Percentage and Fraction Shortcuts for Trading Interviews

A short table of fraction to percent equivalents, plus building percentages from 10 percent and 1 percent, covers most timed trading interview arithmetic.

Percentage and fraction shortcuts matter in trading interviews because prices, spreads and position sizes are almost always expressed as one when the arithmetic underneath is the other, and converting between the two forms fast is what separates a confident quote from a hesitant one. Memorizing a short table of fraction-to-percent equivalents, plus a couple of methods for building an arbitrary percentage out of 10 percent and 1 percent, covers most of what shows up in a timed market-making round or a standalone estimation question on interview questions.

Eighths and sixteenths come up constantly because they map onto how prices were historically quoted and still show up in position-sizing questions. One eighth is 12.5 percent, three eighths is 37.5 percent, five eighths is 62.5 percent and seven eighths is 87.5 percent, all built from the same 12.5 percent step repeated. One sixteenth is 6.25 percent, half of the eighth step, which is the fastest way to derive it rather than memorizing it separately.

Thirds and sixths are the other pair worth knowing cold. One third is 33.33 percent repeating, two thirds is 66.67 percent, one sixth is 16.67 percent and five sixths is 83.33 percent. Recognizing a ratio as close to one of these, even when it is not exact, gives a fast mental anchor to correct from rather than starting a percentage calculation from nothing.

A price moving from 84 to 91 is a change of 7 on a base of 84, and 7 divided by 84 simplifies to 1 divided by 12, which is 8.33 percent. Spotting that 7 goes into 84 exactly 12 times is faster than dividing 7 by 84 directly, and it works because the numerator is deliberately a clean factor of the denominator in most interview-style examples.

When the numbers do not simplify as neatly, round to the nearest clean fraction first and correct afterward. A move from 118 to 127 is a change of 9 on a base of 118, close to 9 divided by 120, which is 7.5 percent, and the actual answer of 7.63 percent is close enough to that estimate to be usable immediately, with a smaller correction added only if the question demands precision.