What the 90 % habit actually pins down
Nothing formally. No standards body publishes the 90 % rule, nobody audits it, and no listing is obliged to disclose which basis it used. It is a habit of the trade, arrived at because it describes the pool in use — and habits of the trade are precisely the figures this site treats with the most care, because there is no document to appeal to.
What the convention does in practice is fix the order of magnitude of the gap. On a round frame pool 3.66 m across and 0.76 m deep, the geometric volume is close to 8 000 litres; the figure on the box is usually somewhere near 6 500. The difference is not an error, a rounding, or a marketing choice. It is freeboard, and it is the reason a pool does not overflow when four people get in.
What a litre figure cannot be asked to say
It does not promise that any two pools quoting the same litres hold the same water. Two makers can arrive at 6 500 litres from different wall heights, different shapes and different assumptions about where the skimmer sits.
It does not promise that the figure was calculated at all. Some listings quote a round number that matches no arithmetic on the stated dimensions, and where a page here can show that, it says so rather than reconciling the two.
And it does not promise anything about dosing. A dose per cubic metre is a statement a chemical manufacturer makes about their own product. Applying it requires knowing the volume of water in front of you, which is a measurement nobody else can make for you.
The two figures printed next to it
The figure only means something beside two others. The first is the geometric volume of the pool’s own published dimensions — the number the page calculates from length, width and depth, and prints next to the stated one so the gap is visible rather than implied. The second is the wall height, because it is what turns a percentage into centimetres of water.
Where a maker publishes a filtration recommendation, that is a third: a pump sized against a volume tells you which volume the maker had in mind, and occasionally the two disagree inside one listing.
Every family that prints a litre figure
Every family that prints a litre figure, and every family whose product is dosed against one. On the equipment side that is frame pools, steel wall pools and outdoor spas, where the number is printed on the carton. On the consumable side it is the entire chemical shelf, where a dose per cubic metre is only as good as the cubic metres it is multiplied by — and the liner shelf, where a replacement is specified by dimensions rather than by volume, which is its own source of confusion.
What readers keep writing in to ask
Why do makers quote the smaller figure at all?
Because it is the useful one. A pool filled to the rim has no freeboard, no working skimmer and nowhere for displaced water to go when somebody gets in. The convention quotes the volume at the working water line, and on most above-ground pools that line sits near 90 % of wall height.
Is 90 % a rule I can rely on?
No, and that is why this site prints both figures. The proportion varies with skimmer type and with how the maker chose to measure — some quote 80 %, some 90 %, some give no basis at all. Where a maker states the basis, the page repeats it; where none is stated, the page says so.
Which volume should I dose against?
The one your pool actually holds, which is a matter for you and the instructions on the product. This page reports what makers publish and stops there — a dose per cubic metre is a statement about a chemical, not advice about a particular body of water.
Does the same gap exist on inflatable spas?
It does, and it is usually wider. A spa’s quoted capacity is normally given for the water line marked inside the shell, which on most models sits well below the rim because the seats displace volume. The geometric figure is meaningless there, and pages on spas quote the maker’s line only.
Last reviewed 21 September 2026