US customary and metric measurement on the job

The United States uses US customary units, a close relative of the old British imperial system, for nearly all building construction. Drawings, building codes, lumber, pipe and fasteners are all in inches and feet. But metric is part of the job too. Engineering and scientific data, many imported tools and fixtures, automotive and equipment fasteners, some federal projects and much manufacturer data are metric, and a growing number of drawings show both. A US tradesperson needs to be fluent in customary units and comfortable converting to and from metric. This lesson sets out the units, the conversion factors, and a method that makes conversions reliable.

The SI system in one page

The International System of Units (SI) builds every unit from a few base units and a set of prefixes that move by powers of ten. For trades work, the units that matter are these:

Because every step is a power of ten, converting within the metric system is a matter of moving the decimal point. 2.385 m is 2385 mm; 750 mm is 0.75 m; 1,500 L is 1.5 m³ because one cubic meter holds 1,000 liters. That simplicity is the main argument for metric: there are no 12s, 3s, 16s or 5,280s to remember.

A writing rule that prevents errors on metric drawings: lengths are usually given in millimeters without writing the unit on each dimension. A dimension of 2400 means 2400 mm. If you see a number with a decimal point, such as 2.4, it is in meters, usually an elevation or a large site dimension. Check the drawing notes, which state the units.

The customary and imperial units

Imperial and US customary length units are the same: 12 inches to a foot, 3 feet to a yard. Volume units differ between the British imperial and US systems, which matters for liquids. The US gallon is about 3.785 liters; the British imperial gallon, still seen on some older Canadian equipment and documents, is about 4.546 liters. On any modern North American product, gallon means the US gallon unless it says otherwise.

The awkward multipliers (12, 3, 144, 27, 1,728) are where errors creep in. Area and volume are the worst: a square foot is 144 square inches, not 12, and a cubic yard is 27 cubic feet, not 3. Lessons 4 and 5 return to this.

Conversion factors

Some conversion factors are exact by definition; others are rounded. The inch is defined as exactly 25.4 mm, which makes every length conversion exact if you start from that number.

Use enough decimal places for the job. For a pipe length, 3.2808 ft per meter is plenty. For rough estimating in your head, 1 m is a little more than 3 ft 3 in, 1 in is about 25 mm, and 1 m³ is about 1.3 cu yd.

The unit-factor method

The surest way to convert is to multiply by a fraction that equals one, written so that the unit you want to get rid of cancels. Because 1 ft equals 0.3048 m, the fraction 0.3048 m over 1 ft equals one, and so does its upside-down version. Choose the one that puts the old unit on the bottom.

Writing the units in every step looks slow, but it catches the most common error, multiplying when you should divide. If the units do not cancel to what you want, the setup is wrong.

Nominal and actual sizes

Many building products are named by a nominal size that is not their actual measurement. You must know the actual size to lay out and fit them.

US rebar is designated by number: #3, #4, #5 and so on, where the number is the diameter in eighths of an inch, so a #5 bar is 5/8 in in diameter. Some mills also mark bars with metric soft-conversion sizes (#13 for #4, for example). Electrical conduit uses trade sizes such as 1/2 in and 3/4 in, which are names rather than exact diameters. When a drawing and a supplier use different systems, convert carefully and confirm with your supervisor.

Hard and soft conversion are terms worth knowing. A soft conversion changes the numbers but not the product: a 4 ft by 8 ft sheet is described as 1220 by 2440 mm. A hard conversion changes the product to a round metric size, such as a 1200 by 2400 mm sheet. Mixing the two causes trouble on metric projects. A layout drawn on a 600 mm module will not line up with sheets that are really 1220 mm wide, so the drawings and the materials must agree.

A conversion helper in code

This program stores conversion factors in a table and converts a list of site values. The structure mirrors the unit-factor method: one multiplier per pair of units.

FACTORS = {
    ("in", "mm"): 25.4, ("ft", "m"): 0.3048, ("m", "ft"): 1 / 0.3048,
    ("sqft", "m2"): 0.3048 ** 2, ("cuyd", "m3"): 0.9144 ** 3,
    ("gal", "L"): 3.785411784, ("lb", "kg"): 0.45359237, ("psi", "kPa"): 6.894757,
}

def convert(value, frm, to):
    if (frm, to) in FACTORS:
        return value * FACTORS[(frm, to)]
    if (to, frm) in FACTORS:              # use the reverse pair if needed
        return value / FACTORS[(to, frm)]
    raise ValueError(f"no factor for {frm} -> {to}")

jobs = [(174, "in", "mm"), (2.4, "m", "ft"), (850, "sqft", "m2"),
        (6, "cuyd", "m3"), (100, "psi", "kPa"), (25, "kg", "lb")]
for v, a, b in jobs:
    print(f"{v:>7} {a:<4} = {convert(v, a, b):9.2f} {b}")

    174 in   =   4419.60 mm
    2.4 m    =      7.87 ft
    850 sqft =     78.97 m2
      6 cuyd =      4.59 m3
    100 psi  =    689.48 kPa
     25 kg   =     55.12 lb

Every factor is built from the exact definitions: 0.3048 m per foot and 0.9144 m per yard. Squaring or cubing the length factor gives exact area and volume factors, which avoids rounding errors.

Converting rates: flow, weight per length and speed

Many numbers on a job are rates: one unit divided by another. Pump capacity is volume per minute, pipe and rebar are sold by weight per length, and a conveyor or a hoist moves at a distance per second. The unit-factor method from this lesson works on rates too. You convert the top unit and the bottom unit separately, multiplying by a factor that equals one each time.

The gallon in the table is the US gallon of 3.785 L. The imperial gallon used in the United Kingdom, and on some older Canadian equipment, is about 4.546 L, roughly 20 percent bigger. When a pump or tank label from another country gives gallons, check which one it means before converting.

Knowing the weight of what you are about to move is a safety calculation as much as a math one. TRD-105 covers estimating load weights for rigging; the arithmetic is exactly this.