What math conversions do nursing students need to know?

A long conversion table can look like a lot to memorize. Most of it comes down to about fifteen facts, and some of those facts are exact while others are rounded.

This guide is for studying only. Real doses follow the order, the label and your facility’s policy, never a study chart.

What are the core math conversions for nursing?

Nursing math rests on a short chart of metric, household, weight, length and temperature facts. The chart below marks each one as exact or rounded. An exact value is true by definition. A rounded value is a friendly number that courses use for easier arithmetic.

You know	It equals	Type
1 kg	1,000 g	Exact
1 g	1,000 mg	Exact
1 mg	1,000 mcg	Exact
1 L	1,000 mL	Exact
1 tbsp	3 tsp	Exact
1 fl oz	2 tbsp	Exact
1 cup	8 fl oz	Exact
1 lb	16 oz	Exact
1 in	2.54 cm	Exact
°C	(°F − 32) × 5/9	Exact
1 kg	2.2 lb	Rounded
1 tsp	5 mL	Rounded
1 tbsp	15 mL	Rounded
1 fl oz	30 mL	Rounded
1 cup	240 mL	Rounded

The exact rows match NIST Handbook 44 (2026). The rounded rows are the values Open RN lists.

How do the metric units fit together?

Each metric step you need is a factor of 1,000, so you move the decimal point three places. The ladder runs kg, g, mg, mcg, and separately L to mL.

Conversion	The number gets	Move the decimal
kg to g	bigger number	3 places right
g to mg	bigger number	3 places right
mg to mcg	bigger number	3 places right
mcg to mg	smaller number	3 places left
mL to L	smaller number	3 places left

A smaller unit needs more of itself, so the number gets bigger. For example, 0.5 L is 500 mL, and 0.4 g is 400 mg.

The symbol for micro in the SI system is µ, so you may see µg in some references. Open RN writes it as mcg, and so does this guide.

How do household measures convert to mL?

Household units form an exact chain, but the step to milliliters is a rounded value that your course picks. NIST also lists a more precise mL value for each unit.

Unit	Chain	Rounded value	NIST value, shortened
1 tsp	1/3 tbsp	5 mL	4.93 mL
1 tbsp	3 tsp	15 mL	14.79 mL
1 fl oz	2 tbsp	30 mL	29.57 mL
1 cup	8 fl oz	240 mL	236.6 mL
1 pint	2 cups, 16 fl oz	Our math: 480 mL (16 × 30)	473.2 mL
1 quart	2 pints, 32 fl oz	Our math: 960 mL (32 × 30)	946.4 mL
1 gallon	4 quarts, 128 fl oz	Our math: 3,840 mL (128 × 30)	3,785 mL

Open RN lists the rounded values for tsp, tbsp, fl oz and cup. It gives no mL value for a pint, quart or gallon, so those three are only our arithmetic at 30 mL per fl oz. Check your program’s table before you memorize them.

The NIST column shortens the longer values in the Guide to the SI (SP 811, Appendix B.9). Handbook 44 also gives a “measuring” teaspoon as 5 mL and a “measuring” tablespoon as 15 mL, which fits the rounded values above.

Open RN also lists 1,000 mL = 1,000 cc, and it gives a range for drops: 15–20 gtt = 1 mL.

A fluid ounce (fl oz) measures volume. An ounce (oz) measures weight. They are not the same unit, so 8 fl oz is a cup and 16 oz is a pound.

What about weight, length, temperature and time?

These groups need only a handful of facts. Weight and length use the same multiply-or-divide habit as the household chain.

Group	Fact	Type
Weight	1 lb = 16 oz	Exact
Weight	1 kg ≈ 2.2 lb (more closely, 1 lb ≈ 0.4536 kg)	Rounded
Weight	1 oz ≈ 28.35 g	Rounded
Length	1 in = 2.54 cm = 25.4 mm	Exact
Length	1 ft = 12 in (30.48 cm)	Exact

Temperature uses a formula. NIST gives °C = (°F − 32) ÷ 1.8 and °F = (°C × 1.8) + 32, and ÷ 1.8 is the same as × 5/9. As a check, 0 °C is 32 °F and 100 °C is 212 °F.

Military time runs on a 24-hour clock. According to Open RN, 12:00 AM is 0000 and 12:00 PM is 1200.

For an AM time, remove the colon and put a zero in front if needed, so 6:30 AM becomes 0630. For 1:00 PM and later, remove the colon and add 1200, so 7:15 PM becomes 1915. You can check your own answers with our military time converter.

How do you convert step by step?

Write the starting number with its unit, then multiply by fractions equal to 1 until only the unit you want is left. This is dimensional analysis. Open RN describes it as placing the unit you want to remove on the diagonal so it cancels.

Use these steps every time:

1. Write the goal unit first.
2. Write what you have, with its unit.
3. Multiply by a fraction from your chart, with the old unit on the bottom.
4. Cross out the units that cancel, then do the arithmetic.
5. Round only at the end, then ask whether the answer makes sense.

Example 1: mcg to mg. Convert 250 mcg to mg.

250 mcg × (1 mg / 1,000 mcg) = 0.25 mg. Check: mg are bigger than mcg, so the number gets smaller.

Example 2: lb to kg. Convert 154 lb to kg.

154 lb × (1 kg / 2.2 lb) = 154 ÷ 2.2 = 70 kg. Check: the kilogram number is smaller than the pound number.

Example 3: a chain in mL. A study problem says a person drank 1 cup of broth and 6 fl oz of juice. Find the total in mL.

1. Broth: 1 cup × (8 fl oz / 1 cup) × (30 mL / 1 fl oz) = 240 mL.
2. Juice: 6 fl oz × (30 mL / 1 fl oz) = 180 mL.
3. Total: 240 + 180 = 420 mL.

Example 4: °F to °C. Convert 102.2 °F to °C.

1. Subtract 32 first: 102.2 − 32 = 70.2.
2. Multiply by 5/9: 70.2 × 5 = 351, and 351 ÷ 9 = 39 °C.

What mistakes cause wrong conversions?

Most wrong answers come from using the right fact in the wrong direction or mixing up similar units. Here are the common ones, with a quick check for each.

• Multiplying lb by 2.2 to get kg. Pounds to kilograms means dividing. Kilograms are bigger units, so the number must shrink.
• Confusing mcg and mg. 1 mg is 1,000 mcg, so a mix-up is a 1,000-fold error. Write both units on every number.
• Confusing tsp and tbsp. A tablespoon is 3 teaspoons. An NCPDP summary hosted by the FDA calls a teaspoon and tablespoon mix-up a 3-fold error. It calls a teaspoon and mL mix-up a 5-fold error.
• Mixing oz and fl oz. One is weight and one is volume. Some courses write oz for fluid ounces, so read the context.
• Rounding too early. Open RN says not to round until the final answer.
• Wrong zeros. Open RN says to write a zero before the decimal point (0.6 mg) and to drop trailing zeros (6 mg, not 6.0 mg).
• Treating ounces as decimals. 2 lb 5 oz is not 2.5 lb, because a pound has 16 ounces, not 10.

For more practice with the household chain, see our lesson on household measures. The TEAS lesson on unit conversions covers the metric ladder and temperature in depth.

Do exact numbers and rounding rules differ by program?

Yes, so follow the table and rounding policy your program gives you. Courses can use different friendly values, and even references differ. Open RN uses 1 kg = 2.2 lb, while Handbook 44 lists 2.205 lb.

Open RN tells students to follow agency policy on rounding. It also gives one example rule for adult oral liquids over 1 mL: round to the tenth, so 17.276 becomes 17.3. Your course may teach a different rule.

If your answer differs slightly from a key, check which conversion values the problem told you to use. Practice with the dosage calculation practice page, or review the TEAS math flashcards to drill these facts.

Quick answers

Which conversions are exact? Metric prefix steps, 1 in = 2.54 cm, 1 lb = 16 oz, and the household chain from tsp to gallon. Conversions between household units and mL, and between kg and lb, are usually rounded.

Why do some charts say 1 fl oz = 30 mL and others 29.57 mL? 30 mL is the rounded value Open RN lists. 29.57 mL is the more precise NIST value. Use whichever your problem or program states.

How do I convert pounds to kilograms? Divide pounds by 2.2. For example, 154 lb ÷ 2.2 = 70 kg.

How many mcg are in 1 mg? There are 1,000 mcg in 1 mg, and 1,000 mg in 1 g.

How do I change 7:15 PM to military time? Remove the colon and add 1200 to get 1915.