Do Water Weights Get Lighter in the Pool?

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Because lifting and running are loading you through two completely different terms, and water happens to help with one and punish the other. Lifting works against gravity, and water cancels part of gravity by pushing up with the weight of the fluid you displaced, so lifting genuinely does get easier. Running works against drag, and drag goes as the square of your speed through a fluid eight hundred times denser than air, so running gets dramatically harder. Same water, opposite answers, and nothing contradictory about it.

Which sets up the question that matters if you own any of this equipment. A foam aqua dumbbell does not resist you with its weight at all. It resists you with its buoyancy, which is set by its volume. And the one number its listing publishes is its mass, which is the term that gets subtracted. We carry a pair where the published figure is 200 grams and the resistance is about nine times that, and the figure that produces the nine is not on the label.

Archimedes, stated as the thing you can feel

Put an object in water and the water pushes it up with a force equal to the weight of the water that the object has pushed out of the way. Not the weight of the object. The weight of the displaced water. That is the whole of Archimedes' principle and it is worth saying slowly, because the substitution people make without noticing is to assume a light object gets a small push.

It does not. A two litre float displaces two litres of water whether it is made of foam, wood or lead, as long as it is fully under. Two litres of water has a mass of two kilograms, so the upward force is 2 times 9.81, which is 19.6 newtons, regardless of what the float is made of. What the material changes is the downward term, which is the object's own weight, and that is the only place the material appears.

So the net force trying to push a submerged float back to the surface, which is exactly the force you feel resisting you when you push it down, is the displaced weight minus its own weight. For a two litre float weighing 200 grams: 19.6 minus 1.96, which is 17.65 newtons, or about 1.8 kilograms force.

Read that expression once more and notice the sign. Mass is subtracted. A heavier float of the same volume resists you less. If you want more resistance you want more volume, and if you want less you want more mass. That is the exact opposite of how every other piece of weight training equipment works, and it is why the specification convention borrowed from dumbbells gives you the wrong number.

Our own aqua dumbbells, and a listing that contradicts itself

A pair of YOTAN water weights for pool exercise is the whole argument in one product, and its listing contains a genuine physical contradiction that is worth taking apart rather than passing over. Its record publishes: a two piece set, "high-density PE", "about 200g" each, anti-slip textured handles, and in its features "High buoyancy. Dense PE creates resistance for effective water workouts."

Density and buoyancy are opposites. Buoyancy is what you get when an object is less dense than the fluid, and the less dense it is the more it pushes up. So "dense PE creates resistance" has the causation exactly backwards, and the phrase "high-density PE" paired with "high buoyancy" cannot both describe the same object in the sense the words normally carry.

Work it and the scale of the problem is clear. High density polyethylene has a density around 0.95 grams per cubic centimetre, which is only just below water. If these dumbbells were solid HDPE, then 200 grams would occupy 200 divided by 0.95, which is 210 cubic centimetres. Displace 210 cubic centimetres and the buoyant force is 0.210 times 9.81, which is 2.06 newtons, against a weight of 1.96 newtons. Net upward force: 0.1 newtons. Ten grams force. You would not be able to feel it, and the product would not work at all.

So they cannot be solid HDPE, and they are not. An aqua dumbbell of this type is closed cell foam, and a realistic volume for a pair of hand-sized floats is around two litres each. Two hundred grams in two litres is a density of 0.10 grams per cubic centimetre, which is one ninth that of HDPE and means the object is roughly ninety percent air by volume. The polymer in the cell walls may well be high density polyethylene, and that is presumably what the phrase is reaching for, but as a description of the thing you hold it is impossible. The product works precisely because it is not dense.

Against that two litre estimate, the net resistance is the 17.65 newtons computed above, which is 1.8 kilograms force, or about nine times the published 200 gram mass. And the honest part: two litres is my estimate, not a published figure. Make it one litre instead and the net becomes 9.81 minus 1.96, which is 7.85 newtons, or 0.80 kilograms force. Halving the volume cut the resistance by a factor of 2.25 rather than 2, because the weight term is subtracted and does not halve with it. So the unpublished number does not merely scale the answer, it bends it, and nothing on the listing lets you pin it down.

Two things in that record are honest and correct, and they are the two that matter. The cons say "Resistance is light, not heavy loading" and "Effort depends on water and movement speed". Both true, and the second is the drag term arriving on schedule. Our YOTAN water weights review has the rest.

The force curve has a shape, and it is flat

Here is the part that no listing in this category describes and that changes how you would program the exercise.

Push a float down from the surface and the force builds while it is entering the water, because the displaced volume is growing. That build happens over roughly the height of the float, say twelve centimetres. Once it is fully submerged, the displaced volume stops changing, and so does the buoyant force. Push it to twenty centimetres down or to a metre down and it resists you with exactly the same 17.65 newtons. Buoyancy is independent of depth.

So the force curve ramps over the first few inches and then goes perfectly flat for the rest of the stroke. That is an unusual and genuinely useful property. Compare it with the thing we wrote about most recently in this category: a resistance band's force is its spring constant multiplied by its extension, so a band gets harder every inch and is at its easiest exactly where your joint is weakest. Our post on whether a 50 lb resistance band is really 50 pounds works through that force curve and the ratios the labels hide. A float is the opposite animal: constant through the range, with no accumulating sting at the end.

And then the asymmetry, which is the thing people find strangest about pool work once it is pointed out. On the way down you fight the 17.65 newtons. On the way up the same 17.65 newtons is helping you. The difference between the two directions at the same speed is therefore 35.3 newtons, and the upward phase has no loading from buoyancy at all. In gym terms there is no eccentric phase in the direction you would expect one: the water assists the return. If you want the return to cost you something, you have to turn the movement over and push the float down in the other direction, which is why pool routines are built from opposed pairs rather than from lifts.

Where the speed term actually takes over

Drag is the other half and it deserves a number rather than a warning, because the warning overstates it at the pace this exercise is done.

Drag force is one half times the fluid density, times a drag coefficient, times the frontal area, times the square of the speed. Water is 1000 kilograms per cubic metre. Take two dumbbell heads about twelve centimetres across, which is two times pi times 0.06 squared, or 0.0226 square metres of frontal area, and a drag coefficient near 1.1 for a blunt shape.

At a slow, controlled 0.3 metres per second: 0.5 times 1000 times 1.1 times 0.0226 times 0.09, which is 1.1 newtons.

At 0.6 metres per second: four times that, 4.5 newtons, because the speed doubled and the force goes as the square.

At 1.0 metres per second: 12.4 newtons.

Set those against 17.65 newtons of buoyancy. At a controlled pace the drag is about six percent of the total resistance. It only equals the buoyancy when 12.43 times the speed squared reaches 17.65, which gives a speed squared of 1.42 and a speed of 1.19 metres per second. That is a brisk, deliberately fast movement, faster than low impact pool work is normally done.

Which means our own guidance is right and right for a slightly different reason than it gives. Our page on how to start low impact water workouts at home says "Speed increases resistance, so let your effort set the intensity rather than heavy loads." That is correct, it is good advice, and the arithmetic refines it: at the pace that page recommends, buoyancy is supplying almost all of the resistance and speed is the trim rather than the dial. If you want meaningfully more work, moving faster will eventually get you there, but it takes you out of the low impact pace the exercise exists for. Buying more volume is the controlled way to do it, and the speed route arrives with the joint loading you came to the pool to avoid.

The post of ours that already had this right

Credit where it is due, and it is the same short page. Our low impact water workouts guide opens the gear section with this: "Buoyant tools like pool exercise water weights add resistance as you push them under the surface." That sentence is entirely correct and it is more careful than it looks. It does not say the weights are heavy, it does not say they add weight, and it correctly identifies the direction in which they resist you, which is downward against the surface. Everything on this page is an elaboration of it rather than a correction.

The same page also says the buoyant setting "reduces impact while still challenging your muscles", which is the right framing, and recommends "a steady, controlled pace", which the crossover figure above supports. Of the four posts on this site that mention buoyancy at all, this is the one that mentions it five times, and it is the one that got it right.

A float is the same sum run backwards, and our dog guide asks for a figure that does not exist

Turn the problem over and it becomes a flotation question, which is the other half of our catalogue here.

A BINGPET inflatable dog pool float has to hold a dog's top surface clear of the water. The useful thing to realise is that this needs far less displacement than the dog's weight, because a dog is itself close to neutrally buoyant: mammal tissue sits near the density of water, which is why dogs swim rather than sink. So the float is not supporting the animal's whole weight, only the part held above the surface, plus whatever margin keeps it stable. A thirty kilogram dog does not need thirty litres of float.

But here is the correction to our own writing, and it is a specific one. Our guide to choosing a pool float for your dog says: "Universal designs cover small through extra-large breeds, but always check weight and length first." That is sensible advice and it asks you to check a figure the product does not publish. The BINGPET record's specification list runs material, edges, valves, inflation, fits, surface, storage and use. Its "Fits" field says "Small to extra-large dogs". There is no weight capacity anywhere in it, and no dimension either. So a reader who follows our instruction finds nothing to follow it with, and the guide should be telling them that the figure is routinely absent and what to do instead.

What to do instead is the practical part. Since the float only has to lift the portion held clear, the test that matters is stability rather than capacity: whether the dog's weight sits inside the float's footprint, which is a shape question, and whether it stays level when the dog shifts. That you can only judge in use. The BINGPET record is otherwise refreshingly direct about its real limits, saying "Untrimmed nails can still puncture the surface", "Claw resistance means resistant, not indestructible" and "Supervision needed around deep water". All three are the things that actually decide whether the product lasts a summer. See our BINGPET dog pool float review.

And the contrast case, because it is the one product here where none of this applies. A Quppa foldable dog pool with sprinkler does not float on water, it contains it, so Archimedes has nothing to say about it. Its mechanics are hydrostatic instead: the pressure on the wall grows with depth, which is why a shallow pool needs no rigid frame and a deep one does, and why these are always wide and low rather than tall. Worth including because the two products sit next to each other in the same category and are governed by different physics, which the category name hides. Its record is clear that the "PVC is not fully puncture-proof against claws" and that you should "never leave a dog unsupervised around water", both of which are the right cautions. Details in our Quppa foldable dog pool review.

The honest limit, and the free measurement that settles it

Not one product in this category publishes a displaced volume, and volume is the only specification that would let you compare two of them. The aqua dumbbells publish a mass, which is the subtracted term. The dog float publishes neither a capacity nor a dimension. Nobody publishes a frontal area, which is what the drag term needs, and nobody publishes a drag coefficient, which nobody could reasonably be expected to. So the figures on this page are worked from one published mass and two stated assumptions, and the text says so wherever they appear.

The good news is that the missing number is the easiest measurement in this whole batch of articles, and it costs nothing. Fill a bucket or a sink to the brim, stand it in a washing up bowl, push the float fully under, and catch what comes over the edge. Measure that in a jug. That volume in litres, multiplied by 9.81 and with the object's own weight in kilograms times 9.81 subtracted, is the resistance in newtons. Divide by 9.81 again if you would rather have it in kilograms force. For the two litre case above the answer is 1.8 kilograms force, and you will know yours to within a few percent rather than to within a factor of two.

Do it for both dumbbells in a pair, because that is the other thing nobody checks. Two floats from the same box with different volumes resist differently, and since the resistance scales with volume while the published mass is identical, there is no way to detect the mismatch from the label. A jug of water finds it in a minute.

And the technique that is worth more than any of this. Since buoyancy is constant with depth and drag goes as the square of speed, the one variable entirely under your control and not under the maker's is tempo. Slow the movement down and you are almost purely on the flat buoyancy curve, which is the joint-friendly regime the exercise exists for. Speed it up and you are adding a term that rises steeply and brings impact-like peak forces back with it. The dial is in your hands and it is not on the box.