IV flow rates: millilitres per hour and drops per minute
- Calculation
With a pump the rate is set in mL/h; with a gravity set, drops per minute are counted using the set's drop factor.
What it is
An infusion is ordered as a volume over a time (for example, one litre over 8 hours). With an infusion pump it is set in millilitres per hour. Without a pump it is regulated with the roller clamp by counting drops per minute, and for that you need the set's drop factor, printed on the package: macrodrip sets give 10, 15 or 20 drops per mL and microdrip sets 60 microdrops per mL. Drops cannot be split: round to the nearest whole number.
Formulas
- mL/h = total volume (mL) ÷ hours.
- gtt/min = volume (mL) × drop factor ÷ minutes.
- Equivalent: gtt/min = mL/h × drop factor ÷ 60.
- Macrodrip (20 gtt/mL): gtt/min = mL/h ÷ 3.
- Microdrip (60 microdrops/mL): microdrops/min = mL/h.
- Reverse: mL/h = gtt/min × 60 ÷ drop factor.
How to calculate it
- Read the order: total volume and time — these are the data for the formula.
- Find the set's drop factor on the package — it changes the result.
- Convert hours to minutes if you are calculating drops per minute — the formula uses minutes.
- Apply the formula and round to a whole number — there are no fractions of a drop.
- Check with the shortcut (mL/h ÷ 3 or mL/h) — two methods that agree confirm the answer.
- Set the clamp while counting the drops in the drip chamber, then recheck — the ordered volume must run in the ordered time.
Worked examples (educational, not a clinical order)
| Data | Set-up | Answer |
|---|---|---|
| 1000 mL over 8 h, 20 gtt/mL macrodrip | 1000 ÷ 8 = 125 mL/h; 1000 × 20 ÷ 480 min = 41.7 (shortcut: 125 ÷ 3 = 41.7) | 125 mL/h ≈ 42 gtt/min |
| 500 mL over 6 h, 60 microdrops per mL microdrip | 500 ÷ 6 = 83.3 mL/h; 500 × 60 ÷ 360 min = 83.3 | 83 microdrops per min |
| 100 mL over 30 min, 20 gtt/mL macrodrip | 100 × 20 ÷ 30 = 66.7; in one hour 200 mL would run (200 ÷ 3 = 66.7) | 200 mL/h ≈ 67 gtt/min |
| 1500 mL over 24 h, 20 gtt/mL macrodrip | 1500 ÷ 24 = 62.5 mL/h; 1500 × 20 ÷ 1440 min = 20.8 | 62.5 mL/h ≈ 21 gtt/min |
| You count 28 gtt/min on a 20 gtt/mL macrodrip. How many mL/h, and how long will 1000 mL last? | 28 × 60 ÷ 20 = 84 mL/h; 1000 ÷ 84 = 11.9 h | 84 mL/h; about 11 h 54 min |
Common mistakes
- Using the wrong drop factor (20 instead of 60, or the reverse).
- Dividing by hours when the formula asks for minutes.
- Not counting the actual drops after setting the clamp.
- Not rechecking the drip and the IV site during the infusion.
Double-checking
- Calculate with the full formula and confirm with the shortcut.
- Compare the volume infused with what was expected every hour.
- High-alert infusions go on a pump, not by gravity.
Test yourself
Think of the answer, then tap the question to see it.
What is the formula for drops per minute?
Volume (mL) × drop factor (gtt/mL) ÷ time (minutes).
With a 60 gtt/mL microdrip set, how many microdrops per minute equal 40 mL/h?
40 microdrops/min, because with a microdrip set microdrops/min equal mL/h (educational example).
Educational example: 90 mL/h with a 20 gtt/mL macrodrip set. How many drops per minute?
90 ÷ 3 = 30 gtt/min (made-up numbers, educational, not a clinical order).
Connections
Educational content for students; it does not replace your institution's protocols, supervised clinical training or a prescriber's order. Dose-calculation examples use made-up numbers: they are educational, not a clinical order.
Sources: Nursing Skills (Open RN), 5.14 IV Infusion by Gravity (Open RN, Chippewa Valley Technical College (LibreTexts), 2022); Nursing Skills (Open RN), 5.15 IV Infusion by Pump (Open RN, Chippewa Valley Technical College (LibreTexts), 2022); Nursing Skills (Open RN), 5.16 IV Completion Time (Open RN, Chippewa Valley Technical College (LibreTexts), 2022); Nursing Skills (Open RN), 23.2 IV Therapy Basics (Open RN, Chippewa Valley Technical College (LibreTexts), 2022).
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Representative model, for educational purposes only.