Rule of 72 doubling time API

Estimate years to double an investment with the Rule of 72 (yearsToDouble ≈ 72/ratePercent) and contrast it with the exact log(2)/log(1+r) answer plus the approximation error — a quick mental-math heuristic made precise. Answers 'how long to double my money at 7%','rule of 72 for 9%','exact doubling time'.

Price$0.01per request

MethodPOST

Route/v1/calc/pf-rule-of-72

StatusLive

MIME typeapplication/json

Rate limit120/minute

CacheNo cache

calcrule-of-72doubling-timeinvestingcompoundheuristicpersonal-financereturns

API URLhttps://x402.hexl.dev/v1/calc/pf-rule-of-72

Integration docs

Example request

{
  "annualRatePercent": 7
}

Example response

{
  "formula": "yearsToDouble ≈ 72 / ratePercent",
  "yearsToDouble": 10.29,
  "exactYearsToDouble": 10.24,
  "approximationError": 0.041,
  "breakdown": {
    "annualRatePercent": 7,
    "numerator": 72
  }
}

Input schema

{
  "type": "object",
  "required": [
    "annualRatePercent"
  ],
  "properties": {
    "annualRatePercent": {
      "type": "number",
      "description": "annual growth rate in percent",
      "examples": [
        7
      ]
    },
    "numerator": {
      "type": "number",
      "description": "rule numerator, default 72 (use 69.3 or 70 for continuous)",
      "examples": [
        72
      ]
    }
  }
}

Output schema

{
  "type": "object",
  "additionalProperties": true
}