A few days into the global outbreak and it seemed like most people didn’t get the memo. Supermarket supplies were still ample and we were all shopping as we normally would. There was no panic buying in stores, anywhere that we could see or that was being reported on the news. A few days later, the realisation that the virus could spread and be labelled as a pandemic saw shoppers rushing to buy anything they could, or you would think so.

The first things to go: toilet paper, soap and hand sanitiser. I didn’t join into the shopping frenzy; by the time I went to the store, all these were gone. Oddly though, bars of soap and shower gels were still in abundance. So surely no one is so desperate. This panic shopping is a fear that everyday essentials would not be available, and not quite at the stage of being triggered to find replacement items.


If you are struggling to find hand soap, you can always try to use shower gel before any other type of soap-replacement. It has the same ingredients as hand soap, though some might not have the ingredients dermatologically tested for sensitive hands. In real desperate times, dishwashing liquid (dish soap, the kind you use by hand, not by dishwasher) is good too. Though as they are commercially made to tackle hard-to-removed grease and oils, they will also remove your naturally occurring skin oils. So using dishwashing liquid repeatedly, is not the most healthy approach. If you must use it, apply hand moisturiser after each wash to help your skin retain some oils. On this note, the same applies for hand sanitiser (if you can actually find some), especially to people with sensitive skin. It will always be best to wash your hands whenever possible.


Anyway, I happened to buy one of the last liquid hand soaps available, I must admit that I myself am a bit lazy to use the bar of soap. It soon became very clear that soap was not coming back to the shelves any time in the near future. So the bottle of 500 mL liquid soap became a treasured item. So I needed to make sure it would last a few weeks.

Method and Rationale

First, I had to take into account that it is not certain that each bottle comes in at 500 mL of liquid soap; some bottles have more, others have less. I had also used some, which I did not measure. I estimated that the bottle contained at least 400 mL of liquid soap when I started my measurements.

Then, using a small graduated tube I poured in a squirt from the bottle (squirt sounds like the wrong word to use). The liquid is very viscous, and it took some time for it to settle to the bottom of the tube. Once it settled the volume was at 1.5 mL, though I imagine there to be some uncertainty. I only had one tube, so I could only have the one replicate. Approximately the same amount of liquid weighed 2 g. Though the sensitivity of this balance isn’t high, so we can imagine this as being less or more than 2 g.

I would consider a small squirt somewhere at 1/4 and 1/6 of the total volume of one full squirt. This means that every time I was my hands, the minimum that I would be using is about 0.25 to 0.40 mL of liquid soap. Since some times I might use a bit more or a bit less, on average I would say I use around 0.35 mL of liquid soap. This creates a nice lather, which works well with my hand size. In addition, I typically do two washes per hand washing activity. This means, I could potentially be using anywhere between 0.5 mL to 0.8 mL of hand soap per session.

Okay, now for the deviations of activity. Sometimes I will forget what I am doing or what I am measuring, and I will take a full squirt from the bottle. This is counted as double of what I usually use. When it is a quick wash, I also might only dispense once. Other times during the day, I might wash hands in the kitchen sink with dishwashing liquid. So counting “times washing hands” is only the number of times when hands are washed in the sink (and specifically, the sink where the soap will remain for the duration of this study).

Measuring the activity. I am quite forgetful, and I will not remember the number of times I dispense from the soap bottle. Therefore, I have placed a mechanical tally counter away from the sink and every time I dispense a small amount of liquid from the bottle (0.75 mL, one unit), I record as one dispensing.

I have established the amount I dispense every time is 0.75 mL (made up from the average of minimum and maximum dispensed per squirt, one unit). Note, that it doesn’t appear that liquid is released linearly to how the dispensing mechanism works.

Preliminary Results

The system seems to work well. On the first day, the bottle was used 9 times (0.75 mL per time). Then we can expect that my behaviour had at least 1 time that I either did not dispense twice (indicative of a quick wash) or that I forgot to record the activity. The former seems to me to be the case, as I was paying a lot of attention to logging my activity.

The second day, I recorded 10 washes.

Approximately 50 uses corresponds to a difference in volume equivalent to a change in 1.2 cm (liquid line). This suggests that I am underestimating the volume used per wash (originally, one unit estimated to be 0.25 mL). If this is the case, then I could be using 3 times more than I estimate unit. Therefore, let’s suppose that I am using 0.75 mL per wash (1.5 mL per washing session), then in 50 uses I have used nearly 40 mL of liquid soap. The bottle has about 11 cm left of liquid, which is approximately 9.2 more 50-set uses. That means there is about 370 mL left over at this stage (working on this assumption, we can also estimate that there was 420 mL of liquid soap to start off with).

Hypothesis

Estimation A: Assuming a starting volume of 400 mL and using 0.75 mL per use.

Based on assumption A (evidence from volume of liquid soap), I expect the bottle of soap to last 530 times, which is equivalent to 270 hand washing sessions. This could be spread over the course of about 27 days or just under a month.

Estimation B: Assuming left over amount of 370 mL based on approximation to height of liquid line.

Based on evidence provided through assumption B (evidence from height of liquid line), there is about 370 mL remaining for liquid soap, this is because in 50 dispenses there is about 40 mL of liquid soap. Assuming a starting volume of 370 mL, then I can use the bottle for another 490 times. If we take the starting volume to be 420, as calculated from these estimates, then, the total number of times I used one unit of liquid soap is 540 times (540 tally counts). This is comparable to my estimations by measuring by volume (Estimation A).


I will have plenty to be doing every day for the next couple of months, and I will report back what I found. As someone who in the past has had extreme OCD, and who has mild symptoms now, my behaviour now, given guidance from the government, doesn’t deviate much from my typical behaviour. I typically will wash my hands before leaving my flat and after arriving from the outside, and any other time when I feel that I have come into contact with something that is dirty or is covered in germs.

I will repeat this experiment with the next full bottle of soap, whenever that might be.

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