Drops a minute

Drops a minute is the millilitres an hour times the set's drop factor, divided by 60.

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What it is for

When there is no pump, the rate is set by counting drops against a watch. The drop factor is printed on the giving set's packet, it is different between sets, and using the wrong one is the whole error.

How to think about it

Get the millilitres an hour first. Multiply by the drop factor from the packet, which is drops per millilitre. Divide by 60 because the rate was per hour and drops are counted per minute. Round to the nearest whole drop: half a drop cannot be counted.

Worked example

gtt/min = (mL/h x gtt/mL) / 60

The formula, with the drop factor taken off the set's packet.

(80 mL/h x 15 gtt/mL) / 60 = 20 gtt/min

1,200 divided by 60.

(100 mL/h x 60 gtt/mL) / 60 = 100 gtt/min

On a 60 drop set the two numbers match, which is why it is used for small volumes.

Round to a whole drop

You cannot count a fraction of a drop, so the answer is always a whole number.

Your turn

A set delivers 10 gtt/mL and the rate is 150 mL/h. Write the drops a minute.

(150 x 10) / 60 =  gtt/min

The trap

Leaving the 60 out. The answer comes out sixty times too fast, and a drip that fast is a bag emptied in minutes.

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