Written and maintained by CASRAI Editorial Board
Published · Last updated
On this page
Not medical advice. This page explains the arithmetic that connects a volume, a time and the drop factor printed on a gravity IV set. It does not say what volume, time or rate anyone should use; that comes from a clinician’s order. All numbers in the examples are arithmetic illustrations.
Short answer: drip rate (drops per minute, gtt/min) = volume in mL ÷ time in minutes × drop factor in gtt/mL. First work out mL per hour (volume ÷ hours), then drops per minute (mL per hour ÷ 60 × drop factor). Read the drop factor off the set’s label; do not assume it.
What you need
- The total volume in mL and the total time the infusion is meant to run.
- The drop factor of the set, the number of drops in 1 mL, written gtt/mL. BD’s tip sheet says microdrip tubing is typically 60 gtts/mL and macrodrip tubing typically 10-20 gtts/mL, so the number differs from set to set.
- A calculator.
Where to look: the drop factor is printed on the set’s labelling and appears in its specifications (for example, a BD gravity set listed as SKU 42293E shows “Drop: 20” and a roller clamp in its specifications). See IV drip chamber parts and label meanings and gravity tubing for where these details sit on a set.
Steps: from volume and time to drops per minute
- Find mL per hour. Total volume in mL ÷ total time in hours = mL per hour. BD’s example: 1000 mL over 4 hours = 250 mL/hr.
- Convert hours to minutes (hours × 60), if you want the one-step version. 4 hours = 240 minutes.
- Read the drop factor from the set’s label (gtt/mL).
- Calculate drops per minute. Volume (mL) ÷ time (minutes), then × drop factor (gtt/mL) = gtt/min. This is the BD formula. The two-step form is identical: mL/hr ÷ 60 × drop factor.
- Round. BD rounds 41.66 gtt/min to 42 gtt/min, because drops are counted in whole numbers.
To see how long a given volume takes at a given rate rather than the reverse, use how long an IV bag takes. For what the drop factor means on a small-drop set, see microdrip.
Worked examples by drop factor
BD’s published example. 1000 mL over 4 hours with a 10 gtt/mL macrodrip set: 4 × 60 = 240 minutes; (1000 ÷ 240) × 10 = 41.66, rounded to 42 gtt/min.
Same volume and time, different sets (our arithmetic). 1000 mL over 4 hours is 250 mL/hr, which is 4.1667 mL per minute. Multiply by each drop factor:
| Drop factor | Calculation | Drops per minute |
|---|---|---|
| 10 gtt/mL | 4.1667 × 10 | 41.7, about 42 |
| 15 gtt/mL | 4.1667 × 15 | 62.5, about 62 or 63 |
| 20 gtt/mL | 4.1667 × 20 | 83.3, about 83 |
| 60 gtt/mL | 4.1667 × 60 | 250 |
The 10 and 20 gtt/mL results match BD’s reference chart row for 250 mL/hr (42 and 83 drops per minute). The chart shows 62 for 15 gtt/mL, so a chart and a calculator can differ by one drop depending on how each rounds.
The microdrip shortcut. With a 60 gtt/mL set, the factor 60 cancels the 60 minutes in an hour, so drops per minute equals mL per hour. BD’s chart shows this in its last column (for example 100 mL/hr = 100 gtt/min).
BD reference chart, to check your answer
Drops per minute for common mL/hr values, from BD’s Appendix A “Reference Gravity Chart of Drops per Minute” (2020).
| mL/hr | 10 gtt/mL | 15 gtt/mL | 20 gtt/mL | 60 gtt/mL |
|---|---|---|---|---|
| 25 | 4 | 6 | 8 | 25 |
| 50 | 8 | 12 | 17 | 50 |
| 125 | 21 | 31 | 42 | 125 |
| 250 | 42 | 62 | 83 | 250 |
| 500 | 83 | 125 | 167 | 500 |
Working backwards
If you know the drops per minute and the drop factor and want mL per hour, rearrange the same formula (our algebra, not from BD): mL/hr = gtt/min × 60 ÷ drop factor. For example, 42 gtt/min × 60 ÷ 10 gtt/mL = 252 mL/hr, close to the 250 mL/hr it came from. The small gap is the rounding.
What can go wrong
- Wrong drop factor. The same mL/hr gives 21 gtt/min on a 10 gtt/mL set and 125 gtt/min on a 60 gtt/mL set (BD chart, 125 mL/hr). Always read the label of the set in use.
- Forgetting the unit conversion. The formula divides by minutes. Using hours gives a result 60 times too small.
- The real flow drifts from the calculated figure. BD lists factors that may alter gravity flow rate: height of the infusion bag, distance to the patient, size of the catheter, and viscosity of the infusion. The calculation gives a target, not a guarantee of what the line delivers. For what a roller clamp is, see IV roller clamp.
- Rounding. Whole drops only, so very low or very high rates carry proportionally more rounding error.
When to get professional help
BD’s worked examples start from a rate ordered by a clinician, and this page does not replace that order or a facility’s own procedures. If the volume, time or set is unclear, or the figures do not agree with the order, ask the prescriber or a pharmacist before changing anything. Gravity sets and pumps differ; see IV equipment: parts, pumps and FDA classes for how the equipment is grouped.
FAQ
How do you calculate drip rate per minute?
Divide the total volume in mL by the total time in minutes, then multiply by the drop factor in gtt/mL. BD’s example: (1000 mL ÷ 240 min) × 10 gtt/mL = 41.66, rounded to 42 gtt/min.
What is the drip rate formula?
Drops per minute = (volume in mL ÷ time in minutes) × drop factor. The equivalent two-step form is mL/hr ÷ 60 × drop factor.
How do you find the drop factor?
It is the number of drops in 1 mL, written gtt/mL, and it is printed on the set’s labelling or listed in its specifications. BD describes microdrip tubing as typically 60 gtts/mL and macrodrip tubing as typically 10-20 gtts/mL; the label on the set in use is what counts.
How is drip rate different from mL/hr?
mL/hr is volume per hour. Drip rate is drops per minute, which depends on the set’s drop factor. The same mL/hr gives different drip rates on different sets. With a 60 gtt/mL set the two numbers are equal.
Sources
- BD (Becton, Dickinson and Company), “Calculating Flow Rates,” tip sheet BD-17095, updated April 22, 2020 (BD Alaris pump module sets; marked archived on bd.com), date checked 2026-11-04: bd.com tip sheet PDF.
- BD gravity administration set, SKU 42293E, product specifications (drop 20, roller clamp), date checked 2026-11-04: bd.com product page.
- Arithmetic in the 1000 mL table and the reverse formula is CASRAI’s own calculation from the formula above.
