Nursing Dosage & Med-Math Quick Reference
A quick-reference guide to common medication-math conversions, dosage formulas, IV calculations, rounding reminders, and nursing math conventions.
These are mathematical, educational references. They are not medication-specific dosing recommendations, and every example uses fictional values.
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Medication mass: mcg, mg, g
1 g = 1,000 mg
1 mg = 1,000 mcg
1 g = 1,000,000 mcg
| Convert | Do this |
|---|---|
| g to mg | multiply by 1,000 |
| mg to g | divide by 1,000 |
| mg to mcg | multiply by 1,000 |
| mcg to mg | divide by 1,000 |
| g to mcg | multiply by 1,000,000 |
| mcg to g | divide by 1,000,000 |
Going down from g to mg to mcg (larger to smaller unit), multiply by 1,000 at each step. Going up from mcg to mg to g (smaller to larger unit), divide by 1,000 at each step.
Examples
- 0.5 g × 1,000 = 500 mg
- 1.25 g × 1,000 = 1,250 mg
- 250 mg ÷ 1,000 = 0.25 g
- 0.5 mg × 1,000 = 500 mcg
- 250 mcg ÷ 1,000 = 0.25 mg
- 2 mg × 1,000 = 2,000 mcg
- 0.001 g × 1,000 = 1 mg
kg is different here. kg is a unit of mass, but in nursing dosage problems it is commonly used for patient body weight rather than as a medication-dose unit in the mcg ↔ mg ↔ g chain. That is why patient weight (lb ↔ kg) has its own section below.
Volume: mL and L
1 L = 1,000 mL
| Convert | Do this |
|---|---|
| L to mL | multiply by 1,000 |
| mL to L | divide by 1,000 |
Examples
- 0.5 L × 1,000 = 500 mL
- 1.25 L × 1,000 = 1,250 mL
- 250 mL ÷ 1,000 = 0.25 L
Patient weight: lb and kg
1 kg = 2.2 lb
| Convert | Do this |
|---|---|
| lb to kg | divide by 2.2 |
| kg to lb | multiply by 2.2 |
Examples
- 110 lb ÷ 2.2 = 50 kg
- 154 lb ÷ 2.2 = 70 kg
- 50 kg × 2.2 = 110 lb
Dosage Calcs uses the nursing educational relationship 1 kg = 2.2 lb consistently in its calculators, practice and lessons, so answers match typical coursework.
Time: hours and minutes
1 hr = 60 min
| Convert | Do this |
|---|---|
| hr to min | multiply by 60 |
| min to hr | divide by 60 |
Examples
- 0.5 hr × 60 = 30 min
- 1.5 hr × 60 = 90 min
- 45 min ÷ 60 = 0.75 hr
- 120 min ÷ 60 = 2 hr
Decimal hours are not minutes. 0.5 hr × 60 = 30 min, not 50 min.
Dosage formulas
Desired over Have
X equals D divided by H, times Q
- D
- desired (ordered) dose
- H
- dose on hand (available)
- Q
- quantity that contains the available dose
- X
- amount to administer, mathematically
D and H must use compatible units before solving.
Fictional example — ordered 500 mg; available 250 mg per 5 mL.
(500 ÷ 250) × 5 mL = 10 mL
This is arithmetic on supplied values. It does not determine whether the ordered dose is appropriate.
Weight-based dose
Ordered dose per kilogram times weight in kilograms equals ordered dose
The dose-per-kg value must be given by the fictional problem or order. Dosage Calcs does not determine an appropriate mg/kg amount.
Fictional example — supplied order 5 mg/kg; weight 44 lb.
44 lb ÷ 2.2 = 20 kg
5 mg/kg × 20 kg = 100 mg
IV formulas
IV flow rate (mL/hr)
Volume in milliliters divided by time in hours equals milliliters per hour
1,000 mL ÷ 8 hr = 125 mL/hr
Gravity IV drip rate (gtt/min)
Volume in milliliters times drop factor in drops per milliliter, divided by time in minutes, equals drops per minute
8 hr × 60 = 480 min
1,000 mL × 15 gtt/mL ÷ 480 min = 31.25 gtt/min → 31 gtt/min
Use the drop factor supplied by the problem or printed on the tubing package. Never assume one. Gravity drip answers are expressed in whole drops in these educational calculations.
Infusion time
Volume in milliliters divided by rate in milliliters per hour equals time in hours. Then the fractional hour times 60 equals minutes.
500 mL ÷ 125 mL/hr = 4 hr
250 mL ÷ 100 mL/hr = 2.5 hr; 0.5 hr × 60 = 30 min → 2 hr 30 min
Drop factor quick reference
Always use the drop factor specified by the problem or the actual tubing package information.
| Drop factor | Note |
|---|---|
| 10 gtt/mL | Seen in nursing math problems |
| 12 gtt/mL | Seen in nursing math problems |
| 15 gtt/mL | Seen in nursing math problems |
| 20 gtt/mL | Seen in nursing math problems |
| 60 gtt/mL | Commonly identified as microdrip tubing in nursing references |
Factors vary by manufacturer and set. This list shows values that appear in coursework; it is not a guide for choosing a factor from memory.
Rounding
Follow the rounding instruction given by the problem, course, instructor, facility or applicable policy. Do not round intermediate steps unless instructed or required by the method. Dosage Calcs does not define a universal clinical rounding rule.
| Round to | Example |
|---|---|
| Whole numberGravity drip rates, when the problem or context calls for whole drops. | 31.25 → 31 |
| Nearest tenthMathematical formatting example only. | 2.46 → 2.5 |
| Nearest hundredthMathematical formatting example only. | 1.234 → 1.23 |
Rounding vs. truncating
Rounding looks at the next digit. Truncating simply drops digits. Mathematical examples:
- 3.67 rounded to the nearest tenth = 3.7
- 3.67 truncated to the tenths place = 3.6
Number safety
Leading zero
Write: 0.5 mg
Not: .5 mg
Use a zero before a decimal value less than one.
No trailing zero
Write: 5 mg
Not: 5.0 mg
Avoid an unnecessary trailing zero in medication-dose notation.
Based on The Joint Commission's dose-designation guidance for medication orders and documentation.
Unit abbreviations used in med math
| Abbreviation | Meaning |
|---|---|
| g | gram |
| mg | milligram |
| mcg | microgram |
| kg | kilogram |
| L | liter |
| mL | milliliter |
| hr | hour |
| min | minute |
| gtt | drop(s) |
| gtt/mL | drops per milliliter |
| mL/hr | milliliters per hour |
| mg/kg | milligrams per kilogram |
| mcg/kg | micrograms per kilogram |
For abbreviations to avoid in documentation, see nursing abbreviations and safety.
Nursing military time quick reference
For 1 PM through 11 PM, add 12 to the hour. 12 PM (noon) stays 1200. 12 AM (midnight) becomes 0000.
| 12-hour | 24-hour |
|---|---|
| 12:00 AM | 0000 |
| 1:00 AM | 0100 |
| 2:00 AM | 0200 |
| 3:00 AM | 0300 |
| 4:00 AM | 0400 |
| 5:00 AM | 0500 |
| 6:00 AM | 0600 |
| 7:00 AM | 0700 |
| 8:00 AM | 0800 |
| 9:00 AM | 0900 |
| 10:00 AM | 1000 |
| 11:00 AM | 1100 |
| 12-hour | 24-hour |
|---|---|
| 12:00 PM | 1200 |
| 1:00 PM | 1300 |
| 2:00 PM | 1400 |
| 3:00 PM | 1500 |
| 4:00 PM | 1600 |
| 5:00 PM | 1700 |
| 6:00 PM | 1800 |
| 7:00 PM | 1900 |
| 8:00 PM | 2000 |
| 9:00 PM | 2100 |
| 10:00 PM | 2200 |
| 11:00 PM | 2300 |
Examples
- 2:30 PM is 1430
- 7:45 PM is 1945
- 12:30 PM is 1230
- 12:30 AM is 0030
What do I do?
- g to mg: multiply by 1,000
- mg to mcg: multiply by 1,000
- mcg to mg: divide by 1,000
- mg to g: divide by 1,000
- lb to kg: divide by 2.2
- kg to lb: multiply by 2.2
- L to mL: multiply by 1,000
- mL to L: divide by 1,000
- hr to min: multiply by 60
- min to hr: divide by 60
Memory aids
- Larger metric unit → smaller unit: the number gets bigger (multiply).
- Smaller metric unit → larger unit: the number gets smaller (divide).
- kg is heavier than lb, so a weight in kg is always a smaller number than the same weight in lb.
- Understand the conversion factor rather than relying only on moving the decimal point. Ask: should the answer be bigger or smaller?
Quick self-check
Optional. No timer, no score, and nothing is saved.
Sources
- Nursing Skills — Math Calculations — Open RN / NCBI Bookshelf
- Pharmacology for Nurses — 2.4 Dosage Calculations — OpenStax
- Writing SI Metric System Units — NIST
- Standards FAQ — abbreviations and dose designations — The Joint Commission
Educational Use & Safety
Educational reference only. These tables and formulas explain nursing medication math and unit conversions. They do not determine whether a medication, dose, concentration, route, frequency, or infusion rate is appropriate for a patient.
Educational Use & Safety