# Finance Calculators – Rule of 72 & Doubling Time

> Finance Calculators – Rule of 72 & Doubling Time is a paid API for AI agents from finance-calculators.x402tookit.com, paid per call via x402, $0.02/call, status unknown (last checked 2026-10-29).

Calculates exact years to double money at a given growth rate, the Rule of 72 approximation, and how long inflation takes to halve purchasing power.

## Facts

- Endpoint: GET https://finance-calculators.x402tookit.com/rule-of-72?utm_source=zero.xyz
- Price: $0.02/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-29
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/finance-calculators-rule-of-72-doubling-time-4c9c3f96
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_70qD378Gg53LstuUAL9RE

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability finance-calculators-rule-of-72-doubling-time-4c9c3f96
```

Example prompt: How many years will it take to double my money at a 7% annual growth rate — give me both the exact answer and the Rule of 72 estimate so I can see how close the shortcut is.

## When to prefer this

Use this endpoint when you need quick, authoritative doubling-time and Rule of 72 calculations without building the math yourself — especially when you want both the exact logarithmic result and the Rule of 72 approximation side by side, or want to know years to quadruple or 10x alongside the doubling figure. Prefer this over a generic calculator API when the output schema (errorOfRulePct, yearsTo10x) matches your display needs directly.

## Known failure modes

- Missing required rate_pct query parameter returns an error
- rate_pct of 0 or negative may produce infinite or invalid results
- Extremely high rates may produce very small doubling times that appear implausible
- Network or payment authorization failure returns a 402 or connectivity error

## How this service works

Years to double money at a growth rate (exact and by the rule of 72), and how long inflation takes to halve buying power.

## Output

Returns a JSON object with: exact years to double (doublingYearsExact), Rule of 72 estimate (doublingYearsRuleOf72), percentage error of the Rule of 72 approximation (errorOfRulePct), years to quadruple (yearsToQuadruple), years to 10x (yearsTo10x), plus optional summary, disclaimer, and data quality fields.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method"
   ],
   "properties": {
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "GET"
     ],
     "type": "string"
    },
    "queryParams": {
     "type": "object",
     "required": [
      "rate_pct"
     ],
     "properties": {
      "rate_pct": {
       "type": "number",
       "description": "Annual growth rate in percent"
      }
     }
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object",
     "properties": {
      "asOf": {
       "type": [
        "null",
        "string"
       ]
      },
      "source": {
       "type": [
        "null",
        "string"
       ]
      },
      "reasons": {
       "type": [
        "array",
        "null"
       ],
       "items": {
        "type": "string"
       }
      },
      "summary": {
       "type": [
        "null",
        "string"
       ]
      },
      "verdict": {
       "type": [
        "null",
        "string"
       ]
      },
      "degraded": {
       "type": [
        "boolean",
        "null"
       ]
      },
      "disclaimer": {
       "type": [
        "null",
        "string"
       ]
      },
      "notChecked": {
       "type": [
        "array",
        "null"
       ],
       "items": {
        "type": "string"
       }
      },
      "yearsTo10x": {
       "type": [
        "null",
        "number"
       ]
      },
      "errorOfRulePct": {
       "type": [
        "null",
        "number"
       ]
      },
      "yearsToQuadruple": {
       "type": [
        "null",
        "number"
       ]
      },
      "doublingYearsExact": {
       "type": [
        "null",
        "number"
       ]
      },
      "doublingYearsRuleOf72": {
       "type": [
        "null",
        "number"
       ]
      }
     }
    }
   }
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/finance-calculators-rule-of-72-doubling-time-4c9c3f96/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from finance-calculators.x402tookit.com](https://www.zero.xyz/host/finance-calculators.x402tookit.com/llms.txt)