About Trades Calculators
HVAC, plumbing, hydronics, and refrigeration are trades where guessing the numbers costs real money: an oversized furnace short-cycles and burns through ignitors, an undersized return duct starves the air handler and ices the evaporator coil, a pipe sized one nominal step too small drops shower pressure when the dishwasher kicks on, a pump that does not match the head curve never moves the design flow, and a refrigerant charge that is half a pound off slashes capacity and shortens the compressor’s life. The AllCalculators Trades hub brings the design math that HVAC contractors, plumbers, hydronic installers, refrigeration techs, and serious DIYers reach for on every job into a single set of fast tools. Duct sizing converts a room-by-room CFM requirement (driven upstream by a Manual J load calculation) into rectangular or round duct dimensions at a target friction rate, typically 0.08–0.10 inches of water column per 100 feet, the figure the ACCA Manual D method assumes.
Static-pressure math then verifies that the chosen blower can actually move the design CFM through the chosen duct, filter, and coil at the available external static. Pipe-flow and pressure-loss calculators handle both supply mains and branch runs in copper, PEX, and PVC, returning velocity (kept under 5 fps in copper to avoid erosion and noise) and head loss in feet per 100 feet of pipe at a target GPM. Water-pressure-loss accounts for length, fittings, elevation change, and meter losses so the fixture at the end of the run still delivers spec flow. Pump sizing matches a circulator or well pump to the system curve and the required total dynamic head.
Water-heater sizing returns a tank size or tankless flow rate that matches peak hour demand: a household with two showers, a dishwasher, and a clothes washer running at 7 a.m. is a very different sizing problem than a single bathroom. Radiant-loop math sizes manifold loops, supply temperatures, and flow per loop for a target heat output. Refrigerant-charge gets the subcooling and superheat numbers a tech needs to verify a system is properly dialed in, instead of just adding gas until the pressures look familiar.
CFM-per-room, tankless sizing, sump-pump capacity, and septic drain-field sizing round out the kit. None of these tools replace a licensed contractor or a code-required engineered design where one applies, but they answer the routine sizing questions accurately enough to bid, install, and troubleshoot with real confidence.
When to Use a Trades Calculator
- Sizing residential supply and return ducts from a room-by-room CFM requirement at a target friction rate
- Verifying that the chosen blower can move the design CFM at the actual external static pressure
- Sizing copper, PEX, or PVC supply pipe to keep velocity under 5 fps and deliver fixture flow without pressure complaints
- Matching a circulator or well pump to the system curve and required total dynamic head
- Choosing a tank or tankless water heater that covers peak-hour demand for the household’s fixture count
- Designing radiant-floor loops with the right length, flow per loop, and supply temperature for a target output
- Verifying refrigerant charge by subcooling and superheat instead of guessing from gauge pressures
Frequently Asked Questions
Is bigger always better when sizing a furnace, AC, or boiler?
No, and oversizing is by far the more common mistake. An air conditioner sized 30% larger than the load short-cycles, never reaches steady state, fails to dehumidify, and leaves the house clammy. An oversized furnace blasts heat, satisfies the thermostat in minutes, and shuts off before the heat exchanger and ducts equalize, producing big temperature swings and burning through ignitors and inducer motors. An oversized boiler on a low-mass radiant system condenses and short-cycles. The correct sequence is always a real load calculation first (Manual J for forced-air residential, ASHRAE-style heat-loss for hydronic), then equipment matched within roughly 10–15% of that load. The duct, pipe, and pump calculators here assume you have already done the load work upstream.
Why is duct static pressure such a big deal?
Because most residential air handlers are rated to move their nameplate CFM against about 0.5 inches of water column of total external static pressure, and a typical installed system, with a high-MERV filter, an aging coil, several elbows, a long flex-duct run, and an undersized return, can easily measure 0.8–1.2 inches w.c. At that point the blower is moving 60–80% of design CFM, the coil ices in cooling, the heat exchanger overheats in heating, and the homeowner blames the equipment. The static-pressure calculator adds up the pressure drops across each component so you can identify the choke point (usually the filter or the return) before installing a bigger blower that will not solve the underlying problem.
Do I really need to size pipe, or will the plumbing code table cover it?
Code tables (IPC Appendix E, UPC Appendix A, the Hunter fixture-unit method) give a safe, conservative answer that satisfies inspection, and for a typical single-family house that is often fine. But code tables do not optimize for velocity, noise, or pressure-drop comfort at the fixture, and they do not handle long runs, elevation changes, or non-standard pressure inputs especially well. A pipe-flow calculator lets you check the actual velocity (under 5 fps in copper to avoid erosion and water-hammer-amplifying noise) and the head loss across the longest run, so the fixture at the end of the line still delivers full flow when something else opens up.
Can I charge a refrigeration system by gauge pressures alone?
You can get it close, but for any system with a TXV or EEV the correct charge is verified by subcooling, and for fixed-orifice systems by superheat, not by matching pressures to a chart that assumes one specific operating condition. Outdoor temperature, indoor wet-bulb, and airflow all shift the right pressures, and a system that "looks fine" on the gauges can be 20% undercharged or overcharged. The refrigerant-charge calculator returns the target subcooling or superheat for the metering device and operating conditions so the system delivers its rated capacity and the compressor sees the suction-line conditions it was designed for.