Why published drip volumes differ by a factor of five
Published figures for a single drop range from about 0.05 to 0.25 ml, a factor of five, and that is not two measurements of the same thing disagreeing. The higher figure was measured on real taps. The lower one is a laboratory dispensing standard defined for a dropper about three millimetres across. Drop volume is proportional to the width of the rim the drop forms on, so both are correct for their own orifice, and only one of them is about taps.
Last reviewed 14 August 2026
The short answer
Two numbers are quoted side by side across the internet as though they were rival estimates of the same quantity. They are not measuring the same thing.
| Figure | What it is | Measured on |
|---|---|---|
| 0.25 ml | The volume of a drop from a domestic tap, used by the United States Geological Survey for its own calculations. | Real taps. A measured range of 0.2 to 0.33 ml across kitchen and bathroom sink taps. |
| 0.05 ml | The pharmacopoeial standard drop, a convention for dispensing medicine at 20 drops to the millilitre. | A standardised dropper with an orifice about 3 mm across. Not a tap. |
A range that spans a measurement and a convention is not a range of uncertainty. It is two different quantities printed next to each other.
How this works in practice
The physics settles it, and it has been settled since 1864. A drop hanging from a rim grows until its weight exceeds the surface tension holding it there, and then it detaches. Tate's law gives the weight of that drop.
W = 2πrγ, where r is the radius of the rim the drop is forming on and γ is the surface tension of the liquid.
The consequence is the whole argument: drop volume is proportional to the radius of the orifice. A wider rim holds a bigger drop before its weight wins, so a wider outlet makes a bigger drop, and the same water dripping from two different openings produces two different drop sizes.
- A pharmacopoeial dropper tip is about 3 mm across.
- A domestic tap outlet is several times wider than that.
- So Tate's law predicts a tap drop several times larger than a dropper drop, and measurement confirms it.
Which means the two published figures are not in conflict and never were. Each is right for the orifice it was defined or measured on. Quoting them as a range implies an uncertainty about taps that does not exist, and it makes the honest tap figure look like the top of a wide band rather than the only measurement anyone has taken.
What changes the answer
The most quoted UK figure for a dripping tap is 5,500 litres a year, and it is worth working through carefully, because what is wrong with it is not what people assume.
It is published by WaterSafe, with Waterwise, from Water Saving Week 2019. Their page states the figure and a comparison to filling a paddling pool. It does not state a drip rate, a drop volume, a method, a calculation, or a citation to one.
So work backwards. 5,500 litres a year is about 15 litres a day. What drip rate produces that?
| At a drop volume of | You need | Which is |
|---|---|---|
| 0.25 ml, the measured tap figure | 42 drips a minute | A fast, badly worn tap. Entirely plausible. |
| 0.05 ml, the dropper convention | 209 drips a minute | About three and a half a second. At that rate drops merge and it is a trickle, not a drip. |
So the figure is reachable, at the top of both variables at once, and the words doing the work are "up to". This page is reporting that no method is published alongside it. It is not saying the number is wrong, because nobody knows whether it is wrong, and the narrower claim is the one the evidence supports.
What an ordinary drip produces is much smaller. At 10 drips a minute, the figure is 1,314 litres a year. the conversion from a count to a volume has the full table.
Assumptions and sources
Drip Calculator: how much water does a leaking faucet waste?
United States Geological Survey, Water Science School · primary source
- Faucet drip volume, adopted: 0.25 ml
- Faucet drip volume, measured range: 0.2 to 0.33 ml
- Bath tap drip volume: 0.5 ml
- Drips per US gallon: 15140 drips
- Drips per litre: 4000 drips
The pharmacopoeial metric drop, 20 drops to 1 millilitre
United States Pharmacopeia, British Pharmacopoeia and European Pharmacopoeia convention · primary-standard source
- Standard drop volume: 0.05 ml
Tate's law: the weight of a falling drop
Thomas Tate, Philosophical Magazine, 1864, with the later drop-weight literature · primary source
- Drop weight: W = 2 * pi * r * gamma equation
Millilitres in a litre
Exact by definition
SI prefix definition. Milli is a factor of one thousandth.
Measured figure
United States Geological Survey, Water Science School. The upper end of the measured range for kitchen and bathroom sink taps, the lower end being 0.2 ml.
The tap figure is from the United States Geological Survey, which states plainly that there is no scientific definition of the volume of a tap drip and that it measured a number of kitchen and bathroom sink taps to arrive at its own. Two limitations travel with it and both are stated here because they are real: they were American taps, and no sample size was published. The page itself has been read directly, including the source of its own calculator, which is where the conversions are visible rather than merely asserted. Those conversions are self-consistent, and the check is worth showing anyway: at 0.25 ml a litre is exactly 4,000 drips, and that is the figure it publishes. The stronger check is the one in the other unit, because it is arithmetic against a number rather than a restatement of ours: at the same drop volume a US gallon works out at 15,142 drips against the 15,140 published, a difference of about a hundredth of a per cent and consistent with rounding. No other candidate drop volume comes close, so the published conversions pin the figure independently of how the wording reached us.
The dropper convention is real, named and attributable to the pharmacopoeias, but their texts are paywalled and were not read for this page. So it is described as the pharmacopoeial standard drop and no section number is quoted, because quoting one would imply a document this site has not opened.
Tate's law is a classical result from 1864 and the mechanism is not in dispute. It is stated here as physics rather than as a statistic, and any physical chemistry text covering surface tension carries it.
The 5,500 litre figure is recorded on this site as untraceable to a method rather than as incorrect. That distinction is the whole point: we know it is unsupported, and we do not know that it is wrong.
What to do next
Three things, and the last one is the useful habit rather than a step.
- Count your own tap rather than using anybody's annual figure. A published number describes a tap somebody else had.
- Change the drop volume on the calculator between 0.2 and 0.33 ml and see whether your conclusion survives. If it flips, the measurement is not precise enough to support the conclusion, and knowing that is worth more than the number.
- When you meet a water-saving figure anywhere, look for the method next to it. If there is no drip rate, no drop volume and no calculation, the figure is an assertion rather than a measurement, whoever published it.
That last habit is what this whole site is built to make possible. what we found when we tried to trace the figures everyone else uses sets out the two cases we found and why publishing our own constants is the response to them.
Common questions
- Which figure should I use for a tap?
- 0.25 ml, because it is the only published figure that was measured on taps. The range behind it is 0.2 to 0.33 ml for kitchen and bathroom sink taps, with bath taps larger at about 0.5 ml.
- Is the 0.05 ml figure simply wrong?
- No. It is correct for what it describes, which is a standardised dropper used for dispensing medicine. It is in the same range as tap figures only because it is what a general search returns for the volume of a drop, and it has been carried across into water contexts by people who did not check what it was defined for.
- Why not just average the two?
- Because the average of a measurement and an inapplicable convention describes no physical object. A drop of 0.15 ml is not a smaller tap drop or a larger dropper drop. It is a number that would be produced by an orifice neither source measured.
- What about the figure of 5,500 litres a year?
- It is published by WaterSafe with Waterwise, and it is reachable: about 42 drips a minute on the measured tap drop volume, which is a fast and badly worn tap. The page states the number and nothing else, with no drip rate, no drop volume, no method and no citation, so the words carrying the claim are "up to". This page reports that no method is published with it. It does not say the number is wrong.
- Does the type of tap change the drop volume?
- Yes, and that is exactly the point the physics makes. A wider outlet produces a larger drop, which is why bath taps measure larger than basin taps and why a dropper measures smaller than both. If your tap has an unusually wide or narrow outlet, the adjustable field on the calculator is there for that.