How this calculator works
A drop is not a fixed unit of volume. Its size depends on the liquid, the dropper, the angle and the temperature, which is why medicine droppers are usually calibrated. Two common definitions exist. The metric drop is 0.05 mL, so 20 metric drops make one millilitre. The US customary drop is one sixtieth of a US fluid dram, or 1/480 of a US fluid ounce, which is about 0.0616 mL. This page converts a counted number of drops to volume using the definition you choose, or a custom volume per drop from a calibrated dropper.
Formula
millilitres = drops x millilitres per drop. 1 metric drop = 0.05 mL. 1 US customary drop = 29.5735295625 / 480 = 0.061611519921875 mL. Litres = millilitres / 1,000. US teaspoons = millilitres / 4.92892159375. US fluid ounces = millilitres / 29.5735295625.
Worked example
20 metric drops is exactly 1 mL, which is 0.001 L, 0.202884 tsp and 0.0338140 fl oz. 100 metric drops is 5 mL or about 1 teaspoon. 20 US customary drops is 1.23223 mL, so the US drop is about 23 per cent larger.
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Frequently asked questions
Why is a drop not exactly 0.05 mL?
Drop size depends on the surface tension and viscosity of the liquid and on the shape of the dropper. The metric drop is a convention for pharmacy and laboratory work, not a physical constant, so a calibrated dropper or the label on the product should be used when accuracy matters.
Should I use the metric or the US drop?
Use the metric drop for pharmacy and laboratory work and for products that state 20 drops per mL. Use the US customary drop only when a US source defines drops that way. The two differ by about 23 per cent.
Can I use this to measure a medicine dose?
Only with the dropper supplied with that medicine or with a stated volume per drop. Do not assume a household spoon or an uncalibrated dropper gives the labelled amount, and follow the instructions on the product.