Let us calculate the quantity of water present in the open well for the given drawing.
Given data:
Depth of water = h = 2700mm = 2.7m.
Outer diameter of RCC ring = D = 1200mm.
Ringwall thickness = 70mm.
The formula for calculating the quantity of water in the open well
= [( π × d2 × h ) ÷ 4]
Here, d = inner diameter of RCC ring.
First, we will find the value of d.
Inner dia. of RCC ring
d = [outer dia. of the ring  ( 2nos. × ring wall thickness )]
= [1200mm  ( 2nos. × 70mm. )]
= 1060mm.= 1.060m.
Quantity of water in the open well
=[ (π × (1.060m)2× 2.7m.) ÷ 4 ]
= [ 9.532 cum. ÷ 4 ]
= 2.383 cum.
As you know, 1cum = 1000 liters.
So, the quantity of water in the open well
= 2.383 × 1000
= 2383 liters.
Easy alternate method:
Given data:
Depth of RCC ring = h = 300mm = 0.3m.
Outer diameter of RCC ring = D = 1500mm.
Ringwall thickness = 80mm.
Inner dia. of RCC ring
d = [outer dia. of the ring  ( 2nos. × ring wall thickness )]
= [ 1500mm  ( 2nos. × 80mm. )
= 1340mm.= 1.34m.
The quantity of water that a single RCC ring can hold
= [(π × d2 × h) ÷ 4 ]
= [(3.142 × 1.342 × 0.3m.) ÷ 4]
= [1.692 cum. ÷ 4 ]
= 0.423 cum
= 423 liters.
Suppose if you have installed 40nos. of RCC rings of 1500mm dia. in your open well, and if the 9 rings are immersed in the water,
then, the quantity of water present in the open well
= No. of immersed rings × water holding capacity of one ring.
= 9 nos. × 423 liters
= 3807 liters.
I have given a table of RCC open well rings of different diameters, for your easy calculation purpose.
Sl. No. 
RCC ring outer dia. (D) in mm. 
Ring wall thickness in mm.

RCC ring inner dia. (d)in mm. 
Depth of the ring (h)in mm. 
Water holding capacity in litres. 
1 
1000 
65 
870 
300 
178 
2 
1200 
70 
1060 
300 
265 
3 
1500 
80 
1340 
300 
423 
4 
1800 
90 
1620 
300 
618 
Note:
The wall thickness and ring depth may vary according to the available RCC ring in your region.
By replacing the value of inner dia. and height in the above formula, you can calculate the water quantity for the open well, according to the installed ring dimension.
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