Dosage calculation: free calculator and practice problems

Work out tablets, liquids, weight-based doses, IV rates in mL/hr and gtt/min, infusion times and heparin or insulin units/hr. Every answer shows the steps and the rounding rule.

Tablets and liquids

Desired over have: D ÷ H × Q.

Dose ordered (D)Dose unitDose on hand (H)Dose unitQuantity that holds it (Q)Form

The client is a child: round mL to hundredths

Answer

10 mL

Before rounding: 10 mL

Worked steps

The dose ordered and the dose on hand use the same unit. No conversion is needed.
Divide the dose ordered by the dose on hand, then multiply by the quantity.D ÷ H × Q = 250 mg ÷ 125 mg × 5 mL = 10 mL
Round the final answer only. 1 mL or more rounds to tenths. Less than 1 mL rounds to hundredths.10 mL → 10 mL

For exam preparation only. This tool does not replace a prescription, a drug reference, a pharmacist or your facility's policy. Before you give a medication, check the order, use approved references, and get an independent double check where your policy requires it.

How to do each dosage calculation

Nursing programs teach three methods: the formula method (desired over have), ratio and proportion, and dimensional analysis. They give the same answer. Use one method to solve and a second one to check. Write the unit next to every number, and round only the final answer.

Tablets and liquids: desired over have

Amount to give = D ÷ H × Q

D is the dose ordered. H is the dose on hand, the strength on the label. Q is the quantity that holds H: 1 tablet, or the number of mL on the label.

D and H must be in the same unit before you divide. If the order is in mcg and the label is in mg, convert first. 1 mg = 1,000 mcg and 1 g = 1,000 mg.

Check the answer for sense. One to two tablets is usual. A dose of 8 tablets or 30 mL from one injection vial points to a decimal or unit error.

Order: cefaclor 250 mg. Label: 125 mg per 5 mL. 250 ÷ 125 × 5 = 10 mL.

Weight-based doses and safe ranges

Dose = dose per kg × weight in kg   (÷ number of doses when the order is per day)

Convert pounds to kilograms first. Divide pounds by 2.2. Keep every digit until the end, unless the question tells you to round the weight.

A reference may give a safe range, such as 10 to 15 mg/kg/dose. Multiply both ends by the weight. Then compare the dose ordered with that range. A dose above the range is not given. Contact the prescriber.

When the range is per day, find the daily range, then divide it by the number of doses. That gives the range for each dose.

A child weighs 22 lb (10 kg). Order: acetaminophen 15 mg/kg. 15 × 10 = 150 mg. Label: 160 mg per 5 mL. 150 ÷ 160 × 5 = 4.6875, given as 4.69 mL.

IV pump rate in mL/hr

mL/hr = total volume (mL) ÷ time (hours)

If the time is in minutes, divide the minutes by 60 to get hours. A pump runs in mL/hr even when the order says 30 minutes.

Textbooks round mL/hr to a whole number. Some pumps take tenths. On the exam, the question tells you which.

500 mL over 45 minutes. 45 ÷ 60 = 0.75 hours. 500 ÷ 0.75 = 666.67, so 667 mL/hr.

IV drip rate in gtt/min

gtt/min = total volume (mL) × drop factor (gtt/mL) ÷ time (minutes)

The drop factor is printed on the tubing package. Macrodrip tubing gives 10, 12, 15 or 20 drops per mL. Microdrip tubing gives 60 drops per mL.

If you already know the mL/hr, use mL/hr × drop factor ÷ 60. With microdrip tubing, gtt/min equals mL/hr.

Round to a whole number of drops.

1,000 mL over 8 hours with 15 gtt/mL tubing. 1,000 × 15 ÷ 480 = 31.25, so 31 gtt/min.

Infusion time and end time

Hours = volume (mL) ÷ rate (mL/hr)

The part after the decimal point is a part of an hour. Multiply it by 60 to get minutes.

Add the time to the start time on a 24-hour clock. Past 2400, the infusion ends the next day.

500 mL at 75 mL/hr. 500 ÷ 75 = 6.667 hours. 0.667 × 60 = 40 minutes. Started at 0800, it ends at 1440.

Heparin, insulin and mcg/kg/min infusions

mL/hr = dose per hour ÷ concentration      dose per hour = mL/hr × concentration

Find the concentration first: the drug in the bag divided by the bag volume. Heparin 25,000 units in 250 mL is 100 units/mL.

For a dose per kg, multiply by the weight in kg. For a dose per minute, multiply by 60 to get a dose per hour. Convert mg to mcg when the order is in mcg.

Heparin and insulin infusions are high-alert medications. Many facilities require an independent double check of the order, the calculation and the pump settings.

Heparin 25,000 units in 250 mL. Order: 1,000 units/hr. 1,000 ÷ 100 = 10 mL/hr.

Conversions to know

1 g = 1,000 mg. 1 mg = 1,000 mcg. 1 L = 1,000 mL. 1 kg = 2.2 lb. 1 tsp = 5 mL. 1 Tbsp = 15 mL. 1 oz = 30 mL.

To go to a smaller unit, multiply. To go to a larger unit, divide. Units (heparin, insulin) and mEq (potassium) do not convert to mg.

Rounding rules

Round only the final answer. The NCLEX FAQ says the same for calculation items.
On the NCLEX, each calculation item tells you to give a whole number or one to two decimal places. If it asks for decimals, you must type the decimal point.
Liquids for adults: 1 mL or more rounds to tenths. Less than 1 mL rounds to hundredths.
Liquids for children: round to hundredths.
Drops per minute: round to a whole number.
Write a leading zero (0.5 mg). Never write a trailing zero (5 mg, not 5.0 mg).
At work, your facility's policy decides. Follow it.

Practice problems

Ten problems in the style of a fill-in-the-blank calculation item. Use the calculator above to check your working, not to skip it.

Question 1Fill-in-the-blank calculationTablets
Metoprolol tartrate 25 mg by mouth is ordered. The tablets on hand are 50 mg and scored. How many tablets will the nurse give?Your answer, in tablets
Question 2Fill-in-the-blank calculationOral liquids
Amoxicillin 400 mg by mouth is ordered. The suspension on hand is 250 mg per 5 mL. How many mL will the nurse give?Your answer, in mL
Question 3Fill-in-the-blank calculationInjections
Morphine 3 mg IV is ordered for an adult. The vial on hand is 4 mg/mL. How many mL will the nurse draw up? Round to the nearest hundredth.Your answer, in mL
Question 4Fill-in-the-blank calculationConversions
Digoxin 125 mcg by mouth is ordered. The tablets on hand are 0.25 mg. How many tablets will the nurse give?Your answer, in tablets
Question 5Fill-in-the-blank calculationWeight-based dosing
A child weighs 44 lb. A medication is ordered at 20 mg/kg/day by mouth, divided into two equal doses every 12 hours. How many mg is each dose?Your answer, in mg
Question 6Multiple choiceSafe dose range
A child weighs 12 kg. The reference range for a medication is 10 to 15 mg/kg/dose. The prescription is 200 mg a dose. What should the nurse do?
Give 200 mg, because it is close to the range.Hold the dose and contact the prescriber.Give 180 mg, the top of the range.Give half the dose now and the rest in one hour.
Question 7Fill-in-the-blank calculationIV pump rate
1,000 mL of 0.9% sodium chloride is ordered to infuse over 8 hours. At what rate will the nurse set the pump, in mL/hr?Your answer, in mL/hr
Question 8Fill-in-the-blank calculationIV drip rate
1,000 mL of lactated Ringer's is ordered over 6 hours. The tubing drop factor is 15 gtt/mL. How many drops per minute? Round to the nearest whole number.Your answer, in gtt/min
Question 9Fill-in-the-blank calculationInfusion time
An IV antibiotic of 250 mL is running at 100 mL/hr. How many hours will it take to infuse? Give the answer to one decimal place.Your answer, in hours
Question 10Fill-in-the-blank calculationUnits per hour
Heparin 25,000 units in 500 mL of D5W is ordered at 1,200 units/hr. At what rate will the nurse set the pump, in mL/hr?Your answer, in mL/hr

Questions about dosage calculation

What is the formula for dosage calculation?

The formula method is D ÷ H × Q. D is the dose ordered, H is the dose on hand and Q is the quantity that holds it. Convert D and H to the same unit first. For IV rates, use mL/hr = volume ÷ hours, and gtt/min = volume × drop factor ÷ minutes.

When should I round in a dosage calculation?

Round only the final answer. Each NCLEX calculation item says whether to give a whole number or one to two decimal places. Without an instruction, textbooks round liquids of 1 mL or more to tenths, liquids under 1 mL to hundredths, and drops to whole numbers.

How do you calculate an IV drip rate in drops per minute?

Multiply the volume in mL by the drop factor of the tubing, then divide by the time in minutes. Round to a whole number. For example, 1,000 mL over 8 hours with 15 gtt/mL tubing is 1,000 × 15 ÷ 480 = 31 gtt/min.

How do you convert pounds to kilograms for a dose?

Divide the weight in pounds by 2.2. A child of 44 lb weighs 20 kg. Keep all the digits until the final answer unless the question tells you to round the weight.

Is there math on the NCLEX?

Yes. The 2026 NCLEX-RN test plan lists "perform calculations needed for medication administration" under Pharmacological and Parenteral Therapies. NCSBN does not set a fixed share of calculation items. You may see none or several, depending on your exam.

Can I use this calculator for real patients?

No. It is a study tool for exam preparation. At work, follow your facility's policy, use the approved references and devices, and get an independent double check where your policy requires it.

Keep practising every day

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Sources checked 18 October 2026