Why Is A Manhole In Circle Shape Only?

" All manholes are circular in shape " is not true. Man Holes in Boilers are elliptical in shape. Even for sewer man holes rectangular in shape can be easily seen. But I agree that sewer manholes should be of circular shape.

Manhole, as the name suggests , is meant for a man to get in it and come out easily. A circular opening requires minimum surface area for a given maximum length that can be lowered.

For example if a man requires minimum of x cm opening to allow him to go into the man hole, Then a circular opening will need a surface area = π (x/2)² = (π/4) x² = 0.785 x² cm²

Now let us consider a square hole of side x cm. It has an are of x² cm² . The rectangular hole has still bigger area.

This clearly shows that Area of the opening providing the same maximum dimension is least in case of a circular opening.

Again if the two shapes of the man hole have the same thickness, and are of the same material, the quantity of the material (may be Mild steel plate or RCC) will be minimum for a circular hole. Ie it will be cheaper.

Dr. Pramod Kumar Tandon

Have you ever lifted one of those covers?  They are heavy.  They often have to be because cars are running over the cover; the cover has to be strong enough to bear the weight of big HGVs.  The smaller the hole the less chance that the cover will cave in from that weight.  

Circular shapes are also easier to make , and easier to reinforce the sides of the tube underneath (concrete line the sides to keep from caving in).

A circle is a more efficient shape.  People are roughly circular, too (more circular than we are square or rectangular). You can squeeze a person in with a minimal opening at the top, especially if they are carrying any gear with them.

Also, the way the holes are bored out quickly is by drilling a circular hole; so makes sense to top it with a circular cover.

That said, I have seen square and rectangular man-hole (sewer) covers.  They are just the exception, rather than the rule. See here.

The real reason manholes are round is that it is the only shaper wherein the cover will not fall into the hole, no matter how you twist or turn it.

So it won't ever fall into the hole.

It is easier for humans to enter and leave a circle than any other shape.  Also there are no sharp corners on a circle so that traffic can pass over without any damage .

For safety..