Free HVAC engineering calculators

Airside & Ductwork 13

Duct SizerSize round and rectangular ducts by equal friction or velocity. Darcy-Weisbach with Colebrook friction, corrected for air temperature and duct material.Airside & DuctworkOpen → Duct Static PressureBuild the critical air path segment by segment, add equipment losses, and total the external static pressure for fan selection.Airside & DuctworkOpen → Duct Area / Size CalculatorDuct area and size from CFM and target velocity, round-to-rectangular equivalents, and a velocity check by application.Airside & DuctworkOpen → Static Pressure BudgetAdd up coils, filters, louvers, dampers, and ductwork into a total static budget, with the external static (outside the unit casing) split out for fan selection.Airside & DuctworkOpen → Fan Power CalculatorBrake horsepower, next NEMA motor size, motor input, and annual energy from CFM, total static pressure, and efficiencies.Airside & DuctworkOpen → Fan Energy Cost CalculatorFan BHP, motor size, annual kWh and operating cost, and an upper-bound VFD savings estimate from the fan affinity law.Airside & DuctworkOpen → Velocity CalculatorAir velocity in round and rectangular ducts and water velocity in pipe, checked against typical design ranges.Airside & DuctworkOpen → Filter Pressure DropClean, change-out, and mid-life pressure drop for MERV 8–15 pleated, bag, and V-bank filters and HEPA, from published filter data at your face velocity.Airside & DuctworkOpen → Diffuser Count EstimatorHow many ceiling supply diffusers a space needs from total CFM and neck size, with the actual CFM each and a spacing check.Airside & DuctworkOpen → Grille & Register SizingSize supply registers and return or exhaust grilles from CFM and target face velocity.Airside & DuctworkOpen → VAV Box SizingZone cooling max, minimum, and heating airflow from loads and supply temperatures, and a recommended VAV inlet size.Airside & DuctworkOpen → Fan Affinity Law CalculatorNew CFM, static pressure, and BHP when fan speed changes — or the RPM needed for a target airflow.Airside & DuctworkOpen → Duct Leakage CalculatorDuct leakage CFM from SMACNA leakage class, surface area, and test pressure, checked against the ASHRAE 90.1 class-4 test limit and your spec, plus fan energy.Airside & DuctworkOpen →

Hydronics & Piping 12

Hydronic Pipe SizerSize chilled, hot, and condenser water piping in steel, copper, or plastic by friction rate or velocity. Darcy-Weisbach with glycol.Hydronics & PipingOpen → Pipe Head LossTotal pump head for a hydronic circuit — add pipe segments (equivalent length incl. fittings) and equipment losses; read head in ft and psi.Hydronics & PipingOpen → Pump SelectorWater and brake horsepower, next NEMA motor size, and annual energy from GPM and total dynamic head.Hydronics & PipingOpen → Pump Head EstimatorBuild total dynamic head from pipe friction, coils, control valves, chiller, strainers, and elevation, with a BHP estimate.Hydronics & PipingOpen → Pipe Velocity + Friction CheckVelocity and friction loss for one pipe size and flow, checked against typical hydronic design limits.Hydronics & PipingOpen → Glycol Correction FactorCorrect GPM and pump head for ethylene or propylene glycol, with freeze and burst protection by concentration.Hydronics & PipingOpen → Expansion Tank SizingSize a closed-system diaphragm expansion tank for water or glycol from system volume, temperatures, fill and maximum pressures, and pipe material.Hydronics & PipingOpen → GPM / Tons / ΔT CalculatorSolve the hydronic heat equation for GPM, tons, or ΔT, with glycol correction.Hydronics & PipingOpen → Balancing Valve Cv CalculatorRequired Cv for a balancing valve from flow and pressure drop, corrected for glycol, with a valve size suggestion.Hydronics & PipingOpen → Control Valve Cv & AuthorityControl valve Cv, valve authority, and equal-percentage flow characteristic for hydronic coils.Hydronics & PipingOpen → Cooling Tower CalculatorHeat rejection, range, and approach, plus the water balance: evaporation, drift, blowdown, and make-up.Hydronics & PipingOpen → Pipe Insulation Heat LossBare and insulated pipe heat loss or gain from cylindrical conduction plus surface film, with a condensation check on cold pipe.Hydronics & PipingOpen →

Loads & Envelope 8

Quick Load EstimatorRule-of-thumb cooling tons, supply CFM, and heating MBH from floor area and building type — a starting point, not a load calc.Loads & EnvelopeOpen → Heating BTU CalculatorHeating capacity three ways: airside sensible heat, a space rule of thumb, and boiler or furnace input with fuel use.Loads & EnvelopeOpen → Sensible vs Latent SplitSensible heat ratio from loads or airflow conditions, and what the split means for coil and dehumidification choices.Loads & EnvelopeOpen → Supply Air Temp CalculatorSolve Q = 1.08 × CFM × ΔT for supply temperature, room temperature, airflow, or load, with altitude correction.Loads & EnvelopeOpen → Infiltration Load CalculatorSensible and latent infiltration loads for heating and cooling from the air-change method, corrected for altitude.Loads & EnvelopeOpen → Lighting Heat Gain CalculatorLighting cooling load from installed watts, use and allowance factors, and the radiant / convective split.Loads & EnvelopeOpen → Wall Assembly U-ValueOverall R-value and U-factor of a multi-layer wall, with framing correction and air films.Loads & EnvelopeOpen → Window Solar Heat GainPeak window solar and conduction gain by orientation, latitude, SHGC, and shading.Loads & EnvelopeOpen →

Ventilation 5

Ventilation CalculatorASHRAE 62.1 Ventilation Rate Procedure for one zone: Vbz = Rp·Pz + Ra·Az and Voz = Vbz / Ez, with rates by space type.VentilationOpen → ACH CalculatorConvert between air changes per hour, CFM, and room volume, with typical ACH ranges by space type.VentilationOpen → OA / RA % CalculatorOutdoor and return air fractions from CFM, or the OA CFM for a target percentage, with mixed air temperature and an economizer flag.VentilationOpen → Mixed Air Temp CalculatorMixed air temperature, humidity ratio, and enthalpy from outdoor and return streams, with an economizer check.VentilationOpen → DOAS Sizing CalculatorSize a dedicated outdoor air system: OA CFM by space type, coil loads, and energy-recovery pre-conditioning.VentilationOpen →

Psychrometrics & Coils 7

Psychrometric CalculatorMoist-air properties from dry bulb plus RH, wet bulb, or dew point: humidity ratio, enthalpy, specific volume, and more.Psychrometrics & CoilsOpen → Dew Point CalculatorDew point from dry bulb and RH, and a condensation check on common duct, pipe, and building surfaces.Psychrometrics & CoilsOpen → Wet Bulb / Dry Bulb / RHSolve any air state from dry bulb plus RH, wet bulb, or dew point — and get the rest.Psychrometrics & CoilsOpen → Enthalpy CalculatorMoist-air enthalpy for two air states and the total heat between them, for coil and air-handling checks.Psychrometrics & CoilsOpen → Humidity Ratio CalculatorHumidity ratio in gr/lb, lb/lb, and g/kg from dry bulb, RH, and elevation, with dew point, vapor pressure, and a comfort check.Psychrometrics & CoilsOpen → Coil Leaving Air CalculatorLeaving-air temperature, humidity, RH, and wet bulb from entering air, airflow, capacity, and SHR, with apparatus dew point and impossible-state checks.Psychrometrics & CoilsOpen → Cooling Coil CalculatorSensible, latent, and total coil capacity from entering and leaving air, with SHR, condensate, and face velocity.Psychrometrics & CoilsOpen →

Equipment & Plant 5

Boiler Size EstimatorFeasibility-stage boiler output from floor area, building type, and climate, with a DHW load, one pickup margin, and altitude-derated gas input.Equipment & PlantOpen → Chiller Size EstimatorFeasibility-stage chiller plant size from floor area, building type, and climate, with ASHRAE 90.1-2022 chiller kW and chilled-water flow at your ΔT.Equipment & PlantOpen → Equipment Load SplitSplit a load across multiple units with N+1, N+2, or 2N redundancy and get a draft equipment schedule.Equipment & PlantOpen → System Type RecommenderAnswer a few questions about the building and get ranked HVAC system types with pros, cons, and reasoning.Equipment & PlantOpen → Motor Full Load Amp CalculatorMotor full-load current from NEC Tables 430.248/430.250, with 125% conductor ampacity, 75 °C copper size, and maximum inverse-time breaker.Equipment & PlantOpen →

Gas & Refrigerant Piping 4

Natural Gas Pipe SizerNatural gas and propane pipe sizing by capacity with the IFGC sizing equations, at low pressure or 2 and 5 psig, in Schedule 40 steel.Gas & Refrigerant PipingOpen → Gas Pipe Quick EstimatorQuick gas pipe size for one load and run in Schedule 40 steel — low pressure or 2/5 psig, IFGC sizing equations, with the capacity table.Gas & Refrigerant PipingOpen → Refrigerant Pipe SizerSize DX suction, discharge, and liquid lines in ACR copper by pressure drop, with oil-return checks, for R-410A, R-454B, R-32 and more.Gas & Refrigerant PipingOpen → Refrigerant Line SizerQuick ACR copper sizing for suction, discharge, and liquid lines by pressure drop, with oil-return velocity checks.Gas & Refrigerant PipingOpen →

Energy & Cost 3

Energy Cost EstimatorAnnual operating cost for fans, pumps, and cooling, with and without variable-speed control.Energy & CostOpen → VFD Energy Savings CalculatorMotor energy and cost savings from a variable frequency drive on a fan or pump, with payback.Energy & CostOpen → Simple Payback & ROISimple payback, NPV, and year-by-year cash flow for an efficiency upgrade.Energy & CostOpen →

Conversions & Quick Checks 4

CFM / Tons / BTU ConverterConvert between tons, BTU/hr, MBH, and kW, and estimate airflow from capacity. Edit any field.Conversions & Quick ChecksOpen → HVAC Unit ConversionsTwo-way converters for temperature, airflow, water flow, pressure, energy, and power.Conversions & Quick ChecksOpen → Design Sanity CheckerEnter tons, CFM, MBH, and GPM and see which fall outside rule-of-thumb ranges for the building type.Conversions & Quick ChecksOpen → Rule-of-Thumb CheckerA quick-reference sheet of common HVAC rules of thumb: velocities, airflow ratios, load densities, and design constants.Conversions & Quick ChecksOpen →

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