ForHosting KIT · Developer Utilities

d20 success chance vs DC calculator

In Dungeons and Dragons style games and many other d20 systems, almost every attack roll, skill check, and saving throw asks the same probability question: if I roll one twenty-sided die, add my modifier, and need to meet or beat a difficulty class, how often do I succeed?

● BetaFree · in your browser

Run — free How it works & API

Use it from WebAPIEmailTelegramApp soon

Mental arithmetic gets messy when modifiers turn negative, when the DC sits outside the usual 10 to 20 band, or when you must compare two builds quickly at the table. This d20 success chance versus DC calculator answers with an exact fraction of the twenty faces that work, not a simulation and not a rule-of-thumb chart. You supply the integer modifier and the target DC; the engine counts every natural result from one through twenty that satisfies face plus modifier greater than or equal to the DC, then returns probability, percent, the minimum natural roll you need, and flags for auto-success and auto-fail. The same pure module powers the free browser widget and the API path, so a GM toolkit, a character builder, and a blog widget never disagree on a DC fifteen with plus five.

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/hobby/game-d20-success

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/hobby/game-d20-success \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"modifier":5,"dc":15}'

const res = await fetch("https://api.kit.forhosting.com/hobby/game-d20-success", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "modifier": 5,
    "dc": 15
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/hobby/game-d20-success",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "modifier": 5,
    "dc": 15
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/hobby/game-d20-success", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"modifier":5,"dc":15}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"modifier":5,"dc":15}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/hobby/game-d20-success", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "modifier": 5,
  "dc": 15
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "hobby.game_d20_success",
  "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 →