How do you convert drops per minute to mL/h?
Multiply the drops per minute by 60, then divide by the drop factor. It is the drip rate formula run backwards: you start from the drops you counted and find the volume per hour they represent.
Rate (mL/h) = Drops per minute (gtt/min) × 60 ÷ Drop factor (gtt/mL)
The × 60 turns drops per minute into drops per hour; the ÷ drop factor turns drops into millilitres. With a standard 20 gtt/mL set the result is simply drops × 3. With a 60 gtt/mL microdrip set it equals the number of drops. To go the other way, from a prescribed rate to the drops to set, use the drip rate calculator.
How to use the calculator
- Count the drops falling into the drip chamber for one minute (or for 15 seconds, then multiply by 4).
- Enter the number in Drops counted per minute.
- Select the Drop factor of the giving set printed on the packaging: 10, 15, 20 or 60 gtt/mL.
- Read the Flow rate in mL/h and compare it with the prescribed rate.
Worked examples
Example 1 – a 15-second count. You count 10 drops in 15 seconds on a 20 gtt/mL set; the prescription is 125 mL/h. 10 × 4 = 40 gtt/min, then 40 × 60 ÷ 20 = 120 mL/h. That is slightly below the prescribed 125 mL/h, which needs 42 gtt/min on this set.
Example 2 – a 15 gtt/mL set. You count 30 gtt/min. 30 × 60 ÷ 15 = 120 mL/h. The same flow as Example 1 from fewer drops, because each drop is larger.
Example 3 – microdrip. You count 25 gtt/min on a 60 gtt/mL set. 25 × 60 ÷ 60 = 25 mL/h.
Quick reference
Flow rate in mL/h for counted drops and common drop factors.
| Counted | 10 gtt/mL | 15 gtt/mL | 20 gtt/mL | 60 gtt/mL |
|---|---|---|---|---|
| 10 gtt/min | 60 | 40 | 30 | 10 |
| 20 gtt/min | 120 | 80 | 60 | 20 |
| 30 gtt/min | 180 | 120 | 90 | 30 |
| 40 gtt/min | 240 | 160 | 120 | 40 |
| 50 gtt/min | 300 | 200 | 150 | 50 |
| 60 gtt/min | 360 | 240 | 180 | 60 |
Once you have the real mL/h, the infusion time calculator tells you when the bag will finish, and the fluid balance calculator helps you total what went in over the shift.
Common mistakes
- Entering the 15-second count directly. If you counted for 15 seconds, multiply by 4 first. Otherwise the result is four times too low.
- Choosing the wrong drop factor. 40 gtt/min is 240 mL/h on a 10 gtt/mL set but only 40 mL/h on a microdrip set.
- Counting too briefly at slow rates. At 8–10 drops a minute, a 15-second count of 2 drops cannot tell 8 from 11. A full-minute count is more reliable.
- Counting while the line is moving. After adjusting the clamp or repositioning the patient, it’s best to let the drip settle before you count.
A quick sense check
With a 20 gtt/mL set the mL/h should be about three times the drops per minute; with a microdrip set, the same number. If the measured rate is far from the prescription, it’s worth checking the line, the cannula and the bag height before you adjust the clamp.
Frequently asked questions
How do I convert gtt/min to mL/h?
Multiply the drops per minute by 60, then divide by the drop factor of the set. 40 gtt/min with a 20 gtt/mL set is 40 × 60 ÷ 20 = 120 mL/h. The × 60 turns drops per minute into drops per hour, and the drop factor turns drops into millilitres.
Is there a shortcut with a 20 gtt/mL set?
Yes: with 20 gtt/mL, the mL/h are the drops per minute multiplied by 3. So 40 gtt/min is 120 mL/h. The same logic gives × 6 with a 10 gtt/mL set and × 4 with a 15 gtt/mL set. It is a useful mental check on the calculator at the bedside.
Is there a shortcut for a microdrip set?
Yes. With 60 gtt/mL the drops per minute and the mL/h are the same number, because the × 60 and the ÷ 60 cancel out. 25 gtt/min is 25 mL/h. This makes microdrip sets easy to check, but only at low rates, where the drops are still slow enough to count.
How should I count the drops?
It helps to hold your watch next to the drip chamber at eye level and count for a full minute. Or count for 15 seconds and multiply by 4. Longer counts are more accurate, especially at slow rates. After you adjust the clamp or move the patient, it’s best to wait for the drip to settle.
Why does my count not match the prescribed rate?
Gravity flow changes with the height of the bag, the position of the arm or cannula, kinks in the line and how full the bag is. A small difference is common. A large one is worth checking: we suggest looking at the line, the cannula site and the bag height before you adjust the clamp.
What if I do not know the drop factor?
It is printed on the giving-set packaging or on the label of the set itself. Without it the conversion cannot be done, because the same count means very different volumes. 40 gtt/min is 240 mL/h on a 10 gtt/mL set but only 40 mL/h on a microdrip set. Guessing the drop factor is best avoided.
Can I use the result to work out when the bag will finish?
Yes. Take the mL/h from here and enter it, with the volume left in the bag, into the infusion time calculator. It gives the remaining time in hours and minutes and, if you add a start time, the clock time the bag should finish. If the rate changes, it’s worth counting again.