In C.G.S. system, the tonnage in metric tonnes is calculated by multiplying the specific gravity of the economic mineral with the total volume in cubic meters. In F.P.S. system, the total volume in cubic feet has to be divided by tonnage factor. The tonnage factor represents the number of cubic feet in a tonne of ore. Tonnage factor can be calculated by the formula: 2240/ 62.5Xg cubic feet, where g is the specific gravity of the ore.

The total reserves in an area can be known by dividing the volume by tonnage factor. Weight calculations should be taken care of due to the presence of pore spaces, fractures, and cavities in the ore bodies.

To estimate the reserves, the first step is to plan to divide the mineral deposit area in to a number of ore reserve blocks. On the plan paper, each bore hole or pit is an unit.

For each block, the area of influence needs to be calculated. The volume can be obtained by multiplying the area with the true width of the ore body. The attitude of the deposits determine the method of calculation of area.

The total tonnage can be calculated by adding tonnage in the individual blocks. The average grade (A) is given by the formula:

T1A1+T2A2+T3A3… T10A10/ T1+T2+T3… T10

If 10 ore reserves are there in a deposit and tonnage of each is taken as, T1,T2,T3, and the grades are represented by A1, A2, A3.

There are mainly two methods to find the area of the blocks as 1. Geometric, and 2. Graphic.

Included area method, extended area method, triangle method, and polygon method fall under the former category. In the latter category, there are isochore mapping method, stratum contours, transverse section method.

Prior to the usage of any method, the mineralized areas need to be demarcated based on geological knowledge of the area such as continuity of the ore, structural features, pitch, etc.

If the bore holes or pits are drilled in grid pattern, the included area method is highly advisable. In this method, the area of influence and hence the weightage in the central pits or bore holes are twice and four times the sides and corner pits. All the blocks have equal area of influence in the extended area method. Triangle and polygon methods are mostly applicable for non-grid pattern of sampling points.