Why we spill coffee when we walk

I came across a paper that looked at the vexing question of why we so often spill coffee when walking with a cup of it, and what might be done to prevent it. Of course, coffee shops give us lids but quite often we use actual mugs that do not come with lids. The research was stimulated when a physicist at a conference watched his colleagues walk gingerly while holding cups of coffee and started wondering about it.

To read the actual paper in the journal Physical Review E requires a subscription but this article gives you some idea of its main contents.

A fluid’s back-and-forth movement has a certain natural frequency, and this is determined by the size of its container. In their paper published last week in Physical Review E, Krechetnikov and Mayer show that everyday mug sizes produce natural frequencies that just happen to match those of a person’s leg movements during walking. This means that walking alone, without any other interference, is tuned to drive coffee to oscillate in a mug. But the researchers also found that even small irregularities in a person’s walking are important: These amplify the wilder oscillations, or sloshing, which bumps up the chance of a spillage.

So how does one avoid a spill? Krechetnikov and Mayer’s answers may not come as a big surprise. Starting your walk slower—that is, accelerating less—will help. So will leaving a decent gap between the top of the coffee and the mug’s rim; this should be at least one-eighth of the mug’s diameter-for a normal mug, about a centimeter should do it. But the researchers’ “take home” advice is to look at what you’re doing—so long as your mug isn’t filled too high, a watched mug almost guarantees a clean run.

Who says that physics research does not produce useful results?


  1. says

    I had many relatives in the Navy and the merchant marine, and they told me the exact opposite: don’t look at the coffee cup while you’re walking. That always works for me, and it makes sense. You’ve got two modalities at work here, proprioception, which is telling you where your hand is, and vision, which you’re also trying to use to determine where your hand is, and they’re not in perfect sync. So you easily get oscillations and over correction.

    I’ve been on boats on heaving seas, and it makes a huge difference — not looking makes it much easier to avoid spills.

  2. Reginald Selkirk says

    In addition to perhaps not looking at it, do not grip the cup too tightly. Use only one hand, with some bend in the elbow and your arm not overly tensed. If your frame is bouncing, you do not want that motion transmitted to the cup, you want to let your should and elbow function as shock absorbers.

  3. flex says

    My first thought was that if mug diameters are roughly related to the natural walking frequency, the solution would be to determine what each person’s natural walking frequency is, and then design mugs which do not match it.

    You could even have a custom mug business where people send you their measurements and you match the mug diameter to the person.

    I guess both my Engineering degree and MBA are showing….

  4. Bruce H says

    I have some significant experience at sea, and I can confirm PZ’s observations. When you walk, look where you’re going. This is also the advice given to restaurant servers. When carrying a tray of drinks, watch where you’re going, not the tray. The key is to walk deliberately and carefully.

    Really, that’s just good advice for living.

  5. StonedRanger says

    The same problem exists when filling an ice cube tray and putting it in the freezer. Many people have issues walking back to the freezer from the sink without spilling. Many many years ago I read something about it that said the same thing, don’t try to watch the ice cube tray while you walk. I almost never spill now and if I do its when trying to put the tray onto the shelf in the freezer.

  6. says

    Usually a thermos solves that problem, but so does tape if it’s a disposable cup with a plastic lid. The 7-11 lids round my way only have open holes if you peel back part of it.

  7. Menyambal says

    You could put a spoon in the cup. I read that a long time back, and experimented with it a bit. Orientation is a factor.

  8. blf says

    One of the weird minor side-effects of working for the N.American BigDumbCo in France is the company-supplied (and gift) coffee mugs are all the “American”-sized, that is, several gallons too big for the usual espresso shots drunk locally. Upshot is there is considerable room in the mug not occupied by coffee, so it’s close-to-never spilled.

  9. Lassi Hippeläinen says

    Use the spoon, Luke. It interferes with the sloshing. The bigger the better. And as Menyambal@11 says, orientation matters.

  10. =8)-DX says

    @siobhan #9

    Obviously the key to not spilling it is to drink it!!!

    HONK! HONK! Wrong answer! (You’re much more likely to spill coffee if you drink it while carrying it.)
    I’d blame filling the cup too high, that and I’d say mixing well just before carrying will help, because the rotating liquid’s angular momentum should dampen some of the sloshing effects of walking.

  11. felicis says

    I remember experimenting with this while working at PSU (Portland State University -- Go Vikings!) -- slow and deliberate movement works, but also disrupting the oscillation with a sideways motion (I would carry the cup in my right hand, and every few steps shift the cup out and back). I did note that, even with a lid, the oscillations could get large enough to send coffee out the little hole (which I usually kept oriented with my forearm), but by turning it 90 degrees, that was eliminated (making me think my jerky walking made the slosh primarily in line with my walking…)

    Now, however, I work from home and don’t walk with coffee more than a few feet, so there’s not enough space to build up the resonance.

  12. lorn says

    Growing up on navy bases the heroes of the system were the senior chiefs. Typically a bit older, and a rounder in the middle, they had quick minds and feet. It was said you could tell a senior chief by his ability to run at full speed the length of the ship on the DC deck, a hazardous trip because of low pipes and other obstacles, without spilling his coffee. Of course this superhuman feat was made possible by the navy mug. A thick and heavy porcelain unit solid enough to hammer nails, or use as a weapon. The venerable navy mug features prominently in several engagements as an expedient but effective thrown weapon.

    It isn’t the potential for weaponized use that keeps them from spilling but rather the characteristic that makes them a useful weapon, their heft. A heavy mug has a significant moment of inertia and if held loosely it will tend to stay level even as the sailor, and ship, roll. A Styrofoam coffee cup is so light and insubstantial that it easily tips if your attention wanders.

    Then again spilled coffee is not so bad. Spilled coffee, hot strong and black (there is a joke in there about how we like our women), dissolves excess wax and crud so it can be mopped up. The moist grounds make a wonderful sweeping compound nearly unrivaled in its ability to gather up dust and grime.

    Coffee is an institution in the military. They order it by the pallet-load and drink it by the gallon. Wherever you find military personnel you will find someone making and/or drinking coffee. The system runs on it.

  13. Mano Singham says


    Could the unusual heft of the navy cup be the reason why there is disagreement about the best way to avoid spilling? Those who say that you should not look at the cup while walking cite navy experiences as the basis for contradicting the suggestions of the paper. The researchers only looked at cups that are used in everyday life.

  14. lorn says


    I suggest a series of experiments involving a more diverse selection of cup/mug designs. Testing could be divided between observed and and non-observed carry.

    I think that with the proper allocation of resources, is $40,000,000 and ten years too much to answer this important question? The results will define the science of coffee carriage, and drink carriage in general, for generations to come. Think of the children. And science.

    And then we could bring in the engineers. Let’s see what Lockheed Martin and General Dynamics can come up with. We could see an entire new world of nuclear-powered, gyro-stabilized, magnetic containment coffee carriage beverage containers. Man may never needlessly spill a cup of Joe ever again.

  15. sonofrojblake says

    If there’s one thing I love, it’s a ludicrously over-engineered solution to a relatively trivial problem: powered gimbals ftw!

    (Bonus points for swinging boardroom table)

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