumption. Assumptions as of 2026-09-07.

## Formula
FIRE target = annual unfunded spending / withdrawal rate. Real growth = (1 + nominal return)/(1 + inflation) − 1. Coast target = FIRE target / (1 + real growth)^(retirement months/12).

- All spending and savings are in today's money. Lean and Fat use your entered spending target without a hidden multiplier. Barista offsets spending with entered part-time income. Expat ignores pension income.
- Savings may be an entered monthly amount or a percentage of take-home income. Beginning-month contributions grow at the entered real return. Freedom duration is the first whole month reaching the target, capped at 600 months.
- The withdrawal rate is an editable planning assumption, not a guarantee. A pension is treated as an enduring real income offset; enter zero when it will not be available at the target date.

## Inputs
- currency: Currency; default "USD"
- mode: FIRE mode; default "standard"
- principal: Current invested balance; default 100000
- monthlyExpenses: Target monthly spending; default 2000
- withdrawalRate: Annual withdrawal rate (%); default 4
- annualReturn: Annual return (%); default 0
- inflationRate: Annual inflation (%); default 0
- savingMode: Saving input; default "amount"
- monthlyContribution: Monthly invested saving; default 1000
- monthlyIncome: Monthly take-home income; default 5000
- savingsRate: Saving rate (%); default 20
- monthlyBaristaIncome: Monthly part-time income; default 0
- monthlyPension: Monthly expected pension; default 0
- retirementMonths: Months until retirement; default 240
- startMonth: Starting month for the projected milestone date, in YYYY-MM format.; default "2026-09"

## Interfaces
Input schema: https://klarfort.com/tools/fire-calculator/schema.json
Worked request and response: https://klarfort.com/tools/fire-calculator/examples.json
Public table: https://klarfort.com/tools/fire-calculator/table.json
JSON API: POST https://klarfort.com/tools/api/v1/fire-calculator
MCP endpoint: POST https://klarfort.com/tools/mcp
MCP tool name: fire-calculator

## JSON request body
```json
{
  "currency": "USD",
  "mode": "standard",
  "principal": 100000,
  "monthlyExpenses": 2000,
  "withdrawalRate": 4,
  "annualReturn": 0,
  "inflationRate": 0,
  "savingMode": "amount",
  "monthlyContribution": 1000,
  "monthlyIncome": 5000,
  "savingsRate": 20,
  "monthlyBaristaIncome": 0,
  "monthlyPension": 0,
  "retirementMonths": 240,
  "startMonth": "2026-09"
}
```

## Usage and credit
Free for personal, commercial and any other lawful use with credit to KlarFort at https://klarfort.com/. Third-party data terms still apply.

Computed with KlarFort Calculators, https://klarfort.com/

Place a visible credit near the reused material or in the credits for your work or integration. One clear credit per work or integration is sufficient; no separate credit per API request is required. The credit_url field contains the homepage. The citation_url field is an optional calculator reference. Full permission: https://klarfort.com/tools/docs/license/

## Sources
KlarFort calculation methodology: https://klarfort.com/tools/methodology/ (1)