# Valve Flow Coefficient API > Control-valve flow-coefficient (Cv / Kv) maths as an API, computed locally and deterministically. The liquid endpoint sizes a control valve for liquid service using Q = Kv·√(ΔP/SG): give any two of the flow rate (m³/h), the pressure drop across the valve (bar) and the flow coefficient Kv, and it returns the third — the required Kv to size a valve, the flow a valve passes, or the pressure drop it develops — together with the equivalent Cv. The convert endpoint converts between the three flow coefficients in use around the world: the metric Kv, the US Cv = 1.156·Kv, and the SI Av = 2.4e-5·Cv. The opening endpoint computes how far a valve must open to pass an operating Kv against its rated Kvs, for both a linear trim (opening = Kv/Kvs) and an equal-percentage trim (opening = 1 + ln(Kv/Kvs)/ln(R) for a rangeability R), so you can keep the valve in its controllable 20–80 % travel band. Everything is computed locally and deterministically, so it is instant and private. Ideal for process, instrumentation and HVAC engineering tools, control-valve selection and commissioning, hydronic-balancing and plant-design apps, and engineering education. Pure local computation — no key, no third-party service, instant. Live, nothing stored. 3 endpoints. This is control-valve sizing; for pump power and head use a pump API and for orifice-plate metering use an orifice API. ## Authentication All requests require your oanor API key in the `x-oanor-key` header. Get one at https://www.oanor.com/developer/keys. ```bash curl -H "x-oanor-key: oanor_live_…" "https://api.oanor.com/valveflow-api/..." ``` ## Pricing - **Free** (Free) — 2,000 calls/Mo, 2 req/s - **Starter** ($9/Mo) — 15,000 calls/Mo, 6 req/s - **Pro** ($24/Mo) — 80,000 calls/Mo, 16 req/s - **Mega** ($74/Mo) — 450,000 calls/Mo, 40 req/s ## Endpoints ### Valve #### `GET /v1/convert` — Cv / Kv / Av conversion **Parameters:** - `kv` (query, optional, string) — Metric flow coefficient Kv Example: `10` - `cv` (query, optional, string) — Or US flow coefficient Cv **Example:** ```bash curl -H "x-oanor-key: $KEY" \ "https://api.oanor.com/valveflow-api/v1/convert?kv=10" ``` **Response:** ```json { "data": { "cv": 11.56, "kv": 10, "note": "Cv = 1.156·Kv (Cv in US gpm/√psi, Kv in m³/h/√bar). Av = 2.4e-5·Cv (m²) is the SI flow coefficient.", "av_m2": 0.00027744, "inputs": { "kv": 10 } }, "meta": { "timestamp": "2026-06-04T18:38:25.786Z", "request_id": "753ad63a-55bc-4b92-b757-d9fd53f1db28" }, "status": "ok", "message": "Cv/Kv conversion", "success": true } ``` #### `GET /v1/liquid` — Liquid valve sizing **Parameters:** - `flow` (query, optional, string) — Flow rate Q (m³/h) Example: `10` - `pressure_drop` (query, optional, string) — Pressure drop ΔP (bar) Example: `2` - `kv` (query, optional, string) — Flow coefficient Kv - `specific_gravity` (query, optional, string) — Specific gravity (default 1) Example: `1` **Example:** ```bash curl -H "x-oanor-key: $KEY" \ "https://api.oanor.com/valveflow-api/v1/liquid?flow=10&pressure_drop=2&specific_gravity=1" ``` **Response:** ```json { "data": { "cv": 8.17415, "kv": 7.07107, "mode": "solve_kv", "note": "Kv = Q/√(ΔP/SG). Required flow coefficient for the duty; pick a valve whose Kvs exceeds this.", "inputs": { "flow": 10, "pressure_drop": 2, "specific_gravity": 1 } }, "meta": { "timestamp": "2026-06-04T18:38:25.892Z", "request_id": "1cfef2f8-0cb9-4ef3-8403-91bd67482c36" }, "status": "ok", "message": "Liquid valve sizing", "success": true } ``` #### `GET /v1/opening` — Required valve opening **Parameters:** - `rated_kv` (query, required, string) — Rated Kvs (fully open) Example: `20` - `operating_kv` (query, required, string) — Operating Kv at duty Example: `7.07` - `rangeability` (query, optional, string) — Equal-% rangeability R (default 50) Example: `50` **Example:** ```bash curl -H "x-oanor-key: $KEY" \ "https://api.oanor.com/valveflow-api/v1/opening?rated_kv=20&operating_kv=7.07&rangeability=50" ``` **Response:** ```json { "data": { "note": "Linear opening = Kv/Kvs. Equal-percentage opening = 1 + ln(Kv/Kvs)/ln(R). Aim for 20–80 % travel at normal flow.", "inputs": { "rated_kv": 20, "operating_kv": 7.07, "rangeability": 50 }, "kv_ratio": 0.3535, "linear_opening_percent": 35.35, "equal_percentage_opening_percent": 73.4186 }, "meta": { "timestamp": "2026-06-04T18:38:26.001Z", "request_id": "d90a89cb-4cd6-4e62-a3b9-33599f939f23" }, "status": "ok", "message": "Valve opening", "success": true } ``` ### Meta #### `GET /v1/meta` — Spec **Example:** ```bash curl -H "x-oanor-key: $KEY" \ "https://api.oanor.com/valveflow-api/v1/meta" ``` **Response:** ```json { "data": { "notes": "Flow in m³/h, pressure drop in bar, specific gravity relative to water. Turbulent, non-choked liquid flow; flashing, cavitation and gas/steam sizing are not covered.", "service": "valveflow-api", "formulae": { "sizing": "Q = Kv·√(ΔP/SG), Kv = Q/√(ΔP/SG), ΔP = SG·(Q/Kv)²", "opening": "linear = Kv/Kvs, equal-% = 1 + ln(Kv/Kvs)/ln(R)", "conversion": "Cv = 1.156·Kv, Av = 2.4e-5·Cv" }, "endpoints": { "GET /v1/meta": "This document.", "GET /v1/liquid": "Liquid valve sizing: solve Kv, flow or pressure drop from the other two.", "GET /v1/convert": "Convert between the Cv, Kv and Av flow coefficients.", "GET /v1/opening": "Required valve opening (%) for an operating Kv against the rated Kvs." }, "description": "Control-valve flow-coefficient calculator: liquid Cv/Kv sizing, Cv↔Kv↔Av conversion, and required valve opening for linear and equal-percentage trims." }, "meta": { "timestamp": "2026-06-04T18:38:26.122Z", "request_id": "fbf75516-5e87-4e66-a4d0-06a292d5ab8a" }, "status": "ok", "message": "Meta", "success": true } ``` --- Marketplace page: https://www.oanor.com/api/valveflow-api OpenAPI spec: https://www.oanor.com/api/valveflow-api/openapi.json