Every tradesperson carries a mental library of unit conversions. Plumbers know there are 7.48 gallons in a cubic foot. HVAC techs know 1 ton of cooling is 12,000 BTU/hr. Electricians know 746 watts per horsepower. Instrument techs know 27.68 inWC per PSI. These conversions are second nature to people who use them daily, but they trip up everyone else.
Generic unit converters handle the easy ones: feet to meters, Fahrenheit to Celsius, gallons to liters. But they fail on the niche conversions that trades workers need most: SCFM to ACFM (which requires knowing altitude and temperature), grains per gallon to ppm (for water chemistry), Cv to Kv (for valve sizing), and the various pressure units used across different disciplines (inWC, inHg, mmHg, mbar, atm, and PSI are all common in different contexts).
This guide serves as a reference for the most commonly needed trades conversions, with the formulas, context, and gotchas that a converter alone does not provide.
Pressure Conversions: The Most Confusing Unit Category
Pressure is measured in more different units than any other physical quantity in trades work. PSI (pounds per square inch) is the standard for pneumatic systems, hydraulic systems, and pipe pressure. InWC (inches of water column) is the standard for gas piping, HVAC ductwork, and clean room pressurization. InHg (inches of mercury) is used for vacuum systems. Bar, kPa, and mbar are metric units used in European equipment and scientific instruments. MmHg (also called torr) is used in medical and laboratory settings. Atmospheres (atm) appear in chemistry and environmental contexts.
The critical conversions: 1 PSI = 27.68 inWC = 2.036 inHg = 6.895 kPa = 0.0689 bar = 51.71 mmHg = 0.068 atm. Most trades workers need to know the inWC-to-PSI conversion by heart because gas pipe sizing tables, HVAC static pressure readings, and instrument calibration all use inWC, while most pressure gauges read PSI.
The common mistake is confusing gauge pressure and absolute pressure. A gauge reading of 0 PSI means atmospheric pressure (14.696 PSIA at sea level). A gauge reading of 100 PSI means 114.696 PSIA. Vacuum readings add further confusion: -10 inHg gauge means 20.92 inHg absolute (assuming 30.92 inHg barometric). Always clarify whether a pressure specification is gauge (PSIG) or absolute (PSIA) before converting or using it in a calculation.
1 PSI = 27.68 inWC = 2.036 inHg = 6.895 kPa
1 bar = 14.504 PSI = 100 kPa
1 atm = 14.696 PSI = 101.325 kPa = 29.92 inHg
1 inWC = 0.0361 PSI = 0.249 kPa
1 mmHg = 0.0193 PSI = 1 torr
Trades Unit Converter
Purpose-built unit converter for trades and industrial workers. Convert pressure, flow, temperature, torque, length, water chemistry, and electrical units including niche conversions like inWC to PSI and grains/gallon to ppm.
Flow Conversions: SCFM vs ACFM and the Altitude Problem
Liquid flow is straightforward: GPM (gallons per minute) to LPM (liters per minute) is a fixed ratio of 3.785. But gas and air flow conversions have a trap that catches even experienced technicians: the difference between standard and actual conditions.
SCFM (standard cubic feet per minute) is the mass flow rate referenced to standard conditions: 14.696 PSIA, 68 degrees F, and 36% relative humidity per the Compressed Air and Gas Institute (CAGI) definition. ACFM (actual cubic feet per minute) is the volume flow rate at your actual pressure, temperature, and humidity.
The conversion: ACFM = SCFM * (P_std / P_actual) * (T_actual / T_std), where pressures are absolute (PSIA) and temperatures are absolute (Rankine). At sea level and standard temperature, ACFM equals SCFM. At 5,000 feet elevation (12.23 PSIA) and 100 degrees F: ACFM = SCFM * (14.696 / 12.23) * (560 / 528) = 1.274 * SCFM. A compressor rated at 100 SCFM delivers 127 ACFM at those conditions. The same mass of air occupies more volume because it is less dense.
Why does this matter? Pipe and duct sizing is based on velocity, which depends on ACFM, not SCFM. If you use SCFM to size pipe at altitude, you will undersize because the actual velocity is higher than you calculated. Equipment like compressors and blowers is rated in SCFM (mass flow) but delivers ACFM (volume flow). Always convert to ACFM for duct/pipe sizing and back to SCFM for capacity comparisons.
ACFM = SCFM × (P_std / P_actual) × (T_actual / T_std)
Standard: P_std = 14.696 PSIA, T_std = 528°R (68°F)
Example at 5,000 ft, 90°F:
ACFM = 100 × (14.696/12.23) × (550/528) = 125 ACFM
Water Chemistry: Grains, PPM, and Conductivity
Water treatment professionals, plumbers, and boiler operators encounter three different unit systems for water hardness and dissolved solids. Grains per gallon (GPG) is the traditional US unit used by water softener manufacturers. Parts per million (ppm) or milligrams per liter (mg/L) are used in laboratory reports and regulatory standards. Conductivity in microsiemens per centimeter (uS/cm) is a field measurement that correlates with total dissolved solids.
The key conversion: 1 GPG = 17.1 mg/L (ppm). Water hardness is expressed "as CaCO3," meaning the concentration is reported as if all the hardness were calcium carbonate. A water test reporting 10 GPG hardness equals 171 ppm as CaCO3.
Conductivity to TDS is an approximation: TDS (ppm) is approximately 0.5 to 0.7 times conductivity (uS/cm), with the exact factor depending on the ion composition. For most municipal water, 0.65 is a reasonable multiplier. A conductivity reading of 500 uS/cm corresponds to roughly 325 ppm TDS.
Water classification by hardness: soft is below 60 ppm (3.5 GPG), moderately hard is 60-120 ppm (3.5-7.0 GPG), hard is 120-180 ppm (7.0-10.5 GPG), and very hard is above 180 ppm (above 10.5 GPG). Most municipal water supplies fall in the moderately hard to hard range. Groundwater from wells is often hard to very hard.
1 GPG = 17.1 ppm (mg/L) as CaCO3
Hardness classification:
Soft: < 60 ppm (< 3.5 GPG)
Moderate: 60-120 ppm (3.5-7.0 GPG)
Hard: 120-180 ppm (7.0-10.5 GPG)
Very hard: > 180 ppm (> 10.5 GPG)
Valve Cv and Kv: The Transatlantic Confusion
Cv (flow coefficient) is the US standard for valve sizing. Kv is the metric/European equivalent. They measure the same thing (valve flow capacity) but use different units and produce different numbers for the same valve. Mixing them up causes valves to be oversized or undersized.
Cv is defined as the flow of water in US gallons per minute at 60 degrees F that produces a 1 PSI pressure drop across the valve. Kv is defined as the flow of water in cubic meters per hour at 16 degrees C that produces a 1 bar pressure drop. The conversion is: Cv = 1.156 * Kv, or equivalently, Kv = 0.865 * Cv.
A Fisher valve might be specified as Cv = 120. A Samson (German) valve might be specified as Kv = 104. They have approximately the same flow capacity: 120 / 1.156 = 103.8 Kv. If you mistakenly compare the numbers directly and assume the Fisher valve is larger, you will make a poor selection.
Always check whether a valve specification uses Cv or Kv before sizing. The units should be stated on the spec sheet, but they are sometimes omitted or assumed. European manufacturers use Kv. US manufacturers use Cv. Japanese manufacturers may use either. When in doubt, ask the vendor.
Cv = 1.156 × Kv
Kv = 0.865 × Cv
Example: Kv = 50 → Cv = 57.8
Example: Cv = 100 → Kv = 86.5
Quick Reference: Most-Used Trades Conversions
These are the conversions that come up daily in trades work. Memorize the ones you use most.
Length: 1 inch = 25.4 mm. 1 foot = 0.3048 m. 1 mil = 0.001 inch (used for coating thickness, wire insulation, and surface finish). 1 thou = 1 mil = 0.001 inch.
Temperature: F to C: (F - 32) * 5/9. C to F: (C * 9/5) + 32. Quick check: 0 C = 32 F, 100 C = 212 F, -40 is the same in both scales.
Torque: 1 ft-lb = 1.356 Nm = 12 in-lb. 1 Nm = 0.738 ft-lb = 8.85 in-lb.
Power: 1 HP = 746 watts = 0.746 kW. 1 boiler HP = 33,475 BTU/hr (not the same as mechanical HP).
Energy: 1 kWh = 3,412 BTU. 1 therm = 100,000 BTU. 1 gallon propane = 91,500 BTU. 1 gallon #2 fuel oil = 138,500 BTU.
Flow: 1 GPM = 3.785 LPM. 1 cubic foot = 7.48 gallons. 1 acre-foot = 325,851 gallons.
Electrical: Watts = Volts * Amps (DC and single-phase PF=1). Three-phase: Watts = Volts * Amps * 1.732 * PF. 1 MCM = 1 kcmil = 1000 circular mils.
Freezing: 32°F = 0°C
Body temp: 98.6°F = 37°C
Boiling: 212°F = 100°C
Equal point: -40°F = -40°C
Rough F-to-C: subtract 30, divide by 2. Close enough for field work.