Book percentage

GET /v1/market/overround

How fat the books have been, by field size and course.

Available on

Free key not on this plan

Live not on this plan

Archive not on this plan

Complete not on this plan

Complete + Analyst 2017 → upcoming

Send your key in the X-API-Key header, or as ?key= if your tool cannot set one. A date outside your plan's window returns 403 outside_window naming the window, never an empty array.

Parameters

Name	In	Type	Required	Description
by	query	string	no	field_size, course, region, run_type, class or year. Defaults to field_size.
region	query	string	no	Restrict to one jurisdiction.
date_from	query	string	no	YYYY-MM-DD. Clamped to your plan's window.
date_to	query	string	no	YYYY-MM-DD. Clamped to your plan's window.

Request

curl https://api.apihorseracing.com/v1/market/overround \
  -H "X-API-Key: $AHR_KEY"

$ch = curl_init('https://api.apihorseracing.com/v1/market/overround');
curl_setopt_array($ch, [
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_HTTPHEADER     => ['X-API-Key: ' . getenv('AHR_KEY')],
]);
$data = json_decode(curl_exec($ch), true)['data'];

const res = await fetch(
  'https://api.apihorseracing.com/v1/market/overround',
  { headers: { 'X-API-Key': process.env.AHR_KEY } }
);
const { meta, data } = await res.json();

import os, requests

r = requests.get(
    "https://api.apihorseracing.com/v1/market/overround",
    headers={"X-API-Key": os.environ["AHR_KEY"]},
).json()

Response

200 OK application/json harvested 8 days ago

{
    "meta": {
        "request_id": "ea1312c40f20db10",
        "data_as_of": "2026-09-12T05:10:27+00:00",
        "plan": "analyst",
        "window": "2017 → upcoming",
        "count": 4
    },
    "data": {
        "window": {
            "from": null,
            "to": null,
            "note": "The whole archive. Your plan sets no date limit."
        },
        "by": "field_size",
        "field_size": [
            {
                "field_size": "8-11",
                "races": 203671,
                "mean_book": 123.51,
                "margin_pct": 23.51,
                "tightest": 100,
                "fattest": 200
            },
            {
                "field_size": "2-7",
                "races": 128757,
                "mean_book": 117.33,
                "margin_pct": 17.33,
                "tightest": 100.51,
                "fattest": 186.12
            },
            "… 2 more"
        ],
        "note": "100 would be a fair book. The excess is the margin taken out of the field. Level stakes on every runner loses more than the margin, because equal stakes overweight the longshots.",
        "excluded": {
            "races": 144,
            "reason": "Book percentage outside 100 to 200, which is a partial field priced or a bad figure at source."
        }
    }
}

A real response from /v1/market/overround, captured by calling the live API rather than written by hand.

Response fields

Each field is described once and rendered on every endpoint that returns it, so the wording cannot drift between pages.

Field	Type	What it is
window	string	How far back and forward that plan can read.

Errors this endpoint returns

Code	Status	When
unauthorized	401	No key, or one we do not recognise. Check the header name and that the key has not been rotated from your account page.
upgrade_required	403	The endpoint returns computed statistics and your plan does not include them. The body names every plan that does reach it, so you never have to look it up.
invalid_param	422	A date that is not YYYY-MM-DD, a by= dimension we do not have, or a cursor that did not come from us. The message names the parameter.
rate_limited	429	Past your per-minute rate. Retry-After is set, and X-RateLimit-Remaining tells you where you stand on every successful response too.
server_error	500	Ours, not yours. Every response carries a request_id; quote it and we can find the exact request.

Run it live

A real request from your browser straight to api.apihorseracing.com with your own key. Nothing is proxied and nothing is logged by this page.

Your API key

Remembered in this browser so you do not paste it on every page.

by optional

region optional

date_from optional

date_to optional

What this is for

How much margin the books have taken, sliced by field size, course, region, run type, class or year. The median and the percentiles rather than a mean, so one broken race cannot move a row.

The headline pattern is strong and consistent: margin grows with the size of the field. In our archive small fields run around 117% and fields of sixteen or more around 133%. That difference is the reason a strategy that looks marginally profitable in small fields can be hopeless in big ones without anything else changing.

Races with a book below 100% or above 200% are excluded as bad data rather than reported as remarkable. There are only a few dozen of them in the whole archive, and every one is a source error rather than an arbitrage opportunity.

Common uses

Understanding why level stakes loses

The overround is the answer, and it is bigger than most people assume in large fields.

Choosing where to operate

by=field_size, then confine a strategy to the bands where the margin is thinnest.

Watching it change over time

by=year. Margins move, and a backtest run across nine years is quietly averaging several different markets.