First-year nursing · 20-minute session

Calculating IV drip rates

By the end you can work out drops per minute for any infusion, and you will convert hours to minutes every time.

Five parts today, and the time conversion is the one to watch

  1. The formula
  2. A worked example, solved in stages
  3. The common mistakedividing by hours
  4. Rounding to a whole drop
  5. Your turn: a practice question

Drops per minute comes from volume, drop factor and time in minutes

Drops per minute = volume (mL) × drop factor (drops per mL) ÷ time (minutes) Drops per minute = volume × drop factor mLdrops per mL time in minutes, not hours

Volume in mL × drop factor in drops per mL gives total drops. Dividing by minutes spreads them over each minute.

1,000 mL over 8 hours: convert to minutes first, then it is 31 drops per minute

Volume1,000 mL
Time8 hours
Drop factor15 drops/mL

Step 1 · Hours to minutes

8 × 60 = 480 min

Step 2 · Total drops

1,000 × 15 = 15,000

Step 3 · Per minute

15,000 ÷ 480 = 31.25

Step 4 · Round

31 drops/min

Dividing by hours gives 1,875 drops per minute, which is 60 times too fast

Correct rate 31.25 drops per minute against the wrong answer of 1,875, drawn to scale 31.25 drops/min 1,000 × 15 ÷ 480 minutes · correct 1,875 drops/min · 1,000 × 15 ÷ 8 hours · wrong

Bars drawn to scale. If an answer looks far too big, check whether you divided by hours.

Round to a whole drop, because you can't count part of a drop

31.25 sits closer to 31 than to 32, so it rounds to 31 drops per minute 31 32 31.5 31.25

31.25 is closer to 31, so you set 31 drops per minute.

Practice: work out the drip rate for this order

Volume500 mL
Time4 hours
Drop factor20 drops/mL

How many drops per minute? Convert the time first.

The answer is about 42 drops per minute

Step 1 · Hours to minutes

4 × 60 = 240 min

Step 2 · Total drops

500 × 20 = 10,000

Step 3 · Per minute

10,000 ÷ 240 = 41.67

Step 4 · Round

42 drops/min

Got 2,500? That is 10,000 ÷ 4: you divided by hours.

Every time: minutes first, then the formula, then round

1 · Minutes first
Multiply hours by 60. Never divide by hours.
2 · The formula
Volume (mL) × drop factor (drops per mL) ÷ time (minutes).
3 · Round
Set a whole number of drops, because you can't count part of a drop.
Sense check
If the answer looks far too big, check you didn't divide by hours.