One pound is exactly 16 ounces. The sixteen is the defining feature of the avoirdupois system, and it is what separates the everyday ounce from the troy ounce used for precious metals, where twelve go to the pound instead.
About the units
The avoirdupois pound is exactly 0.45359237 kg by international agreement, and the avoirdupois ounce is one sixteenth of it, exactly 28.349523125 g. The name avoirdupois comes from Old French for "goods of weight" and refers to bulk commodities sold by mass rather than by count. Its sixteen-ounce pound competed for centuries with the troy pound of twelve troy ounces, used for gold, silver and gemstones, and with the apothecaries' system used for medicine. The troy ounce is heavier than the avoirdupois ounce — 31.10 g against 28.35 g — but the troy pound is lighter, at 373.24 g, because it contains only twelve of them. That crossover is why precious metal prices are never quoted in pounds.
The exact factor
The factor is exactly 16 in one direction and one sixteenth in the other, so both are exact and both are easy: doubling four times converts pounds to ounces, and halving four times converts back. Because sixteen is a power of two, the fractions that appear in practice are all clean — a quarter pound is 4 oz, an eighth is 2 oz. Compare the troy system, where 1 troy pound is 12 troy ounces, and note that the two systems share the grain: 7,000 grains make an avoirdupois pound and 5,760 make a troy pound.
Where you meet this conversion
American cooking and grocery shopping are the everyday case, where a recipe calls for a pound and a half of beef and the package is labelled 24 oz. Postal rates in the United States switch from ounces to pounds at the one-pound mark. Boxing gloves, fishing line, baby weights and coffee are all sold in one unit and discussed in the other. In babies especially the mixed form is standard: a birth weight of 7 lb 11 oz is 3.49 kg, and reading it as 7.11 lb gives 3.23 kg — an error of a quarter of a kilogram.