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Why Are Manholes Covers Made Circular? Know the Complete Fact About it

By Satabdi MazumdarNovember 14, 2022
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Some Scientific Reasons for Shapes of Sewer


While walking on the road, you will come across many manholes which are round in shape, but you may not pay much attention to them. However, it is pretty interesting to learn why manholes are round. Manholes, sometimes known as maintenance holes, have a useful function as entrances or access points to underground public service, like a sewage or drainage system. They are used by engineers, workers, and inspectors to check, clean, repair, and maintain utilities.  In many countries, manholes are very essential to keep the drainage system intact.

Where Does the Name Manhole Come From?

One of the interesting facts about the manhole name is it originated between 1785-1795. The term covers both- Man and Hole allowing the person to pass through and reach the drain or sewer. The openings to the sewer system were initially only big enough for a light to shine through to see if the sewage was flowing properly. But eventually, the sewer system needed to be accessible to people.

During the rainy season, these manholes play a major role in drain in excess water accumulated on roads. But why are manhole covers round? The answer is that the manholes are round, so it naturally explains why the covers are round too. The longer response is that the choice to create something like a manhole covers are made circular depends on several practical, engineering, and economic decisions.  Over the centuries, these manholes became essential part of the city drainage system.

Brief Reasons Why Sewer Covers are Circular

A "lip" around the hole rim in every manhole supports the cover, making the underlying hole smaller than the cover. No matter how you position it, a round manhole cover is wider than the hole; therefore, it cannot fall through the round aperture. However, suppose a square, rectangle, or oval manhole cover is fitted diagonally into the hole. In that case, it might fall in, which would be dangerous for automobiles and pedestrians who are not paying attention.

 Other practical reasons why sewer covers are circular, after a spherical cover is taken off; moving it by rolling it along the ground is simple. The surface area of a round manhole cover is also lower than that of a square cover with the same width, making the manufacturing process easy, less expensive, and more effective.

Manholes existed since the 19th century when communities began constructing underground drainage systems worldwide. Engineers were looking for routes to access the sewer systems to inspect the pipes as part of routine maintenance. Since then and now, all sewer covers remain circular.

Some More Information About Sewer Hole Cover

Robust, detachable metal plates protect manholes. The sewer hole cover is often built of concrete or cast iron and weigh between 200 and 300 pounds. In addition, each manhole entrance carries an edge lip that the cover rests on. These coverings shield the sewers from garbage, such as trash or biological materials, which often collect on streets and may clog sewers and water systems, as well as from unsuspecting pedestrians accidentally falling into the sewers.

The Math Behind Manholes Circular Shape

If you are wondering shapes of sewer that are circularly limited to access pipes, then you need to consider Maths behind them to. There is the math behind manholes. As mentioned above, using alternative shapes, manhole coverings might fall through the hole. For instance, a square cover with a side length of 1 meter can fit through a square manhole even though the manhole's side length is reduced by the lip (stopper).

To further illustrate, imagine that a 5-cm lip is added to each side of the hole. This results in a square hole with 90-cm-long sides.

Contrarily, a circular cover's constant diameter ensures that it will not fall through the circular hole regardless of how roughly vehicles move it. In addition to the mathematical explanation, manhole covers are difficult to move about but are simple to roll if they are round.

Other unique forms besides circles help for covering manholes. For example, Reuleaux triangle happens when drawing arcs with the three vertices of an equilateral triangle as their centers results. In other words, start with A as the center and draw an arc BC from an equilateral triangle ABC. To make a "triangular-shaped" curve, repeat this technique with B as the center and C as the center.

Manhole Covers

Why Manhole Covers are Round?

Some Benefits of Having Round Shapes of Sewer

There are multiple benefits to why manholes are round-

  • If you use a square cover, the entire thing might easily fall through the manhole, posing a severe threat. If a circular manhole cover were to break or fall through the cover, it would be impossible to fall through the manhole.

  • Due to their ability to be rolled, round covers are significantly easier to handle and move around than square covers.

  • The strength of a manhole cover is a crucial component; thus, a circular cover would be the wisest choice because they handle the weight and speed much better than square covers.

  • A square cover needs proper alignment with the borders of the manhole. However, when it comes to manholes being round, they can be placed at any angle and fit; they do not need to be aligned or placed in a certain manner.


Since now we know the reason why are manhole covers round, it is also important to know that they are an important part of the drainage system. Contractors need to make sure that these manholes don't remain open that invites accidents for pedestrians and drivers. Next time when you walk around streets, do check out manhole covers. You now have all the answers on why are manhole covers round.