IV Drip Rate Lab
Pump rate or gravity drip rate from a volume and a time, with the working shown.
A set volume going in over a set time, like a litre of saline over 8 hours. The drug does not enter the maths.
How much fluid is going in, from the bag or the order.
How long it should take. Half an hour is 0.5.
Printed on the giving set’s packaging. Only needed for a gravity drip.
Pump rate
125mL/h
Gravity drip at 15 gtt/mL
31gtt/min
Practice questions
No pump. Counting drops in the chamber, so the drop factor matters.
Score this session: 0 / 0
The chapter
What a drip rate is, and why it has to be right
An intravenous infusion delivers a volume of fluid, or a dose of a drug dissolved in fluid, over time. The rate is the speed of that delivery. On a pump it is set in millilitres per hour. On a gravity line it is the number of drops you count falling through the chamber each minute.
The rate matters because the body only tolerates a range. Fluid run too fast can overload a heart or a set of lungs that cannot keep up; a drug run too fast can reach a toxic level in minutes. Run too slow, a patient in shock does not get the volume the resuscitation plan depends on, and a pressor never reaches the dose that was ordered. Every rate you set is a decision about how quickly something enters a person.
Every order gives you the numbers you need. For fluid: a volume and a time. For a gravity line: the drop factor of the tubing as well. For a weight-based drug: the dose per kilogram per minute, the patient weight and the concentration in the bag. The three formulas below are only ways of putting those numbers together.
- mL/h
- Millilitres per hour. What a pump is set to.
- gtt/min
- Drops per minute. What you count in the drip chamber on a gravity line.
- gtt/mL
- The drop factor: how many drops of this tubing make one millilitre. Printed on the giving set.
- mcg/kg/min
- A weight-based dose per minute, the way pressors and inotropes are ordered.
Three ways to set a rate
Three calculations cover almost every infusion: how much volume, over how much time, through what equipment. Pick the formula by what the order gives you and what the equipment asks for.
Type 1
Gravity drip
Drops per minute
No pump. You are counting drops in the chamber, so the drop factor of the tubing decides how many drops make a millilitre. It is printed on the bag of the giving set.
gtt/min = (volume mL x drop factor) / time in minutes
Type 2
Pump rate
Millilitres per hour
A pump asks for mL/h and does the rest. The tubing no longer matters to the maths, only the volume and the time.
mL/h = volume mL / time in hours
Type 3
Weight based
mcg/kg/min to mL/h
Pressors and inotropes are ordered by patient weight per minute, but the pump wants volume per hour. The 60 in this formula is the clock: minutes into an hour.
mL/h = (dose x weight x 60) / concentration mcg/mL
The 60-minute clock
A drip calculation divides one hour into equal pieces. The drop factor sets how many, and that number is the divisor. Divide 60 minutes by the drop factor for a fixed divisor, then divide the hourly rate by it to get drops per minute. On a 15 gtt/mL set the divisor is 4, so 120 mL/h is 30 drops a minute. On a 60 gtt/mL microdrip the divisor is 1, so gtt/min and mL/h are the same number.
divisor = 60 / drop factor then gtt/min = (mL/h) / divisor
| Drop factor | Divide mL/h by | Set |
|---|---|---|
| 10 gtt/mL | 6 | Common macrodrip set |
| 15 gtt/mL | 4 | Macrodrip: the true quarter-hour set |
| 20 gtt/mL | 3 | Macrodrip |
| 60 gtt/mL | 1 | Microdrip: gtt/min equals mL/h exactly |
Gravity or pump
A gravity line
The bag hangs above the patient and the roller clamp sets the speed. You work out drops per minute from the volume, the time and the drop factor, then set it by counting: watch the chamber for 15 seconds, multiply the drops by four, and open or close the clamp until the count matches.
A gravity rate drifts. Raising the arm, lowering the bag, a bend at the elbow or a vein that clamps down all change it. Count again after any change in position and before every handover.
A pump
The pump asks for a rate in millilitres per hour and a volume to be infused, and holds the rate whatever the arm does. The drop factor no longer matters. Your work is the arithmetic from the order to mL/h, and a second look at what you keyed in before you press start.
For a weight-based drug the pump still wants mL/h. Convert the dose to a volume per minute from the concentration, then multiply by 60. The worked examples below do exactly that.
Worked examples
Worked example 1: simple drug
1,000 mL normal saline over 8 hours, 15 gtt/mL set
- Get the hourly rate first. 1,000 mL divided by 8 hours is 125 mL/h. If a pump is running, you are already done.
- Find the divisor for the tubing. 60 divided by 15 is 4.
- Divide the hourly rate by it. 125 divided by 4 is 31 gtt/min.
- Sanity check on the clock: 31 drops a minute means about 31 mL every quarter hour, which is 125 mL in the hour. It agrees, so the answer is sound.
Worked example 2: complex drug
Dopamine 400 mg in 250 mL, 80 kg patient, ordered at 5 mcg/kg/min
- Work out the concentration in mcg/mL. 400 mg is 400,000 mcg, divided by 250 mL is 1,600 mcg/mL.
- Work out the dose per minute. 5 mcg/kg/min times 80 kg is 400 mcg/min.
- Convert dose to volume. 400 mcg needed, divided by 1,600 mcg in every mL, is 0.25 mL/min.
- Put it on the clock. 0.25 mL every minute times 60 minutes is 15 mL/h. That is the pump rate.
Norepinephrine 4 mg in 250 mL runs the same way: 4,000 mcg over 250 mL is 16 mcg/mL, so a 70 kg patient at 0.1 mcg/kg/min needs 7 mcg/min, which is 0.44 mL/min, or about 26 mL/h.
Where drips go wrong
Most drip errors are not arithmetic. They are a unit carried in wrong, a number read from memory instead of the equipment, or a rate set once and never looked at again. Each one has a check.
- Hours where minutes belong
- A gravity formula wants time in minutes; a pump formula wants hours. Write the unit beside the number before you divide.
- The wrong drop factor
- A 60 gtt/mL microdrip and a 10 gtt/mL macrodrip give answers six times apart. Read the set, not your memory.
- mg and mcg mixed
- Concentrations come in mg per bag and doses in mcg per minute. Convert the bag to mcg (multiply by 1,000) before anything else.
- The weight left out
- A mcg/kg/min order is per kilogram. Multiply by the patient's weight, and ask whether the weight on the chart is real or estimated.
- Set once, never rechecked
- A gravity rate changes when the arm moves, the bag drops or the vein clamps down. Count again after any change, and before handover.
- A clean answer on the wrong order
- The maths can be right and the rate still wrong. Check the drug, the dose and the patient against the order every time.