HESI A2 Measurement and Conversion study guide

What this topic actually asks of you

Every answer on this topic carries a unit, and a unit that did not travel with the number is how this topic is lost. The arithmetic is a decimal move or a single factor; the reasoning is which factor, in which direction, to how many places.

HESI lists mathematics among the areas the A2 covers. What the publisher does not release is a national content specification — no item count, no clock, no cut score — so nothing on this page claims to mirror an official weighting. The method, not a particular item layout, is what this page teaches.

The conversion factors themselves are not a matter of opinion. The metric prefixes are exact powers of ten, and the exact bridge values — the inch and the pound — trace to definitions held by the International Bureau of Weights and Measures, published for the United States in NIST Special Publication 811. This page uses those; the household values that are clinical shorthand rather than definition (5 mL per teaspoon, 2.2 lb per kilogram) are marked "rounded" in the table, because a question's choices may follow the rounded value.

The eight procedures

1. Move the decimal for metric prefixes — each step is a power of ten, so each conversion is a point that moves three, or two, or one place. → example 1
2. Convert with a factor when the unit is a length, a mass or a volume with a named relationship. → example 2
3. Bridge household and metric with the published equivalents, and know which of them are rounded. → example 3
4. Convert temperature in both directions, with the formula in the right order. → example 4
5. Convert and add time — the one place base-60 arithmetic bites. → example 5
6. Read and write the 24-hour clock. → example 6
7. Read and write Roman numerals, including the subtractive forms. → example 7
8. Chain two conversions when the unit asked for is two steps away. → example 8

The prefix table

The metric system is one table and a decimal move. Learn the table once and the conversions stop being memory work.

Sixty-five kilograms is 65,000 g, and 65,000 g is 65,000,000 mg — two moves, six places, no memorized pair needed.

The bridges between systems

These are the ones a question is built from. The exact ones are exact by definition; the rounded ones are marked, because a question's choices may follow the rounded value.

The two temperature formulas

Both are needed, because a temperature can arrive in either scale.

• Celsius to Fahrenheit: multiply by 1.8, then add 32.
• Fahrenheit to Celsius: subtract 32, then divide by 1.8.

Order matters both times. Adding 32 before multiplying, or dividing before subtracting, gives a number that is wrong by more than rounding.

A four-step method

1. Circle the unit the question wants. The answer's unit is stated in the question; find it before converting anything.

2. Convert one step at a time, writing the factor. Two conversions written out beat one remembered shortcut, because the shortcut hides the direction.

3. Ask whether the number should get bigger or smaller. Milligrams to grams must shrink; grams to milligrams must grow. A conversion that moves the wrong way fails this test immediately.

4. State the unit with the answer, and round only at the end.

The eight worked examples

1. Metric prefixes — the decimal moves

Convert 0.75 g to milligrams, and 2,500 mcg to milligrams.

• Grams to milligrams is three places right: 0.75 → 750 mg.
• Micrograms to milligrams is three places left: 2,500 → 2.5 mg.
• Check the direction: 0.75 g became 750 mg, a larger number for a smaller unit.

The trap: moving two places. Milli is a thousandth and micro is a millionth, so the gap between them is a thousand — three places — not a hundred.

2. Convert with a factor

Convert 1.4 m to centimeters, and 250 cm to meters.

• Meters to centimeters is two places right: 1.4 → 140 cm.
• Centimeters to meters is two places left: 250 → 2.5 m.
• Check both ways: 140 cm ÷ 100 = 1.4 m, the number we started from.

The trap: assuming every metric conversion is three places. The move is the distance between the two prefixes, and centi sits two places from the base unit.

3. Bridge household and metric, and know which factors are rounded

Convert 165 lb to kilograms.

• 1 kg ≈ 2.2 lb, so divide: 165 ÷ 2.2 = 75 kg.
• The same patient by the exact definition (1 lb = 0.45359237 kg) is 74.84 kg.
• Check by multiplying back: 75 × 2.2 = 165 lb, the weight we started from.

The trap: multiplying when the answer must be smaller. A kilogram is heavier than a pound, so a weight in kilograms is always a smaller number than the same weight in pounds.

4. Temperature — both formulas, in order

Convert 37 °C to °F, and 104 °F to °C.

• 37 × 1.8 = 66.6, then 66.6 + 32 = 98.6 °F.
• 104 − 32 = 72, then 72 ÷ 1.8 = 40 °C.
• Check against the anchors: 0 °C = 32 °F and 100 °C = 212 °F, and 37 °C belongs just under 100 °F.

The trap: applying the operations in the other order. (37 + 32) × 1.8 gives 124.2, which is not a temperature any person has.

5. Time — base sixty, so convert before you add

Add 2 h 45 min and 1 h 30 min, and convert 150 min to hours and minutes.

• 2 h 45 min + 1 h 30 min = 3 h 75 min, and 75 min = 1 h 15 min → 4 h 15 min.
• 150 min ÷ 60 = 2 remainder 30 → 2 h 30 min.
• Check in minutes alone: 165 + 90 = 255 min, and 255 ÷ 60 = 4 h 15 min.

The trap: carrying at 100 the way decimals carry. Minutes roll over at 60, so 75 min is 1 h 15 min; writing 1.75 h next to it is the other mistake — a decimal reading of a base-60 quantity.

6. The 24-hour clock

Write 7:45 p.m. in 24-hour form, and read 0730.

• After noon, add 12 to the hour: 7 + 12 = 19 → 1945.
• 0730 is before noon → 7:30 a.m.
• Check the way back: 1945 − 12 = 7, the minutes stay at 45, so 7:45 p.m.

The trap: adding 12 to a morning time (7:30 a.m. is 0730, not 1930). 24-hour times always use four digits, so 7:30 a.m. is written 0730.

7. Roman numerals — read in descending order, subtract when a smaller value leads

Write 48, and read XCIV.

• 48 = 40 + 8 = XL + VIII → XLVIII.
• XCIV = XC (90) + IV (4) → 94.
• Check by adding the values back: XL (40) + VIII (8) = 48, and XC (90) + IV (4) = 94.

The trap: writing 48 as XXXXVIII. Four in a row is not a Roman numeral — 40 is XL, and the subtractive pairs are IV, IX, XL, XC, CD and CM.

8. Chain two conversions

A label gives 1 tsp = 5 mL. How many milliliters are 3 tbsp?

• Tablespoons to teaspoons: 3 × 3 = 9 tsp.
• Teaspoons to milliliters: 9 × 5 = 45 mL.
• Check one step shorter: 1 tbsp = 3 tsp = 15 mL, so 3 tbsp = 3 × 15 = 45 mL.

The trap: stopping after one step and answering 9. The question asked for milliliters; a number without its unit is not an answer.

The traps, collected

• Moving the decimal the wrong number of places between prefixes (three between milli and micro, two around centi).
• Converting in the wrong direction, so a small unit produces a smaller number.
• Using a rounded bridge as though it were exact (2.2 lb per kg), when the choices follow the exact definition.
• Temperature operations in the wrong order, or the formula for the wrong direction.
• Base-60 carried as base-10 when times are added.
• A 24-hour time not written with four digits, when the page asked for 24-hour form.
• Four of the same Roman numeral in a row where a subtractive pair belongs.
• Stopping one conversion short, and answering in the unit before the one asked for.
• A bare number as an answer on a topic where every answer has a unit.

Answer-choice strategies: three moves for when the setup will not come

1. Check the direction with the size of the unit. A gram is bigger than a milligram, so the number of milligrams is always larger. Any choice that got smaller lost the direction.

0.75 g as milligrams must be a bigger number than 0.75 — 750 mg, never 0.00075 mg.

2. Use a familiar anchor value. 0 °C is 32 °F, 100 °C is 212 °F, and 37 °C is close to 98.6 °F. A choice outside that range is gone.

An answer of 204 °F for a body temperature cannot be right, whatever the arithmetic said.

3. Estimate with the coarsest bridge. 1 kg is about 2 lb, so 165 lb is about 80 kg — and the precise answer sits a little under that.

How to practice this

• Write the factor on the line before the number. "× 1,000" above "0.75" is a step you can check later; a converted answer with no factor is a memory test.
• Drill the prefixes as one ladder, not as pairs: kilo → base is ×1,000, base → centi is ×100, centi → milli is ×10, and milli → micro is ×1,000. The step is the distance between the two prefixes, so it is not always three places.
• Convert in both directions on purpose, since every factor appears twice in a real attempt.
• Practice the temperature formula cold, then check it against the two anchor points (0 °C = 32 °F, 100 °C = 212 °F).
• Then use the factors where they are used — in dosage calculation, the unit you convert first is often the one the label did not give you.

Where to go next

• The applied topic — Dosage Calculation: the factors on this page, applied to an order.
• The arithmetic behind the moves — Basic Operations: decimals and rounding, which every conversion ends with.
• Rates — Ratio and Proportion: how a conversion factor becomes a rate.
• The subject's own page — Measurement and Conversion lists what is published for this subject.

Sources. The exact definitions above are taken from NIST Special Publication 811, Guide for the Use of the International System of Units (SI), which restates the SI definitions and the exact metric–inch–pound relationships; the metric prefixes and the exact inch and pound values are those definitions, and NIST publications are in the public domain. The household approximations (5 mL per teaspoon, 15 mL per tablespoon, 30 mL per fluid ounce, 240 mL per cup, 2.2 lb per kilogram) are the rounded clinical values in common use; the table marks each of them as rounded and gives the exact value beside it. The publisher's own account of the areas the exam covers is quoted from its HESI A2 page. Accessed 2026-10-23.

HESI A2 is a trademark of Elsevier Inc. Elk Ridge Academy is an independent study resource and is not affiliated with, endorsed by, or sponsored by Elsevier Inc. Exam names are used descriptively only.