Rectangular vs Kidney pools compared

Swimming: length is everything, and curves eat length

A rectangle of 8 m gives you 8 m of straight water, wall to wall. That is the threshold where an adult can take four or five real strokes, turn and repeat. A kidney drawn in the same 8 × 4 m box does not. Its long axis bends, its waist narrows, and the only straight line you can swim without touching a wall runs about 5–6 m through the lobes. The rule of thumb from the shape guide holds: a kidney loses roughly a third of its length to the curves.

The area tells the same story. In the designer, an 8 × 4 m rectangle holds 32 m² of water; the kidney at the same dimensions holds about 18 m². That is not a flaw in the kidney — the bean simply does not fill its box — but it is the number to know before you fall for a plan view. If you want a kidney with a genuine 8 m swim, draw it at 11–12 m overall. Put both shapes at 8 × 4 in the designer and compare the computed volume; the gap is obvious in seconds.

For cooling off, floating and children, none of this matters. Nobody bobbing on a noodle cares about a straight line. Decide honestly whether anyone in the house will swim lengths, because that single question settles a lot of this page.

How each one sits in the garden

A rectangle wants a straight edge to answer to: a terrace, a house wall, a hedge. Set parallel to something, it looks intended. Set at a slight angle to everything, it looks dropped in. It is a terrace element — paving runs up to it, loungers line up along it, and the geometry of the garden organises itself around its long side.

A kidney wants the opposite. It looks best when the paving stops short and planting comes to the water on at least one side, ideally at the waist. The curve disguises the true size, which is why a small kidney can feel generous while a small rectangle can feel like a tank. If your garden is soft, sloping or irregular, the kidney forgives it. If your garden is a level lawn framed by a modern house, a kidney can look like it wandered in from a different decade.

One practical test: stand where you will see the pool most — usually the kitchen window — and ask whether the view is of architecture or of planting. Match the pool to that answer.

Building it: straight walls are cheap walls

Straight walls take straight formwork. Standard panels, standard rebar spacing, standard coping lengths. A rectangle is the shape every builder has done a hundred times, and the price reflects that. Corners are the only fiddly part, and even they are a solved problem.

Curved walls cost more at every step. The shuttering is either bent plywood or sprayed concrete against a curved cage, the steel needs bending to radius, the coping is cut piece by piece to follow the curve, and the waterline tile goes on in small formats or mosaics that can take the bend. Expect the shell and finishes of a kidney to run a genuine premium over a rectangle of the same water area — typically in the region of 15–30%, depending on how tight the curves are and how the pool is built. Sprayed concrete narrows the gap; block-and-render widens it.

Liner pools change the arithmetic a little. A kidney liner is a stock pattern for many makers, and the walls can be steel panels that follow the curve. Even so, the coping and the cover still cost more than their straight equivalents.

Covers, cleaning and the corners question

This is the rectangle's strongest card. An automatic slatted cover is a rectangle by nature; on a rectangular pool it sits perfectly, seals well, and turns covering into a button press. Solar blankets come off the roll in the right shape. Robot cleaners drive straight lines and find the walls where they expect them.

A kidney takes a custom cover — every time. Slatted covers on free-form pools exist, but they need an oversized rectangular housing and a lot of overhang, or a tailored slat pattern; safety covers and winter covers are made to a template of your pool. Budget for that, and budget for the time to fit it.

On cleaning the kidney wins a point back. It has no corners at all, so the water keeps moving everywhere and nothing settles in dead spots. The rectangle's sharp corners are where leaves collect and where the robot has to work hardest. It is a small daily difference, not a deal-breaker, and rounding the corners of a rectangle — the rounded shape — removes most of it.

Cost drivers, summed up

Three things push the kidney's total above the rectangle's: curved forming, cut-to-shape coping and a bespoke cover. Three things pull it back: it usually holds less water for its footprint, so heating, chemicals and the pump are all sized smaller, and it rarely needs the largest sizes to feel right.

The rectangle's running costs scale with its honest area. An 8 × 4 m rectangle at 1.2–1.8 m depth is about 48 m³ of water. The kidney in the same box is closer to 27 m³. Over years that difference in heating alone is real money. Only compare like with like, though: a kidney enlarged to give the same swim length ends up with a similar volume and all of the curve premium.

Living with it: resale and the long view

Rectangles age well because they are neutral. They suit the next owner's taste as well as yours, they take any cover, and they look right with any finish. The kidney is a statement, and statements date. A well-planted kidney beside a period house will still look right in twenty years; a kidney beside a crisp modern extension may look like a mistake in five.

Neither shape is a resale liability if it is warm, clean and well kept. Buyers react to condition and running cost far more than to outline.

Choose the rectangle if… / choose the kidney if…

Choose the rectangle if anyone will swim lengths, if you want an automatic cover, if the pool sits against a terrace or house, or if the budget is tight and the builder is local and conventional.

Choose the kidney if the pool is for cooling off and play, if the garden is planted, sloping or irregular, if you want the pool to read as a pond rather than a pool, or if you are prepared to pay for curves and a made-to-measure cover to get the look you actually want.

If you are torn, look at the rounded rectangle and the oval. Both soften the box without giving up the swim line, and both are far kinder to covers than a kidney.