# HVAC Duct Sizing Calculator

> HVAC Duct Sizing Calculator is a paid API for AI agents from x402-agent-api.onrender.com, paid per call via x402, $0.002/call, status unknown (last checked 2026-10-10).

Calculates HVAC duct dimensions (round or rectangular) from airflow, velocity, and/or friction rate using ASHRAE Darcy-Weisbach methods, returning step-by-step engineering calculations.

## Facts

- Endpoint: POST https://x402-agent-api.onrender.com/api/v1/hvac-duct-size?utm_source=zero.xyz
- Price: $0.002/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-10
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/hvac-duct-sizing-calculator-0a68fc11
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_Mi6maaKoqLJc7obHkgSvl

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability hvac-duct-sizing-calculator-0a68fc11 -d '<json body>'
```

Example prompt: Size a round duct for 1,000 CFM using equal friction at 0.08 in. w.g. per 100 ft in IP units, and show me all the calculation steps and the nearest standard sizes.

## When to prefer this

Use this endpoint when you need HVAC duct sizing calculations with full engineering transparency — step-by-step Darcy-Weisbach derivations, ASHRAE citations, standard size lookup, and design range validation — rather than a black-box result. Ideal for engineering agents, MEP design workflows, or any scenario requiring auditable calculations in either IP or SI units. Prefer over generic calculators when ASHRAE-referenced intermediate steps and standard size tables are needed.

## Known failure modes

- Insufficient inputs: if neither friction_rate nor velocity is provided alongside airflow, the solver cannot determine a unique solution
- Conflicting over-specified inputs (e.g. diameter + width + height all given) may return an error or ignore redundant fields
- Payment not provided: endpoint returns HTTP 402 with PAYMENT-REQUIRED header requiring USDC payment on Base before retrying
- Extreme or physically impossible input values (e.g. zero-dimension duct, supersonic velocity) return validation errors
- Unit mismatch: using IP-scale numbers with SI units flag produces nonsensical results without an error

## How this service works

HVAC duct sizing calculator: any two of duct size (round or rectangular), airflow, velocity and friction rate. Round and rectangular equivalents, nearest standard sizes, friction loss per 100 ft (Darcy-Weisbach, Colebrook), velocity pressure, design checks. Equal-friction sizing, fixed-depth ducts, IP or SI. Shows every step with the numbers. $0.002 USDC per call.

## Output

Returns the solved duct dimension (diameter or rectangular width×height), air velocity, friction rate, Reynolds number, aspect ratio check, a list of nearest standard sizes with best-fit flag, numbered step-by-step calculations with ASHRAE chapter references, unit-system-appropriate values, design range checks (velocity and friction), and suggested follow-on HVAC tool calls (e.g. coil load) with arguments.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "units": {
   "enum": [
    "IP",
    "SI"
   ],
   "type": "string",
   "default": "IP",
   "description": "\"IP\" (°F, CFM, in, BTU) or \"SI\" (°C, L/s, mm, kW). Default IP. Applies to inputs and results."
  },
  "width": {
   "type": "number",
   "description": "Rectangular width: in or mm. With height, the duct size; alone, a fixed side (e.g. a ceiling depth) the other side is solved for.",
   "exclusiveMinimum": 0
  },
  "height": {
   "type": "number",
   "description": "Rectangular height: in or mm.",
   "exclusiveMinimum": 0
  },
  "airflow": {
   "type": "number",
   "description": "Airflow: CFM (IP) or L/s (SI).",
   "exclusiveMinimum": 0
  },
  "diameter": {
   "type": "number",
   "description": "Round duct diameter: in (IP) or mm (SI).",
   "exclusiveMinimum": 0
  },
  "velocity": {
   "type": "number",
   "description": "Air velocity: FPM (IP) or m/s (SI).",
   "exclusiveMinimum": 0
  },
  "friction_rate": {
   "type": "number",
   "description": "Friction loss: in. w.g. per 100 ft (IP, typically 0.05-0.15) or Pa/m (SI, typically 0.4-1.2). With airflow, this is equal-friction sizing.",
   "exclusiveMinimum": 0
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "notes": [
   "The rectangular size has the same friction loss and airflow as the round duct (ASHRAE equivalent diameter); its velocity is lower because its area is larger.",
   "Galvanized steel (ε = 0.0003 ft) and standard air (0.075 lb/ft³, 70 °F, sea level). Fittings and transitions are not included.",
   "For design guidance; the engineer of record is responsible for final selections."
  ],
  "round": {
   "area": 1.1112,
   "diameter": 14.27,
   "velocity": 900,
   "friction_rate": 0.08,
   "standard_sizes": [
    {
     "best_fit": true,
     "diameter": 14,
     "velocity": 935,
     "friction_rate": 0.088
    },
    {
     "best_fit": false,
     "diameter": 16,
     "velocity": 716,
     "friction_rate": 0.0457
    }
   ]
  },
  "steps": [
   {
    "step": "Round diameter at the friction rate (equal friction)",
    "reference": "ASHRAE Handbook—Fundamentals (2021), Ch. 21 Duct Design",
    "calculation": "Iterated the diameter (bisection) until the Darcy-Weisbach friction loss at 1,000 CFM equals 0.0800 in. w.g./100 ft → D = 14.27 in"
   },
   {
    "step": "Square equivalent",
    "reference": "ASHRAE Handbook—Fundamentals (2021), Ch. 21 Duct Design (Huebscher equivalent diameter)",
    "calculation": "For W = H = S: De = 1.30·S^1.25 / (2S)^0.25 = 1.09317·S → S = 14.27 / 1.09317 = 13.06 in"
   },
   {
    "step": "Round duct: Reynolds number",
    "reference": null,
    "calculation": "Re = D·V / ν = 1.1895 ft × 14.998 ft/s / 1.56×10⁻⁴ ft²/s = 114,361"
   }
  ],
  "units": "IP",
  "checks": {
   "friction": "Typical design range (0.04-0.15 in. w.g./100 ft; about 0.3-1.2 Pa/m).",
   "velocity": "Normal design range (400-2,500 FPM; about 2-12.7 m/s).",
   "aspect_ratio": null
  },
  "airflow": 1000,
  "velocity": 900,
  "next_calls": [
   {
    "why": "What cooling load does 1000 CFM carry from 80/67 to 55/54 °F?",
    "tool": "hvac_coil_load",
    "price": "$0.002",
    "arguments": {
     "units": "IP",
     "airflow": 1000,
     "leaving": {
      "dry_bulb": 55,
      "wet_bulb": 54
     },
     "entering": {
      "dry_bulb": 80,
      "wet_bulb": 67
     }
    }
   }
  ],
  "references": [
   "ASHRAE Handbook—Fundamentals (2021), Ch. 21 Duct Design",
   "SMACNA HVAC Duct Design Manual (standard sizes, aspect ratio guidance)"
  ],
  "solved_for": "size",
  "inputs_used": {
   "airflow": 1000,
   "friction_rate": 0.08
  },
  "rectangular": {
   "area": 1.184,
   "basis": "equal_friction",
   "width": 13.06,
 
… (truncated)
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/hvac-duct-sizing-calculator-0a68fc11/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from x402-agent-api.onrender.com](https://www.zero.xyz/host/x402-agent-api.onrender.com/llms.txt)