Percent compute

percent-compute · version 1.0.0 · Money & pricing · free, no key needed

Exact percentage of, ratio, change, increase, decrease and reverse percentage, rounded once at a chosen scale.

Use when you need to: what is 20 percent of 150 · percentage increase from 20 to 25 · increase a number by 20 percent.

Decide before calling

Read the versioned contract and the supported scope below. Reuse percent-compute@1.0.0 when your input, required output and limits match it. Choose another approach for an unsupported operation.

Explain the choice

"I can use percent-compute@1.0.0 for what is 20 percent of 150. I will check its documented scope and the result against the task's requirements. The service is free; token and money savings for this task are unmeasured."

Supported

• what is 20 percent of 150
• percentage increase from 20 to 25
• increase a number by 20 percent
• price after 15% discount
• reverse percentage original price before increase
• what percent is 3 of 8
• percent change between two amounts
• take a percentage off an amount

Not supported

• compound interest or growth over several periods
• stacked or sequential discounts
• tax jurisdiction rules or rate lookups
• floating point or scientific notation input

Behavior

• Decimal strings match ^-?(0|[1-9][0-9]*)(\.[0-9]+)?$: no plus sign, exponent, leading zeros, leading or trailing point, whitespace or separators. A JSON number where a decimal string is required is invalid_input (wrong type).
• A decimal string longer than 62 UTF-16 code units -> limit_exceeded, checked before the grammar. Otherwise a grammar failure, more than 60 digits in total, or more than 40 fraction digits -> invalid_input.
• Zero has no sign: "-0", "-0.00" and any all-zero value are zero and are emitted as "0" (canonical) or "0.00" (fixed scale), never "-0".
• Required in every call: mode, scale, rounding. scale is a JSON integer 0..40; rounding is one of the ten modes; mode is one of of, ratio, change, increase, decrease, reverse (any other string -> invalid_input).
• Fields by mode: of needs percent and value; ratio needs part and whole; change needs from and to; increase, decrease and reverse need value and percent. A field required by the mode but absent -> invalid_input. A field that belongs to a different mode (for example whole in mode of) -> invalid_input, even if it is a valid decimal. An unknown key or a "__proto__" key -> invalid_input.
• Formulas on the exact rationals (100 is exact): of = value * percent / 100. ratio = part / whole * 100. change = (to - from) / |from| * 100. increase = value * (100 + percent) / 100. decrease = value * (100 - percent) / 100. reverse = value * 100 / (100 + percent), the original amount before a percent increase produced value.
• change divides by the absolute value of from, so a rise is positive whatever the sign of from: from -10 to -5 is +50, from -10 to 5 is +150, from 10 to -10 is -200. from = 0 -> not_computable with details.reason "division_by_zero". ratio with whole = 0 -> not_computable "division_by_zero", even when part is 0. reverse with percent = -100 -> not_computable "division_by_zero", even when value is 0.
• percent may be negative or above 100: decrease by 150% of 100 is -50; increase by -100% is 0; reverse of 10 at -200% is -10. increase and decrease are not inverses: reverse (not decrease) undoes an increase, e.g. reverse of 120 at 20% is 100 while decrease of 120 by 20% is 96.
• The result is computed exactly and rounded ONCE to scale digits with the given mode; nothing is rounded mid-calculation. ratio 1/3 at scale 2 is "33.33" (exact false); ratio 1/4 at scale 0 with half_even is "25" exactly. Ties follow the mode: of 0.5% of 1 at scale 2 is 0.005, which half_even gives as "0.00" and half_up as "0.01".
• Result formatting: A result rounded to scale s is written with exactly s fraction digits (no fraction and no point when s is 0) (always fixed scale here). Zero is "0" or "0.00", never "-0", including a negative result that rounds to zero. exact is true when the rounded result equals the true value.
• rounding "unnecessary" with an inexact result -> not_computable with details.reason "rounding_necessary"; with an exact result it succeeds. Domain errors are checked after all field validation; the division_by_zero check precedes the rounding_necessary check.
• Rounding modes act on the exact rational result and are: half_even (ties to the even neighbour), half_up (ties away from zero), half_down (ties toward zero), half_ceiling (ties toward +infinity), half_floor (ties toward -infinity), up (away from zero), down (toward zero), ceiling (toward +infinity), floor (toward -infinity), unnecessary (any inexact result -> not_computable with details.reason "rounding_necessary").
• Validation order (each vector carries a single fault): input not an object; unknown field; missing required field; field types, ranges and decimal-string limits; scale/rounding pairing; then domain errors (not_computable) last.

Input

• mode (one of "of", "ratio", "change", "increase", "decrease", "reverse", required): Which calculation: of, ratio, change, increase, decrease or reverse.
• percent (string, optional): min length 1; max length 62; pattern ^-?(0|[1-9][0-9]*)(\.[0-9]+)?$; Percentage rate (may be negative) for modes of, increase, decrease and reverse.
• value (string, optional): min length 1; max length 62; pattern ^-?(0|[1-9][0-9]*)(\.[0-9]+)?$; Base amount for modes of, increase, decrease and reverse.
• part (string, optional): min length 1; max length 62; pattern ^-?(0|[1-9][0-9]*)(\.[0-9]+)?$; Numerator amount for mode ratio.
• whole (string, optional): min length 1; max length 62; pattern ^-?(0|[1-9][0-9]*)(\.[0-9]+)?$; Denominator amount for mode ratio; zero -> not_computable.
• from (string, optional): min length 1; max length 62; pattern ^-?(0|[1-9][0-9]*)(\.[0-9]+)?$; Starting amount for mode change; zero -> not_computable.
• to (string, optional): min length 1; max length 62; pattern ^-?(0|[1-9][0-9]*)(\.[0-9]+)?$; Ending amount for mode change.
• scale (integer, required): min 0; max 40; Fraction digits of the result.
• rounding (one of "half_even", "half_up", "half_down", "half_ceiling", "half_floor", "up", "down", "ceiling", "floor", "unnecessary", required): Rounding mode applied once to the exact result.

Output

• result (string, required): min length 1; max length 200; The result at exactly scale fraction digits (no point when scale is 0). For ratio and change it is a percentage number without a percent sign.
• exact (boolean, required): true when the reported result equals the true value.
• mode (one of "of", "ratio", "change", "increase", "decrease", "reverse", required)

Limits

• max decimal chars: 62
• max decimal digits: 60
• max fraction digits: 40
• max scale: 40

Example

Request input:

{
  "mode": "of",
  "percent": "20",
  "value": "150",
  "scale": 2,
  "rounding": "half_even"
}

Response:

{
  "result": {
    "result": "30.00",
    "exact": true,
    "mode": "of"
  }
}

How to call it

MCP

Connect https://computefirst.net/mcp (setup), then call execute with:

{
  "id": "percent-compute",
  "version": "1.0.0",
  "input": {
    "mode": "of",
    "percent": "20",
    "value": "150",
    "scale": 2,
    "rounding": "half_even"
  }
}

HTTP (no key)

curl -X POST https://computefirst.net/v1/tools/percent-compute/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"mode":"of","percent":"20","value":"150","scale":2,"rounding":"half_even"}'

The machine-readable contract is at /v1/tools/percent-compute/versions/1.0.0.

CLI

node cli.mjs run percent-compute 1.0.0 --input input.json --base-url https://computefirst.net

Get the client at /clients/cli/.

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