HHI calculator
Market shares of 40, 30, 20 and 10 percent give an HHI of 3,000, highly concentrated under the 2010 merger guidelines. Enter each firm's share of one market and measure concentration on a scale up to 10,000. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Shares
- 40, 30, 20, 10
- N
- 4
- Sum shares
- 100
- Hhi
- 3,000
- Concentration
- highly concentrated
The Herfindahl-Hirschman Index is the workhorse scalar of market concentration: the sum of the squares of each firm's market share, with those shares written as percentages of a defined relevant market. Competition agencies, merger counsel, and industrial-organization students use it because a monopoly prints ten thousand, a handful of large firms prints a high four-figure score, and a fragmented industry prints a much smaller number. This HHI calculator takes the list of those percentages, checks that they add to one hundred, and returns the index together with a concentration band drawn from the 2010 Horizontal Merger Guidelines.
What the Herfindahl-Hirschman Index measures
The Herfindahl-Hirschman Index, often abbreviated HHI, is the sum of the squares of each firm's market share in a defined relevant market, with those shares expressed as percentages rather than fractions. If four firms hold forty, thirty, twenty, and ten percent, the index is sixteen hundred plus nine hundred plus four hundred plus one hundred, which equals three thousand.
Squaring gives large firms more weight than a simple headcount of competitors would: two firms at fifty percent each score five thousand, while ten firms at ten percent each score one thousand, even though both lists add to one hundred. The theoretical ceiling is ten thousand, the square of a one-hundred-percent monopoly. The theoretical floor for a market of n equal firms is ten thousand divided by n, and an atomistic market of many tiny sellers approaches zero.
United States merger practice still treats the 2010 Horizontal Merger Guidelines bands as the public vocabulary: below fifteen hundred is unconcentrated, fifteen hundred through twenty-five hundred is moderately concentrated, and above twenty-five hundred is highly concentrated. This calculator returns that index from the list you supply; it does not define the product market, the geographic market, or the set of competitors — those are legal and economic judgments you make before the arithmetic.
It also does not compute a delta HHI for a merger, a four-firm concentration ratio, or a Lerner index. Those are different searches. You bring percentages that already describe one market in one period; the engine squares them, adds them, and reports the scalar that agencies, courts, and textbooks all call HHI.
Input contract, the sum-to-100 rule, and the response
Each entry must be a finite number between zero and one hundred inclusive; numeric strings are accepted, empty strings and non-numeric text are not. Negative shares and shares above one hundred fail by index so you can see which firm is out of range. After every entry is accepted, the engine adds them.
There is no silent rescaling to force the list onto one hundred percent, because a list that adds to ninety or to one hundred and five is usually a missing firm, a double-counted firm, or a mix of percentages and fractions, and those mistakes must not produce a concentration number that looks official. Example: forty, thirty, twenty, and ten yield hhi three thousand and the highly concentrated label. Failed validation is not billed.
Where an HHI calculator earns its keep
Antitrust counsel use an HHI calculator to screen a proposed combination before they write the full competitive-effects narrative: paste last year's shares, read the index, and know whether the agencies will treat the market as unconcentrated, moderate, or highly concentrated. Corporate-strategy teams track the same number across annual reports so a slide that claims the industry is fragmenting is pinned to a scalar instead of an anecdote.
Economics and MBA classrooms need a deterministic oracle that rejects a list that does not add to one hundred, which is the most common student error, instead of quietly renormalizing and marking a wrong market definition as correct. Journalists and data desks paste published shares from a statistical release or a company filing and quote the index without a spreadsheet that silently changes rounding.
This is a different job from a four-firm concentration ratio, from a Lorenz curve or Gini of firm size, from a Lerner index of pricing power, and from a merger simulation that needs diversion ratios: those tools start from different inputs and answer different questions. Failed validation does not consume balance. Keep the list finite and adding to one hundred, pin the rounded index in tests, and you get a reproducible HHI for classrooms, merger memos, and edge workers without standing up an industrial-organization stack of your own.
What you can do with it
Antitrust and merger screening
Paste last period's firm shares, read the HHI and the guideline band, and know whether a proposed combination sits in an unconcentrated, moderate, or highly concentrated market.
Competitive-strategy memos
Strategy teams pin a reproducible concentration scalar across annual reports instead of claiming fragmentation from a headcount of logos.
FAQ
What happens if the market shares do not sum to 100?
The message reports the observed sum so you can correct the list. There is no silent rescaling. Validation failures are not charged.
What HHI values count as concentrated under the merger guidelines?
Below 1500 is unconcentrated, 1500 through 2500 is moderately concentrated, and above 2500 is highly concentrated. A monopoly of 100 percent scores 10000, the ceiling of the index.
Can I enter market shares as fractions instead of percentages?
No. This calculator expects percentages that add to 100, not fractions that add to 1. A list such as 0.4, 0.3, 0.2, 0.1 sums to 1 and is rejected; send 40, 30, 20, 10 instead.
What does the HHI calculator cost?
The browser tool runs free on your device.
How is the HHI calculated?
Square each firm's market share, written as a percent, and add the squares. Shares of 40, 30, 20 and 10 give 3,000. Squaring makes large firms weigh more than a simple count of competitors would.
Related tools
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
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/fin/hhi \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"shares":[40,30,20,10]}'const res = await fetch("https://api.kit.forhosting.com/fin/hhi", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"shares": [
40,
30,
20,
10
]
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/fin/hhi",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"shares": [
40,
30,
20,
10
]
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/fin/hhi", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"shares":[40,30,20,10]}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"shares":[40,30,20,10]}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/fin/hhi", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"shares": [
40,
30,
20,
10
]
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "fin.hhi",
"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
Published price — no tokens, no invented credits. A failed task is never charged.
Limits
max_items | 10000 |
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. |
Technical notes
Lists that fail the adding-up test are rejected with invalid_input instead of being silently rescaled, because a broken market definition must not produce a precise-looking concentration number. The same pure arithmetic runs free in the browser and over the API at $0.002 per successful call.
Input contract, the sum-to-100 rule, and the response
POST a JSON body with one required array named shares: the market share of each firm as a percentage of the relevant market. The engine also accepts the aliases market_shares, market_share, percentages, and pct, so a worksheet that already uses those column names does not need a rename step. A missing field, a non-array value, an empty list, or a list longer than the published item cap fails with invalid_input naming shares. The sum must equal one hundred within a tiny floating tolerance; if it does not, the request fails and the message reports the observed sum. On success the body echoes the parsed shares, reports n as the number of firms, sum_shares as the checked total, hhi as the sum of squares rounded to twelve decimal places, a concentration label from the 2010 guideline bands, and a formula string restating HHI equals the sum of s_i squared. Pair the JSON with your own rule for which published series match the market — this endpoint squares the percentages you send; it does not rebuild a relevant market or convert units from fractions.
Integration use cases
Industry-structure homework
Students and graders verify HHI = Σ s_i² for textbook share lists, including the hard rule that percentages must sum to 100.
Technical questions
How is the Herfindahl-Hirschman Index calculated?
Send JSON with shares, an array of each firm's market share in percent. The response returns hhi as the sum of those percentages squared, plus n, sum_shares, a 2010-guideline concentration label, and the formula HHI = Σ s_i².