NCLEX Math Formulas

Essential Conversion Factors

Conversion	Equivalent
1 kg	2.2 lbs
1 grain (gr)	60 mg
1 teaspoon (tsp)	5 mL
1 tablespoon (tbsp)	15 mL (3 tsp)
1 ounce (oz)	30 mL
1 cup	240 mL (8 oz)
1 pint	480 mL (16 oz)
1 quart	960 mL (32 oz)
1 liter (L)	1000 mL
1 gram (g)	1000 mg
1 mg	1000 mcg
1 inch	2.54 cm

Temperature Conversions

Formula	Example
°F to °C: (°F − 32) × 5/9	98.6°F: (98.6 − 32) × 5/9 = 37°C
°C to °F: (°C × 9/5) + 32	38°C: (38 × 9/5) + 32 = 100.4°F

Basic Dosage Formula

D / H × V = Amount

• D = Desired (ordered dose)
• H = Have (available dose on hand)
• V = Vehicle (form it comes in: mL, tablet)

Example: Ordered 750 mg. Available 500 mg/tablet.

750 / 500 × 1 tab = 1.5 tablets

IV Flow Rate Formulas

mL per Hour

Total Volume (mL) / Total Time (hours) = mL/hr

Example: 1000 mL over 10 hrs = 1000/10 = 100 mL/hr

Drops per Minute

(Volume × Drop Factor) / (Time in minutes) = gtt/min

Example: 250 mL over 2 hrs with 20 gtt/mL tubing

(250 × 20) / 120 = 5000/120 = 42 gtt/min

Infusion Time

Total Volume / Rate (mL/hr) = Time in hours

Example: 500 mL at 75 mL/hr = 500/75 = 6.67 hours (6 hrs 40 min)

Weight-Based Calculations

Dose = Weight (kg) × Dose per kg

Pounds to kg: Weight (lbs) / 2.2 = kg

Example: Patient 154 lbs, order 0.5 mg/kg

154 / 2.2 = 70 kg. Dose = 70 × 0.5 = 35 mg

Concentration Calculations

Concentration = Amount of Drug / Volume of Solution

Example: 500 mg in 250 mL = 500/250 = 2 mg/mL

mcg/kg/min Drip Calculations

1. Calculate the dose per minute: Weight (kg) × Dose (mcg/kg/min)
2. Convert to mg/min if needed: mcg/min / 1000
3. Calculate concentration: Drug amount (mg) / Volume (mL)
4. Calculate mL/min: Dose (mg/min) / Concentration (mg/mL)
5. Convert to mL/hr: mL/min × 60

Units per Hour

Formula: Desired units/hr / Concentration (units/mL) = mL/hr

Example: Heparin 800 units/hr. Bag: 25,000 units/500 mL (= 50 units/mL)

800 / 50 = 16 mL/hr

Percent Solution

A percent solution = grams of solute per 100 mL of solution

• 1% solution = 1 g/100 mL = 10 mg/mL
• 0.9% NS = 0.9 g NaCl per 100 mL = 900 mg/100 mL
• D5W = 5 g dextrose per 100 mL

BSA (Body Surface Area) Calculations

Formula (Mosteller):

BSA (m²) = √[(Height in cm × Weight in kg) / 3600]

Used primarily for chemotherapy dosing: Dose = BSA × mg/m²

Intake and Output

• 1 oz = 30 mL
• Ice chips: record half the volume (1 cup ice = 120 mL fluid)
• Include: IV fluids, PO fluids, tube feeding, blood products, IV meds
• Normal adult urine output: 30 mL/hr minimum (0.5 mL/kg/hr)
• Pediatric urine output: 1–2 mL/kg/hr

Quick Reference: Common Calculations at a Glance

What You Need	Formula
Tablets/capsules to give	Desired / Have × 1 tablet
mL of liquid to give	Desired / Have × Vehicle (mL)
Pump rate (mL/hr)	Volume / Time (hrs)
Drops per minute	(Volume × Drop Factor) / Time (min)
Infusion time remaining	Volume remaining / Rate (mL/hr)
Weight-based dose	Weight (kg) × mg/kg
mcg/min to mL/hr	(mcg/min / concentration in mcg/mL) × 60
units/hr to mL/hr	units/hr / concentration (units/mL)