SizingCalcs
Free sizing calculators for HVAC, plumbing, solar, towing and home energy. Every result shows the formula or standard it came from.
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Electrical
Bending geometry, three-phase conversions and generator sizing. Conductor and overcurrent sizing are not published here — see the methodology page for why.
- Generator Size Calculator Size a generator from running watts and the largest motor. Only the biggest motor surge counts — summing every appliance’s starting watts oversizes badly.
- Three Phase Power Calculator Convert between kW, kVA, current and horsepower for single and three phase, and see what ignoring power factor does to a conductor calculation.
- Generator Run Time Calculator Estimate generator run time and fuel consumption from the actual load rather than the nameplate, with propane, diesel and natural gas comparisons.
- Conduit Bending Calculator Offset, saddle and 90° stub marks for EMT, with the shrink figure that decides whether the conduit reaches the box and the 360 degree limit per run.
HVAC & Insulation
Cooling load, duct sizing, static pressure and insulation — with the adjustments shown separately rather than folded into one number.
- BTU Calculator — What Size Air Conditioner Do I Need? Estimate a room’s cooling load in BTU/hr. Shows every adjustment separately and warns when the nearest standard unit would be oversized.
- Mini Split Sizing Calculator Size a ductless mini split head against the room load, and check minimum modulation, multi-zone combination ratio and heating output at your design temperature.
- Furnace Size Calculator Size a gas furnace against the heating load using output rather than input BTU, with AFUE, altitude derate and the airflow needed for the nameplate temperature rise.
- Heat Pump Balance Point Calculator Find the outdoor temperature where heat pump capacity crosses the building load, how much backup heat is needed below it, and what that heat costs.
- Duct Size Calculator Size supply and return ducts from airflow. Shows the resulting air velocity and flags when a duct will be noisy, plus the rectangular equivalent.
- Room CFM Calculator Work out how much airflow a room should get from its share of the cooling load rather than its floor area, with register count and return air requirements.
- Whole House Ventilation Calculator Work out the whole-house mechanical ventilation rate under ASHRAE 62.2 from floor area and bedrooms, with fan sizing for intermittent operation.
- Dryer Vent Length Calculator Check a dryer duct against the IRC 35 foot limit, with the equivalent length each elbow subtracts and the installation rules that cause lint blockages.
- Static Pressure Calculator Add up total external static pressure by component and see whether the filter, coil or ductwork is actually restricting airflow.
- Insulation R-Value Calculator Find how many inches of insulation you need for a target R-value, and whether it physically fits the stud or joist cavity you have.
Water & Plumbing
Pipe sizing by velocity rather than guesswork, pump head that counts friction and pressure as well as lift, and the expansion tank pre-charge everyone gets wrong.
- Pipe Size Calculator Size water pipe from flow rate, using the velocity limits that apply to your material and water temperature, plus Hazen-Williams friction loss.
- Pump Head Calculator Calculate total dynamic head from static lift, pipe friction loss and required discharge pressure — the three parts a pump actually has to overcome.
- Well Pressure Tank Calculator Find the usable drawdown of a well pressure tank from its volume and pressure switch settings, and size it for the pump run time that keeps the motor alive.
- Expansion Tank Calculator Size a thermal expansion tank from system volume, temperature rise and pressures — and check the pre-charge setting that decides whether it works at all.
- Water Heater Size Calculator Size a water heater by first hour rating against your actual peak hour demand, instead of guessing from tank gallons and household size.
- Tankless Water Heater Calculator Size a tankless water heater from temperature rise and simultaneous flow, with the gas or electrical capacity it needs and why the box rating is misleading.
- Water Softener Sizing Calculator Size a water softener from your hardness, iron and daily usage rather than household size, and see what the salt dose setting costs you per year.
- Drain Pipe Slope Calculator Total fall for a drain run at a given slope, checked against the code minimum for the pipe size and against the point where too much slope starts causing blockages.
Solar & Home Energy
Array sizing with real system losses, battery capacity you can actually use, and payback that accounts for both rate escalation and panel degradation.
- Solar Panel Calculator Work out how many solar panels you need from your monthly kWh usage, including the system losses that most calculators leave out.
- Solar String Sizing Calculator Find the maximum and minimum modules per string from temperature-corrected Voc and Vmp, so the array clears the inverter voltage limit on the coldest morning.
- Solar Inverter Sizing Calculator Check the DC to AC ratio of a solar array against its inverter, with realistic peak output after temperature and the annual cost of clipping.
- Battery Bank Calculator Size a solar battery bank from daily usage and days of autonomy, accounting for depth of discharge and round-trip efficiency by chemistry.
- Solar Payback Calculator Calculate solar payback period accounting for rising electricity rates and panel degradation, instead of dividing cost by flat annual savings.
- EV Charging Cost Calculator Work out real EV charging costs including charging losses, and compare against gasoline per mile at your own rates.
Towing & Load
The five ratings that govern a tow rig, checked together. Payload and axle load usually run out before the advertised tow rating does.
- Towing Capacity Calculator Find your real maximum trailer weight. Checks GCWR and payload together — tongue weight counts against payload, the limit most calculators ignore.
- Tongue Weight Calculator Check tongue weight as a percentage of trailer weight against the 10–15% conventional and 15–25% fifth wheel ranges, and get the exact cargo shift that corrects it.
- Weight Distribution Hitch Calculator Work out how much weight a trailer lifts off the front axle and the axle weight to tension a weight distribution hitch to, with the payload point people get wrong.
- Trailer Axle Position Calculator Find the axle position or load placement that gives the tongue weight you need, and how many pounds one inch of axle movement is worth.
- Payload Calculator Work out remaining truck payload from people, accessories, cargo and tongue weight, starting from your door jamb sticker rather than the advertised figure.
- Axle Load Calculator Calculate front and rear axle loads including the lever effect of tongue weight, and check each against GAWR rather than just total weight.
- Trailer Tire Load Calculator Check load per trailer tyre against its stamped capacity, with the pressure and reserve margin that decide whether that capacity actually applies.
Every answer names its source
There is no shortage of calculators on the web. What is scarce is one that tells you where its number came from, so you can check it. Every result on this site names the formula, standard or manufacturer figure behind it on the line that produced it, rather than in a footnote.
That is not decoration. A front axle result that says 278 lb lighter than unhitched, from tongue weight × overhang ÷ wheelbase can be checked on a scale, argued with, or used to decide where the load goes. One that says within limits cannot be checked at all.
Built to answer the whole question
Most calculators solve half a problem and stop. A towing calculator that reports GCWR but ignores payload will call an overloaded truck legal. A water heater tool that goes by tank gallons misses that a smaller tank with a stronger burner delivers more hot water. A tankless unit rated at five gallons a minute delivers under three in a northern winter, with nothing wrong with it.
Each of the 35 calculators here applies every constraint that governs its answer, and says which one is binding. Where an assumption is deliberately left out — Manual J, NPSH, engineered spans — the page says so plainly rather than letting you assume it was included. Tools whose answer would be safety-critical if it were wrong are not published here; the methodology page explains which, and why.
One source of truth
The 4 reference charts are generated from the same data the calculators run on. The pump head calculator imports its friction loss from the pipe size calculator rather than keeping a second copy. Two pages on this site cannot disagree about the same question, because there is only one answer in the code.
The calculations are covered by an automated test suite that runs before anything is published, and some of those tests deliberately assert the behaviour that makes each tool useful. More on how that works on the methodology page.
What these tools are not
They are estimates for planning. They are not engineered designs, permit documents or a substitute for a licensed professional. Codes carry local amendments, editions change, and the authority having jurisdiction in your area has the final say. Every calculator carries a disclaimer describing exactly what it does not model — those are worth reading before you rely on a result.