ForHosting KIT · Developer Utilities

Marginal cost from cost function calculator

Marginal cost is the extra cost of producing one more unit of output. In intermediate microeconomics the usual short-run total cost function is quadratic, TC(Q) = a + bQ + cQ², where a is fixed cost and the remaining terms are variable cost.

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Differentiating those variable-cost terms with respect to Q gives the analytic marginal-cost schedule MC(Q) = b + 2cQ. This calculator accepts the three coefficients of that cost function and a chosen output level, then returns MC evaluated at that Q together with the implied total and variable cost so you can check the arithmetic against a textbook table. Fixed cost does not appear in the derivative, which is why two plants that share the same variable-cost coefficients share the same MC curve even when their rents differ. Students use the identity to sketch MC next to AVC and ATC; operators use it when a fitted quadratic cost curve must become a unit-cost figure at a planned run size. The same pure analytic path powers the free browser widget and the API, so homework keys and production notebooks never disagree on a derivative that is only a few multiplications.

How to use it

Enter your values in the form above. The tool checks them before calculating and shows the result on the same page.

Check your inputs

Use the labels and units shown next to each field. If something is missing or outside the allowed range, the page points to the field to fix.

Use it again or automate it

Use the browser tool for individual checks and the API when you need the same capability in an automated workflow.

What you can do with it

Get an answer now

Enter one set of values and see the result without building a spreadsheet or script.

Compare scenarios

Change one value at a time and rerun the calculation to understand what affects the result.

Automate repeated work

Use the API when the same calculation needs to run inside your product or workflow.

FAQ

How do I use this capability?

Complete the fields above and run it on this page. The form highlights anything that needs attention.

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/econ/marginal-cost-from-cost-function

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/econ/marginal-cost-from-cost-function \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"fixed_cost":100,"linear_coef":20,"quadratic_coef":2,"quantity":10}'

const res = await fetch("https://api.kit.forhosting.com/econ/marginal-cost-from-cost-function", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "fixed_cost": 100,
    "linear_coef": 20,
    "quadratic_coef": 2,
    "quantity": 10
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/econ/marginal-cost-from-cost-function",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "fixed_cost": 100,
    "linear_coef": 20,
    "quadratic_coef": 2,
    "quantity": 10
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/econ/marginal-cost-from-cost-function", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"fixed_cost":100,"linear_coef":20,"quadratic_coef":2,"quantity":10}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"fixed_cost":100,"linear_coef":20,"quadratic_coef":2,"quantity":10}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/econ/marginal-cost-from-cost-function", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "fixed_cost": 100,
  "linear_coef": 20,
  "quadratic_coef": 2,
  "quantity": 10
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "econ.marginal_cost_from_cost_function",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Read the full KIT documentation →