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IV rates · Published by Maksym Shevchuk · Updated

How to calculate mL/hr for an IV infusion

Short answer

To calculate mL/hr, divide the volume in milliliters by the time in hours: mL/hr = mL ÷ hr. With the time in minutes, multiply by 60 min per 1 hr. With an order written as a dose per hour, divide it by the drug in each milliliter of the bag. Round once, at the end, under your program's rule.

How to calculate mL/hr from volume and time

mL/hr = volume (mL) ÷ time (hr).

An infusion pump is usually programmed in milliliters per hour. Open RN notes that in the United States intravenous medications are most commonly given by infusion pump, which the nurse typically programs in mL/hour, and Bergen Community College's module reminds students that infusion pumps are always set at mL/hr.

Open RN's method starts from the unit the answer needs: write mL over hr, put the ordered volume over the ordered time, and divide. When the units already read mL over hr, there is nothing left to convert.

The worked example in this guide is an infusion after surgery, a bag set to run over a stated number of hours. A volume divided by a time is not a dose desired over a dose on hand, so the formula method does not fit it. The example is worked in dimensional analysis and in ratio and proportion, where the proportion finds the milliliters in one hour.

Worked example · Pump rates

After surgery, an infusion of lactated Ringer solution totaling 1,000 mL is to run over 10 hr. At what rate is the pump set, in milliliters per hour?

Dimensional analysis

1,000 × 1 ÷ 10 = 100

Ratio and proportion

  1. 1,000 mL is to 10 hr as x mL is to 1 hr

    1,000 × 1 ÷ 10 = 100

    x = 100 mL

    that is 100 mL per hr

Formula method

The formula method (D ÷ H × Q) does not fit this problem. The chain above is the working.

The answer

The answer under the stated rule

100 mL per hr

Nearest whole number, half rounds up, the stated rule for rates in milliliters per hour. The working comes to 100 mL/hr exactly, so the rule leaves it as it is.

What each slip gives here

  • Decimal point one place out1,000 mL per hr or 10 mL per hr
  • Conversion factor inverted10,000 mL per hr

What this problem is built from

When the time is in minutes

A pump runs in hours, so a time in minutes is converted. Open RN's instruction is that because the pump will be set in mL/hour, the minutes are converted to hours, with a factor of 60 minutes over 1 hour placed so the minutes cancel. Dividing the minutes by 60 first and using the hours gives the same rate. In ratio and proportion the minutes can go straight in, as Bergen Community College's module sets it up: the volume is to the minutes as x mL is to 60 min.

Leaving the minutes in place gives milliliters per minute, a number 60 times too small for the pump. The mL per hour practice problems include one with the time in minutes.

From a dose per hour to mL/hr

Some orders are a rate of drug rather than of fluid. Open RN lists units per hour beside mL/hr and gtt/min among the rates nurses calculate. To turn a dose per hour into a pump rate you need the bag's concentration: the amount of drug in the bag and the bag's volume.

mL/hr = dose per hour × bag volume (mL) ÷ drug in the bag.

As a chain, start from the dose per hour and multiply by the bag's volume over the drug in it, so the drug's unit cancels and mL over hr is left. The same problem fits the formula method, with the hourly dose as D, the drug in the bag as H and the bag's volume as Q, and it fits a proportion: the drug in the bag is to its volume as the hourly dose is to x mL in one hour. Bergen's module works an hourly order all three ways.

When the order is per minute, add 60 min over 1 hr to the chain. Open RN's multi-step example goes further, from an order per kilogram per minute with the weight in pounds: it converts the weight and the drug's unit before turning minutes into hours. The dimensional analysis guide works a per-minute order as one chain.

The worked example in this guide is an hourly order in milligrams, run from a bag that states the milligrams it holds and its volume.

Worked example · Pump rates

A diltiazem infusion contains 250 mg in 550 mL of solution. The order is 10 mg per hr. At what rate is the pump set, in milliliters per hour?

Dimensional analysis

10 × 550 ÷ 250 = 22

Ratio and proportion

  1. 250 mg is to 550 mL as 10 mg per hr is to x mL per hr

    10 × 550 ÷ 250 = 22

    x = 22 mL per hr

Formula method

D (desired)
10 mg per hr
H (have)
250 mg
Q (quantity)
550 mL

D ÷ H × Q

10 × 550 ÷ 250 = 22

The answer

The answer under the stated rule

22 mL per hr

Nearest whole number, half rounds up, the stated rule for rates in milliliters per hour. The working comes to 22 mL/hr exactly, so the rule leaves it as it is.

What each slip gives here

  • Decimal point one place out220 mL per hr or 2 mL per hr
  • Conversion factor inverted5 mL per hr
  • Conversion skipped10 mL per hr

What this problem is built from

Rounding pump rates

Programs and texts differ on how far to round a pump rate, so check the rule in your syllabus.

  • The WisTech dosage text rounds final answers in mL/hr, like those in gtt/min, to the nearest whole number.
  • Mercer County Community College's guidelines allow infusion pump rates to be rounded to the nearest tenth.
  • Open RN's multi-step example rounds a pump rate to the hundredth, “depending on the agency policy and available pump settings”.

Whatever the rule, round once. Open RN's rule is not to round until calculating the final answer. The worked examples in this guide round mL/hr to the nearest whole number, half rounding up, and state the rule next to each answer; the rounding rules guide compares published rules.

Check the rate before you set it

Two checks use only the numbers you already have. Multiply the rate by the hours: it should give back the ordered volume. For a dose per hour, multiply the rate by the drug in each milliliter of the bag: it should give back the ordered dose per hour.

A rate ten times too fast or too slow fails both checks, and so does a rate left in milliliters per minute.

mL/hr, drops per minute and infusion time

Volume, time and rate also answer the other IV questions. Open RN notes that an infusion pump is not always available, so the nurse must also know how to calculate drip rates for gravity tubing. From a rate in mL/hr, drops per minute are mL/hr × drop factor ÷ 60; the IV drip rate guide works that through.

Turned around, the volume divided by the rate in mL/hr gives the hours an infusion takes. The infusion time guide and its practice problems work in that direction.

Where points are lost

  1. 1

    Conversion skipped: The time stays in minutes, so the answer is milliliters per minute, 60 times too small for a pump set in mL/hr. Check that the last units standing are mL over hr.

  2. 2

    Conversion factor inverted: The bag's volume over its drug, or 60 min over 1 hr, goes in upside down. The units stop canceling and the rate lands far from anything the order means.

  3. 3

    Decimal point one place out: The pump rate comes out ten times too fast or too slow. Multiplying the rate back by the hours shows it at once.

  4. 4

    Rounded to the wrong precision: The rate is rounded to a tenth where the program wants a whole number, or the other way round. Mercer County allows tenths for pump rates, while the WisTech text rounds mL/hr to the whole number.

Practice this

mL per hr IV rate practice problems

Each problem opens to its worked solution, the answer under a stated rounding rule and the sources it is built from.

Questions

How do you calculate mL per hour?
Divide the volume in milliliters by the time in hours. If the time is in minutes, divide the volume by the minutes and multiply by 60, or turn the minutes into hours first. Round the rate under your program's rule.
How do you calculate mL/hr from mg/hr?
Divide the ordered milligrams per hour by the milligrams in each milliliter of the bag, which is the drug in the bag divided by the bag's volume. The same steps turn an order in units per hour into mL/hr.
Do you round mL/hr to a whole number?
It depends on the program. The WisTech dosage text rounds mL/hr to the nearest whole number, while Mercer County Community College allows infusion pump rates to the nearest tenth. Use the rule in your syllabus.
What is the difference between mL/hr and gtt/min?
mL/hr is the rate an infusion pump is set to. gtt/min is the rate counted in drops when an infusion runs by gravity, and it equals the rate in mL/hr times the tubing's drop factor, divided by 60.

Sources

  1. [1]Open RN, Nursing Skills 2e, 5.13 Intravenous Infusions (WisTech Open, 2023)wtcs.pressbooks.pub
  2. [2]Open RN, Nursing Skills 2e, 5.15 IV Infusion by Pump (WisTech Open, 2023)wtcs.pressbooks.pub
  3. [3]Open RN, Nursing Skills 2e, 5.4 Equivalencies (WisTech Open, 2023)wtcs.pressbooks.pub
  4. [4]Open RN, Nursing Skills 2e, 5.17 Multi-Step Calculations (WisTech Open, 2023)wtcs.pressbooks.pub
  5. [5]Open RN, Nursing Skills 2e, 5.5 Rounding (WisTech Open, 2023)wtcs.pressbooks.pub
  6. [6]WisTech Open, Nursing Dosage Calculations, Introduction to IV Flow Rates and Infusion Time (2024)wtcs.pressbooks.pub
  7. [7]Mercer County Community College Nursing, Medication Calculation Guidelines (updated September 2020)mccc.edu
  8. [8]Bergen Community College Nursing, Pharmacological Math Computation Skills Self Learning Module (2023)bergen.edu

No licensed clinician has reviewed this guide. How the guides are made. Exam practice, never for patient care. Follow your program's rules.