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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.
Content updated . Assumptions reviewed .
Formula and assumptions
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.
Assumptions reviewed:
Sources
- KlarFort calculation methodology. Free to use with the credit line; vintage 1.
Parameters
| Name | Description | Type | Default | Minimum | Maximum |
|---|---|---|---|---|---|
| currency | Currency | string | "USD" | ||
| mode | FIRE mode | string | "standard" | ||
| principal | Current invested balance | number | 100000 | 0 | 1,000,000,000,000 |
| monthlyExpenses | Target monthly spending | number | 2000 | 0 | 1,000,000,000,000 |
| withdrawalRate | Annual withdrawal rate (%) | number | 4 | 0.1 | 100 |
| annualReturn | Annual return (%) | number | 0 | -99 | 100 |
| inflationRate | Annual inflation (%) | number | 0 | -99 | 100 |
| savingMode | Saving input | string | "amount" | ||
| monthlyContribution | Monthly invested saving | number | 1000 | 0 | 1,000,000,000,000 |
| monthlyIncome | Monthly take-home income | number | 5000 | 0 | 1,000,000,000,000 |
| savingsRate | Saving rate (%) | number | 20 | 0 | 100 |
| monthlyBaristaIncome | Monthly part-time income | number | 0 | 0 | 1,000,000,000,000 |
| monthlyPension | Monthly expected pension | number | 0 | 0 | 1,000,000,000,000 |
| retirementMonths | Months until retirement | integer | 240 | 1 | 600 |
| startMonth | Starting month for the projected milestone date, in YYYY-MM format. | string | "2026-09" |
JSON API
GET accepts one input parameter containing URL-encoded JSON. POST sends the JSON input object. Prefer POST for personal figures. Inputs are limited to 16 KiB. POST results are not stored; GET results may be cached for up to 24 hours.
curl -X POST https://klarfort.com/tools/api/v1/fire-calculator -H "Content-Type: application/json" --data '{"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"}'fetch("https://klarfort.com/tools/api/v1/fire-calculator", {method:"POST", headers:{"Content-Type":"application/json"}, body:JSON.stringify({"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"})})import json, urllib.request
request = urllib.request.Request("https://klarfort.com/tools/api/v1/fire-calculator", data=json.dumps({"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"}).encode(), headers={"Content-Type":"application/json"})
print(urllib.request.urlopen(request).read().decode())Connected assistants
Connect using https://klarfort.com/tools/mcp. Discover this calculator with tools/list, then call fire-calculator. Protocol versions 2026-07-28 and 2025-11-25 are supported. Modern requests carry protocol metadata and matching HTTP headers. Set Mcp-Name to params.name for tools/call and to params.uri for resources/read. Older clients initialize first.
Limits: 120 requests per minute per address and approximately 3,000 per minute per edge location. Simulations are capped at 1,000 paths, 600 months and 150,000 path-months (paths multiplied by months). Shorten the duration or reduce paths when that work limit is exceeded. A limit response returns 429 and Retry-After. No exact remaining-request count is available.
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.
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. Links to individual calculators are optional.
Computed with KlarFort Calculators, https://klarfort.com/
The API returns this text in credit and the homepage in credit_url. The optional citation_url identifies the calculator for reference. See usage and credit for the full permission.
KlarFort is not financial, investment, tax, or legal advice. Verify outputs before making material decisions.