{"uid":"cap_Mi6maaKoqLJc7obHkgSvl","slug":"hvac-duct-sizing-calculator-0a68fc11","name":"HVAC Duct Sizing Calculator","description":"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.","url":"https://x402-agent-api.onrender.com/api/v1/hvac-duct-size?utm_source=zero.xyz","method":"POST","headers":{},"bodySchema":{"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}}},"responseSchema":{"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,"height":13.06,"velocity":845,"aspect_ratio":1,"friction_rate":0.08,"standard_sizes":[{"width":14,"height":14,"best_fit":false,"velocity":735,"aspect_ratio":1,"friction_rate":0.0568,"equivalent_diameter":15.3},{"width":16,"height":12,"best_fit":false,"velocity":750,"aspect_ratio":1.33,"friction_rate":0.0605,"equivalent_diameter":15.11}],"equivalent_diameter":14.27},"unit_labels":{"area":"ft²","load":"MBH (1,000 BTU/h)","grains":"gr / lb dry air","length":"in","airflow":"CFM","density":"lb / ft³","enthalpy":"BTU / lb dry air","pressure":"psia","velocity":"FPM","elevation":"ft","condensate":"gal / h","temperature":"°F","friction_rate":"in. w.g. / 100 ft","humidity_ratio":"lb water / lb dry air","specific_volume":"ft³ / lb dry air","relative_humidity":"%","velocity_pressure":"in. w.g."},"friction_rate":0.08,"primary_shape":"round","friction_factor":0.01885,"reynolds_number":114361,"velocity_pressure":0.0505}},"example":null,"exampleRequest":null,"tags":["x402"],"displayCostAmount":"0.002","displayCostAsset":"USDC","priceDynamic":false,"priceHint":null,"priceStatus":"priced","priceSource":"probe","requiresHandshake":false,"reviewCount":0,"rating":{"score":"0.00","successRate":"0.00","reviews":0,"stars":null,"state":"unrated"},"availabilityStatus":"unknown","priceObserved":null,"sessionDeposit":null,"pricing":{"kind":"static","summary":"$0.002/call","primary":{"kind":"static","protocol":"x402","network":"base","amountUsd":"0.002","per":"call","confidence":"exact"},"accepted":[{"kind":"static","protocol":"x402","network":"base","amountUsd":"0.002","per":"call","confidence":"exact"}]},"paymentMethods":[{"uid":"pm_QwZfVy1z2kevxo1__lvMA","protocol":"x402","methodType":"crypto","chain":"base","mode":"charge","costAmount":"0.002","costPer":"request","priority":0,"asset":"0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913","unit":"request","depositMicros":null,"planRef":null}],"brandName":null,"brandSlug":null,"brandBaseUrl":null,"brandDocsUrl":null,"whatItDoes":"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.","exampleAgentPrompt":"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.","exampleUseCases":[{"title":"Equal-friction round duct sizing","prompt":"What diameter round duct do I need to carry 1,500 CFM at a friction rate of 0.10 in. w.g. per 100 ft? Show me the steps and list the nearest standard sizes in IP units."},{"title":"Fixed-width rectangular duct solve","prompt":"I have a ceiling space that limits me to a 10-inch duct width. What height do I need to carry 600 CFM in IP units? Use a friction rate of 0.08 in. w.g. per 100 ft."},{"title":"SI metric duct sizing check","prompt":"Size a round duct in SI units for 250 L/s at 0.8 Pa/m friction loss, and tell me if the resulting velocity falls within the normal ASHRAE design range."}],"resultDescription":"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.","failureModes":["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"],"whenToPreferThis":"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.","instructions":null,"reviewSummary":null,"reviewSummaryHighlights":null,"reviewSummaryConcerns":null,"reviewSummaryGeneratedAt":null,"activationCount":0,"lastUsedAt":null,"lastSuccessfullyRanAt":null,"lastHealthCheckAt":"2026-10-10T06:39:39.953Z","isFirstParty":false,"canonicalSlug":"hvac-duct-sizing-calculator-0a68fc11"}