> ## Documentation Index > Fetch the complete documentation index at: https://docs.trdrs.co/llms.txt > Use this file to discover all available pages before exploring further. # Size an order in money > Returns the order an amount of the account's money would place, sized as `POST /api/trading/order` sizes an order sent with `size`, with what the smallest order puts in and the collateral the account has free. A ticket uses it to show what an amount buys and to bound a slider from the smallest order to the most the account can put in. Nothing is placed, so it takes no `clientOrderId` and isn't stopped by a risk lock. Required key: Trading API key. A Connect app's backend can also send it with the app's API key, and a venue's backend with its Venue key, which carries `trader:trade`, each naming one of its own traders in `x-trdrs-trader`: it then runs on that trader's own accounts. ## OpenAPI ````yaml /api/openapi.json post /api/trading/order-size openapi: 3.1.0 info: title: trdrs Engine API version: 1.1.0 description: >- ## API Reference Every route the trdrs engine serves, with what to send and what comes back. It covers market data and news, trading and account state for a trader's own software, the Connect API a Connect app's backend calls and the Connect dashboard's routes, Connect pre-registration, the statements that carry a Connect account between environments, the venue routes a prop firm or brokerage runs its accounts through, and a trader's own challenges. Start with the Quickstart for your first call. The API standards hold the rules every route shares: keys, errors, rate limits, idempotency, paging and streaming. servers: - url: https://app.trdrs.co description: Production - url: / description: This engine security: [] tags: - name: Venue platform preview description: >- Run your venue: its providers, instruments, conditions, groups, routes, stages, venue rules, keys, accounts, usage, balance receipts and webhooks. These routes are in preview. They are served on the sandbox to every venue, and production access is arranged when a venue qualifies. Every route here under `/api/venues/` takes a Venue key. The back office reaches the same routes under `/api/back-office/` with a verified owner’s session, because the Venue key stays on your server, and both run the same checks. The team’s routes under `/api/organizations` serve the back office and the Connect dashboard alike, since one organization can run venues and Connect apps with one team. - name: Market data description: >- Search and look up symbols, read price history and quotes, check the server clock, and stream live bars. Crypto prices come from each provider’s public feed. Futures prices are licensed to each user and stream only from that trader’s own futures source: a login on their venue’s production Rithmic system, under their own market data subscription. A Rithmic Test login carries no market data. Futures reach the trader’s own signed-in session, and a venue’s or Connect app’s backend acting for its trader. A Trading API key reads crypto market data only. Without a source, a futures request answers 503 `feed_requires_connection` and the symbol search lists no futures. - name: News description: >- Market news and the economic calendar, from licensed and open sources, tagged with futures roots as they arrive. The content is the same for everyone, and these routes admit the same callers as market data: a licensed origin, a session or a Trading API key. Page headlines by publish time, filter them by instrument root, and stream them live over server-sent events. Thumbnails come through the image route. - name: Trading description: >- Place, change and cancel orders, protect a position with a bracket and change or withdraw it, and close or flatten positions. Every call that places an order takes a `clientOrderId` as its idempotency key. - name: Account description: >- Read an account a trader can trade: its balance, positions, working orders, fills, profit and loss history, and the live account stream. You don’t create accounts here. A trader connects their own account at a provider, or opens their own Demo on the paper book, in the trdrs app. A venue issues accounts on the paper book with Issue an account into a group, and a venue pre-registers accounts at a provider through Connect with Pre-register a trader’s account. - name: Connect description: >- Connect is the account picker a trader opens: in the trdrs app, where it lists the built-in providers and every listed venue, and in hosted Connect on a Connect app’s website, where it lists the tiles the app chose. The pre-registration routes let a venue fill Connect in the trdrs app ahead of time, for a trader who signs in to trdrs. You tell trdrs that a trader has an account at a built-in provider: their sign-in email, and optionally the account number and login name. When that trader signs in, Connect shows the account ready to link, and they sign in to the provider themselves, once. The trader types their own password, and access starts when their own login succeeds. You can list who you pre-registered and who has linked, and cancel a pre-registration that hasn’t been used. A pre-registration expires after 30 days, and sending it again refreshes it. These routes take the pre-registration key. [Register an account](/guides/register-an-account) walks through the whole flow. - name: Connect apps preview description: >- Run a Connect app: your own trading interface, whose traders connect their accounts in hosted Connect on your website and trade them on your screens. The Connect API is what your backend calls with your app’s API key: Create a Connect link, read and close it, and the account, market and trading routes with your trader named in `x-trdrs-trader`. Hosted Connect calls its own routes with the frame session a Connect link opens. The Connect dashboard calls the rest with your Connect sign-in session: your apps, each app’s name and logo, API keys, websites, tiles, white-label paper and Demos, traders and active traders, invoices, conformance runs and its Connect pass. These routes are in preview, served on the sandbox, where they are free. An app’s production opens when production records the app’s Connect pass, which the sandbox signs once the app passes its Connect conformance run. - name: Connect accounts description: >- A Connect client's team signs in to the Connect dashboard with a Connect account of its own, never a trader's, and one login reaches both environments. Production answers a short statement for the signed-in account, naming its verified email and the Connect apps it owns, and sandbox exchanges it for a sandbox session and the counterpart app of each: the sandbox app that stands for the production one. The environments share no credential: production signs the statement with its own key, and sandbox checks it with production's public key alone. These routes take the Connect dashboard's own session, from the dashboard's origin; a trader's session and every key are refused, and a Connect account's session reaches no trading, account, market or AI route. - name: Challenges description: >- List evaluation programs, enroll in one, and read a trader’s own enrollments and their history. **Preview: outside the additive-only promise**, so these shapes can change; see Versioning and stability. **They take a signed-in session, not a key**, and exist only on an engine that runs with `CHALLENGES_ENABLED`. Anywhere else every one answers `404`. paths: /api/trading/order-size: post: tags: - Trading summary: Size an order in money description: >- Returns the order an amount of the account's money would place, sized as `POST /api/trading/order` sizes an order sent with `size`, with what the smallest order puts in and the collateral the account has free. A ticket uses it to show what an amount buys and to bound a slider from the smallest order to the most the account can put in. Nothing is placed, so it takes no `clientOrderId` and isn't stopped by a risk lock. Required key: Trading API key. A Connect app's backend can also send it with the app's API key, and a venue's backend with its Venue key, which carries `trader:trade`, each naming one of its own traders in `x-trdrs-trader`: it then runs on that trader's own accounts. parameters: - name: provider in: query schema: type: string description: >- The provider the account is at, such as `rithmic` or `paper`. Omit it to use your default provider. An unknown value is refused with 400, never replaced with another provider. - name: account in: query schema: type: string description: >- The account number at that provider. It must be one of your own accounts, or the call is refused with 400. requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/OrderSizeRequest' responses: '200': description: >- The order the amount would place now. The price and the free collateral can change before an order arrives, so the order is sized again when it does. content: application/json: schema: $ref: '#/components/schemas/OrderSizeResponse' headers: Server-Timing: $ref: '#/components/headers/ServerTiming' '400': description: >- The instrument, side, size or order type is invalid, the market can't be traded on the account, or the provider sizes no order in money (`size_unsupported`). The body carries a `code`. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' headers: Server-Timing: $ref: '#/components/headers/ServerTiming' '401': description: The key is missing, unknown or revoked. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' headers: Server-Timing: $ref: '#/components/headers/ServerTiming' '503': description: >- `engine_draining` with a `Retry-After` header: the engine you reached is restarting for a release and did nothing. Wait for `Retry-After`, then send the same request again, with the same `clientOrderId` or `Idempotency-Key` where the route takes one: another engine takes it. A write refused before any route takes it says `server draining` as its `error`. content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' headers: Retry-After: $ref: '#/components/headers/RetryAfter' Server-Timing: $ref: '#/components/headers/ServerTiming' security: - tradingApiKey: [] - connectApiKey: [] trader: [] - venueKey: [] trader: [] x-codeSamples: - lang: javascript label: TypeScript source: >- const res = await fetch('https://app.trdrs.co/api/trading/order-size', { method: 'POST', headers: { 'content-type': 'application/json', Authorization: `Bearer ${process.env.TRDRS_API_KEY}`, }, body: JSON.stringify({ "instrument": "ESU6", "side": "buy", "size": { "kind": "margin", "amount": "5000" }, "orderType": "market" }), }) const data = await res.json() - lang: shell label: cURL source: |- curl -X POST 'https://app.trdrs.co/api/trading/order-size' \ -H "Authorization: Bearer $TRDRS_API_KEY" \ -H 'content-type: application/json' \ -d '{"instrument":"ESU6","side":"buy","size":{"kind":"margin","amount":"5000"},"orderType":"market"}' components: schemas: OrderSizeRequest: type: object description: The order to size. properties: instrument: type: string description: The instrument. side: type: string enum: - buy - sell description: Buy or sell. size: $ref: '#/components/schemas/OrderSize' orderType: type: string description: The order type. Omit it for `market`. limitPrice: type: number description: The limit price, for a limit or stop-limit order. triggerPrice: type: number description: The trigger price, for a stop order. withExits: type: boolean description: >- True when the order will be placed with `exits`: it is then sized on the instrument you sent alone, as the order route sizes it. required: - instrument - side - size example: instrument: ESU6 side: buy size: kind: margin amount: '5000' orderType: market OrderSizeResponse: type: object description: >- The order `size` would place now, and the range of money an order can put in. properties: instrument: type: string description: >- The instrument it would be placed on: the one you sent, or its micro or full-size twin. qty: type: - number - 'null' description: Its quantity, or null when the amount is below the smallest order. cost: type: - string - 'null' description: >- What it would put in, its margin with its commission, or null with no quantity. smallest: type: - string - 'null' description: >- What the smallest order puts in, on the instrument or its twin, whichever is less; null when the market takes no order now. available: type: - string - 'null' description: >- The collateral the account has free, the most an order can put in; null when it couldn't be read. currency: type: string description: The currency of every amount, the account's. required: - instrument - qty - cost - smallest - available - currency example: instrument: MESU6 qty: 3 cost: '4626.30' smallest: '1542.10' available: '10000.00' currency: USD ErrorResponse: type: object description: >- The body of every error response. It always carries `error`, an English sentence you can show. A refused trading request also carries `code`, one of the refusal codes, and `params`, the details of that refusal. Translate by `code` and `params`, and show a generic message for a code you don't recognize. Other errors may carry a `code` of their own. properties: error: type: string description: What went wrong, as an English sentence. code: type: string description: >- A stable machine code. On a refused trading request, it is one of the refusal codes. params: type: object description: >- The details of the refusal named by `code`, on a refused trading request. required: - error example: error: invalid_instrument OrderSize: type: object description: >- The account's money an order puts in: its margin with the commission its fill pays, in the account's currency, counted as the paper book counts it against free collateral. trdrs places the largest order that amount pays for, on the market's quantity step, so it puts in at most the amount. For a future with a micro or full-size twin the account trades (ES and MES, NQ and MNQ), it places on the twin where that puts in more of the amount, nearer what you asked for; an order placed with `exits` stays on the instrument you sent, since its exits are set for it. Only the paper book, including a Demo, sizes an order in money; another provider refuses it, `invalid_request` with the reason `size_unsupported`. An amount below the smallest order is refused with the reason `size_below_minimum`; `POST /api/trading/order-size` tells you the smallest beforehand. properties: kind: type: string enum: - margin description: The money is the margin the order puts in, with its commission. amount: type: string description: The amount in the account's currency, a positive decimal string. required: - kind - amount headers: ServerTiming: description: >- The engine's own time for the request, in the W3C Server-Timing form: `engine` from the request's arrival at the engine to its answer, and `db` the part of it the request's work spent in the database, with the statements it sent, each duration in milliseconds. The network's time is outside both. Every answer to a request on a trading route carries it, the engine's refusals before the route among them. schema: type: string example: engine;dur=12.4, db;dur=9.8;desc="statements=3" RetryAfter: description: >- How many seconds to wait before sending the same request again. A `503` carries it only when the engine did nothing, so the request may be sent again as it is. schema: type: integer minimum: 1 securitySchemes: tradingApiKey: type: http scheme: bearer description: >- The Trading API key (`trdrs_sk_…`), for market data, orders and account state on the accounts its owner holds. Use it server-to-server, and keep it on your server. Its scope is named `trading` in the API. connectApiKey: type: http scheme: bearer description: >- A Connect app's API key (`trdrs_ck_sandbox_…` or `trdrs_ck_production_…`), in preview. It belongs to one app in one environment, has a name and an expiry, and carries the whole Connect API. An owner of the app creates it in the Connect dashboard. It creates, reads and closes the app's Connect links under `/api/connect/links`, and on the account, market and trading routes it reads the app's own traders' accounts and market data and routes their orders, each request naming its trader in `x-trdrs-trader`. It stops working when it is revoked or expires, or when the owner who created it stops being an owner. Keep it on your server. trader: type: apiKey in: header name: x-trdrs-trader description: >- Your own id for one of your traders: the id your Connect links name, or the id the accounts your venue issues them use. Send it with your app's API key, or with your Venue key carrying `trader:read`, or `trader:trade` to also route the trader's orders, and the request runs as that trader on their own accounts. An id that names none of your traders is refused with `404` `trader_not_found` and a suspended trader with `403` `trader_suspended`. A request that also carries a cookie or a browser `Origin` is refused with `400` `credential_not_allowed`. venueKey: type: http scheme: bearer description: >- The Venue key (`trdrs_vk_sandbox_…` or `trdrs_vk_production_…`), in preview. It belongs to one venue in one environment and carries the scopes it was created with. The venue's verified owner creates it while signed in. It works on the `/api/venues/{venueId}` routes and, with `trader:read` or `trader:trade`, on the account, market and trading routes, reading the venue's own traders' accounts and routing their orders, each request naming its trader in `x-trdrs-trader`. A `trdrs_sk_…` key can't be used in its place. It reaches its venue's routes and its venue's own traders, and the owner manages keys from a signed-in session. Keep it on your server. ```` This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.