The formula for specific gravity of fine aggregates as per IS 2386 (part 3) 1963, using the pycnometer is by the following: Specific gravity = \(\frac{\mathrm{W} 2-\mathrm{W} 1}{(\mathrm{~W} 2-\mathrm{W} 1)-(\mathrm{W} 3-\mathrm{W} 4)}\)

Where,

W1 = Empty weight of pycnometer

W2 = Weight of pycnometer + sample

W3 = Weight of pycnometer + sample + water

W4 = Weight of pycnometer + water

Concept:

Pycnometer method 

It is used to determine specific gravity and water content both. This method is suitable for cohesionless soils.

Pycnometer is a glass jar of 1-liter capacity that is fitted at its top by a conical cap made of brass. It has a screw type cover and there is a small hole at its apex of 6 mm diameter.

W1 = Mass of empty volume of pycnometer

W2 = Mass of pycnometer + Mass of moist sample

W3 = Mass of pycnometer + soil + water

W4 = Mass of pycnometer full of water

Gs = Specific gravity of soil solids

Water content of the soil sample is given by:

\(w = \left( {\frac{{{W_2} - {W_1}}}{{{W_3} - {W_4}}}} \right)\left( {\frac{{G - 1}}{G}} \right) - 1\)

Specific gravity is usually reported at 27°C and it is given by:

\(G_s=\frac{{{W_2} - {W_1}}}{{\left( {{W_2} - {W_1}} \right) - \left( {{W_3} - {W_4}} \right)}}\).