Infusion Time
Infusion time answers “how long will this take?” when you know the volume and the rate in mL/hr.
In this lesson
- The volume ÷ rate formula
- Turning decimal hours into hours and minutes
- Why 0.5 hr is 30 minutes
What you're solving for
Time — first in hours, then usually written as hours and minutes.
Volume in milliliters divided by rate in milliliters per hour equals time in hours
Decimal hours to hours and minutes
The whole number is the hours. Multiply the decimal part by 60 to get minutes. An hour has 60 minutes, not 100, so 0.5 hr = 30 min, not 50 min, and 0.75 hr = 45 min.
decimal part of the hours times 60 equals minutes
Worked examples
Example 1: A whole number of hours
500 mL at 100 mL/hr.
Step 1: Formula
Volume (mL) ÷ Rate (mL/hr) = Time (hr)
Step 2: Calculation
500 mL ÷ 100 mL/hr
= 5 hr
Answer: 5 hr
Example 2: Half an hour
250 mL at 100 mL/hr.
2.5 hr → 2 hr + 0.5 × 60 min.
Step 1: Formula
Volume (mL) ÷ Rate (mL/hr) = Time (hr)
Step 2: Calculation
250 mL ÷ 100 mL/hr
= 2.5 hr
Step 3: Convert the part hour to minutes
Whole hours: 2
0.5 hr × 60 = 30 min
= 2 hr 30 min
Answer: 2 hr 30 min
Example 3: Hours and minutes
1,000 mL at 120 mL/hr.
Step 1: Formula
Volume (mL) ÷ Rate (mL/hr) = Time (hr)
Step 2: Calculation
1,000 mL ÷ 120 mL/hr
= ≈ 8.3333 hr
Step 3: Convert the part hour to minutes
Whole hours: 8
≈ 0.3333 hr × 60 = 20 min
= 8 hr 20 min
Answer: 8 hr 20 min
Example 4: Less than an hour
150 mL at 200 mL/hr.
Step 1: Formula
Volume (mL) ÷ Rate (mL/hr) = Time (hr)
Step 2: Calculation
150 mL ÷ 200 mL/hr
= 0.75 hr
Step 3: Convert the part hour to minutes
Whole hours: 0
0.75 hr × 60 = 45 min
= 45 min
Answer: 45 min
Common mistakes
Reading decimal hours as minutes
- What goes wrong
- 2.5 hr written as 2 hr 50 min
- Why it fails
- The .5 is half of 60 minutes.
- Correct approach
- 2.5 hr = 2 hr 30 min
Dividing rate by volume
- What goes wrong
- 100 mL/hr ÷ 500 mL = 0.2
- Why it fails
- That's the fraction of the bag per hour, not hours.
- Correct approach
- 500 mL ÷ 100 mL/hr = 5 hr
Quick self-check
900 mL at 200 mL/hr?
Show the answer
900 ÷ 200 = 4.5 hr = 4 hr 30 min
Calculate and practice
Related lessons
- IV Flow Rate (mL/hr) — Volume ÷ time in hours = mL/hr.
- IV Drip Rate (gtt/min) — Gravity drops per minute using the tubing's drop factor.
- Nursing Unit Conversions — Move between metric units, pounds and kilograms, and hours and minutes.
Sources
- Nursing Skills — Math Calculations — Open RN / NCBI Bookshelf
Explanations are original Dosage Calcs wording. See all sources and the methodology.
Educational Use & Safety
Educational content only. These lessons teach medication math using fictional values and do not determine an appropriate medication, dose, infusion rate, or treatment for an actual patient.
Educational Use & Safety