About
SizingCalcs publishes free sizing calculators for HVAC, plumbing, towing and home energy work. Every tool shows the formula, standard or manufacturer figure its answer came from, so you can check the reasoning instead of trusting a number.
We build these because most sizing calculators answer half the question. A towing calculator that reports GCWR but not payload will call an overloaded truck legal. A tankless water heater rated at five gallons a minute delivers less than three in a northern winter, at the same setting. A mini split chosen one size too large cannot modulate at all. In each case the equipment works, the arithmetic looks right, and the answer is wrong in exactly the situation you needed help with.
Where the numbers come from
Most of what these tools do is arithmetic on figures you supply from your own equipment: the ratings on a vehicle certification label, a module datasheet, an inverter specification, a pump curve. That is deliberate — those numbers are specific to your hardware and no table can substitute for them, which is why several calculators ask you to go and read a label.
Where a published standard governs, it is cited by section: ASHRAE 62.2 for whole-house ventilation, NEC 690.7 for the cold-weather voltage rise that sets a solar string length, NEC 358.26 for bends between pull points, the IPC minimum slopes for drainage, IRC M1502 for dryer exhaust. HVAC and plumbing otherwise lean on published engineering practice — Hazen-Williams for friction loss, the erosion-corrosion velocity limits that govern copper tube, Boyle's law for a pressure tank.
How they are built
Each calculator is a small, tested program with an automated test suite that runs before anything is published. Some of those tests deliberately assert the specific behaviour that makes a tool useful — that tongue weight reduces front axle load rather than adding to it, that a twenty gallon pressure tank delivers about six, that a solar string sized on nameplate voltage exceeds the inverter limit on a cold morning — so a change to an assumption cannot pass silently.
The reference data lives in one place and feeds both the calculators and the published charts, which means a chart and a calculator on this site cannot give different answers to the same question.
What we deliberately do not publish
There are no conductor sizing, overcurrent protection, conduit fill or service load calculators here, and their absence is a decision rather than an oversight. Those depend on large National Electrical Code tables where a single wrong cell produces a confident, authoritative-looking answer that could start a fire — and no amount of software testing catches a table that is wrong in a consistent way.
They will be published when those tables have been checked against a verified current code copy. Until then, size conductors and overcurrent devices from your own code book or with a licensed electrician. A calculator that looks authoritative and is wrong about a circuit is worse than no calculator at all.
What these tools are not
They are estimates for planning. They are not engineered designs, permit documents, or a substitute for a licensed electrician, plumber or HVAC contractor. Standards carry local amendments, and the authority having jurisdiction in your area has the final say. Every calculator carries a disclaimer describing what it does not model — Manual J, NPSH, drift loading — and those are worth reading.