Workshop units and conversions

November 3, 2026  ·  Indumor

A shop in Argentina works in two systems at once, and that is not going to change. Drawings come in millimeters, but the lathe was built in 1960 with an inch lead screw, the pipe thread is BSP, the compressor gauge reads psi, the motor nameplate says HP and the missing bolt is a 3/8. This chapter collects the factors you need to move between the two systems without hesitating, with exact values rather than the rounded ones people carry in their heads.

One figure rules all the others: the inch is 25.4 millimeters, exactly. It is not an approximation. It was fixed that way in 1959 by agreement among the English-speaking countries, and industry had already been using it since the 1930s. Every fraction in the first table comes from that number.

The inch and the millimeter

The inch is halved again and again: halves, quarters, eighths, sixteenths, thirty-seconds and sixty-fourths. That is why inch sizes are never round numbers in millimeters. A 1/4 is 6.35 mm, a 3/8 is 9.525 mm and a 1/2 is 12.7 mm. A 3/16 plate is 4.76 mm thick, which is why it is ordered as "4.75 plate" in metric shops. The full table is here; the converter that finds the closest fraction to any size in mm is on the inches to millimeters chart.

To get by without the table you need four anchors: 1/8 is 3.2 mm, 1/4 is 6.35, 1/2 is 12.7 and 1 inch is 25.4. The other way round, 1 mm is 0.03937 in, just over 39 thousandths. Inch shops talk in "thou": one thousandth of an inch is 0.0254 mm, so a "two thou" fit is five hundredths of a millimeter.

Length, area and volume

The foot, the yard and the mile derive from the inch, so they are exact too. Volumes hold a well-known trap: the US gallon and the British imperial gallon are not the same, and the gap is more than 20 %. A 55-gallon drum holds 208 liters because those are US gallons.

Mass, force and torque

The pound is 453.59 grams, also exact under the same 1959 agreement. The usual confusion in the shop is torque: the kilogram-meter (kgf·m) of the old charts, the newton-meter (N·m) of the new ones and the pound-foot (lbf·ft) of American manuals. One kgf·m is 9.81 N·m and one lbf·ft is 1.356 N·m. Watch the pound-inch, which is twelve times smaller than the pound-foot: many small torque wrenches and most precision fastener charts are in lbf·in.

Pressure

Five units live together here and none of them is "the" right one. The compressor gauge and imported equipment sheets speak psi, hydraulics speak bar, boilers and old vessels speak kgf/cm², calculations use kPa or MPa, and water column shows up in pumps and ventilation. For a quick sum: 1 bar is 14.5 psi, 1.02 kgf/cm² and 10.2 meters of water column.

m H₂O: conventional meter of water column (1000 kg/m³ and standard gravity; at a real 4 °C it is 9.806 kPa). In Latin American shops the kgf/cm² is simply called a "kilo" of pressure; 1 bar is 2 % more than 1 kgf/cm².

Power and energy

The mechanical horsepower (HP) and the metric horsepower (CV, PS or cv depending on the country) are not the same, even if the gap is only 1.4 %. An electric motor rated 10 HP on the nameplate delivers 7.46 kW. European motor nameplates are in kW; American pumps and compressors are in HP. Boilers and burners are rated in kcal/h or BTU/h, and the kWh is the unit on the electricity bill.

Temperature

Fahrenheit shows up in American heat treatment manuals and on the thermometers of many imported machines. To go from °C to °F, multiply by 1.8 and add 32; to come back, subtract 32 and divide by 1.8. Two references help check a sum: 0 °C is 32 °F, and the two scales meet at 40 below zero. The kelvin is °C plus 273.15 and is used in gas calculations.

Speed, flow and angle

Cutting speed is written in meters per minute in European catalogs and in surface feet per minute (sfm) in American ones: one sfm is 0.3048 m/min, so an sfm value divided by 3.28 gives m/min. Air flow for compressors and extractors comes in cfm (cubic feet per minute) and water flow in gpm, US gallons per minute.

Angles come in sexagesimal degrees with minutes and seconds: 1° has 60' and each minute has 60". To turn 12° 30' into decimal, divide 30 by 60: 12.5°. Taper drawings also give the slope in millimeters per meter and in percent: 1° is 17.455 mm per meter, and a 1 % slope is an angle of 0.573°.

Frequently asked questions

How many millimeters is an inch, exactly?

25.4 mm exactly, by definition since 1959. Every fraction follows from it: 1/2 is 12.7 mm, 1/4 is 6.35 mm and 1/8 is 3.175 mm.

What is the difference between HP and metric horsepower?

The HP is 745.7 W and the metric horsepower (CV, PS) is 735.5 W. The HP is 1.4 % larger. American motors are labeled in HP and European ones in kW.

Is a bar the same as a kgf/cm²?

Almost. One bar is 1.0197 kgf/cm², 2 % more. To set a compressor or read a gauge they can be taken as equal; for a strength calculation use the factor.

Sources: NIST, Guide for the Use of the International System of Units, SP 811 (2008), Appendix B; BIPM, The International System of Units, 9th edition (2019); ISO 80000-1:2022; International Yard and Pound Agreement (1959).