In one sentence A pound and a kilogram both name the same physical property — body weight — but they are different sizes, so the same patient has one number in pounds and a smaller number in kilograms.
One body weight, two unit names
Body weight is a single quantity: the mass of the patient sitting on the scale. The unit is simply the name we attach to the number. A pound, written lb, is the unit many hospital scales, many older charts and most patients still use in everyday speech. A kilogram, written kg, is the metric base unit and the unit the rest of the hospital uses for medication. Because these two units are different sizes, the same patient produces two different numbers. One kilogram is heavier than one pound, so the kilogram number is always the smaller of the two. Nothing about the patient has changed between the two readings; only the counting unit has changed. This matters clinically because a weight-based order names its unit, and the number you substitute must be measured in that same unit.
A ward scale set to pounds prints 154 lb for a patient. The same patient on a metric scale reads 70 kg. Measure once, and be sure which name the scale is using.
A patient weighs 44 lb. Will the kilogram number be larger or smaller than 44, and why?
Show the answer
Smaller. A kilogram is heavier than a pound, so we need fewer of them to describe the same weight. 44 lb ÷ 2.2 = 20 kg, and 20 is smaller than 44.
Read detailed explanations
- What 2.2 really stands for, and the discrepancy it hides
The factor 2.2 is a rounded value, and knowing where it comes from turns it from a chant into a measurement. The international avoirdupois pound is defined as exactly 0.45359237 kg, so one kilogram equals 1 ÷ 0.45359237 ≈ 2.20462262 lb. Using 2.2 therefore introduces a small systematic discrepancy: 70 kg × 2.2 = 154.0 lb, while the exact computation gives about 154.32 lb, roughly 0.2 % low. That tiny bias propagates into every milligram scaled from the weight. For ordinary adult body-weight dosing the discrepancy sits far inside clinical tolerance, which is precisely why the supplied notes can offer 2.2 as a convenient working estimate. It is not, however, a definition, and no clinician should treat the final decimal as meaningful. Laboratory metrology separates accuracy — closeness to the true value — from precision, the repeatability of a reading. A conversion factor carries its own uncertainty, and a scale carries another. When the drug has a narrow therapeutic index, or the patient is a small infant whose absolute dose is measured in fractions of a milligram, a percentage bias that is negligible in an adult can become clinically relevant. Some practitioners therefore use 2.205 or divide by the reciprocal 0.454 to reduce that bias. The reverse factor deserves the same care: one pound is approximately 0.454 kg. Whichever figure you adopt, state it, use it consistently, and never let a stated precision outrun the measurement behind it.
- Weighing the patient: the number on the chart is a measurement, not a memory
Because the dose is proportional to the weight, the credibility of the weight is the credibility of the dose. A usable weight comes from a calibrated instrument and a defined procedure. Standing, chair and bed scales each carry their own technique: the patient centred on the platform, feet bare, heavy outer clothing removed, the reading taken at eye level, and the instrument zeroed beforehand. Body mass genuinely fluctuates: one litre of retained fluid adds about one kilogram, so peripheral oedema, ascites and intravenous infusions all inflate the reading, while amputations, plaster casts and dehydration deflate it. Some patients exceed the capacity of an ordinary floor scale, and a bed or chair scale has to be used instead. Documentation must carry the unit, the date and, where it exists, the instrument; a pound reading entered without a unit is not a weight at all. The defensible rule is blunt: for any mg/kg order, weigh the patient, write kilograms, and if a pound reading is all you have, convert it and mark the conversion on the chart.
- The silent error: how a wrong unit stays plausible
A wrong weight unit does not announce itself, and that is what makes it dangerous. Suppose a nurse multiplies 6 mg/kg by 154, the pounds figure, and obtains 924 mg. Every keystroke is correct; the product is a plausible-looking quantity; nothing visibly fails. Contrast that with a misplaced decimal in a dilution, which usually produces an absurd volume and provokes immediate suspicion. The reason the unit error hides is that the notation mg/kg encodes a silent instruction: “for each kilogram”. The multiplication is valid only when the second factor genuinely is a number of kilograms, and dimensional analysis is the instrument that makes the hidden instruction visible, because pounds refuse to cancel with the kilograms in the denominator and the surviving unit betrays the mistake. Three separate failures can masquerade as one good number on a chart: the wrong unit, a measurement of the wrong age, and an unmeasured round-number guess. Independent double-checking — two nurses calculating separately and comparing — catches all three only if each checker starts from the source measurement rather than from the other’s arithmetic. A useful cross-check is a clinical benchmark: does the answer fall inside the range you would expect for this drug and this patient? Many medications behave proportionally, so doubling the weight doubles the dose; others do not, and a loading dose may scale with weight while maintenance dosing depends on clearance and volume of distribution instead. Recognising that difference stops a nurse from assuming that every dose must move with the weight. The safest habit remains cheap and unglamorous: convert, write the unit, reverse the conversion, and compare with a reference range before the syringe moves.