Water Properties: Specific weight
Table of Contents
Specific Weight of Water
The specific weight is the weight per unit volume of a material. The specific weight of water is 9.81 kN/m3 or 62.43 Ib/ft3 . The specific weight is defined by the equation below:
\(\gamma = \rho g\) [kN/m3]
where,
\(\rho\) = density [kg/m3 ]
\(g\) = standard gravity [m/s2]
standard gravity, g is the acceleration is due to gravity, which is taken as 9.81 m/s2 on Earth.
The equation for density is defined below:
\(\rho = m/V\) [kg/m3]
where
\(\rho\) = density, units are in \(kg/m^3\), or \(Ib/ft^3\), or \(g/cm^3\)
\(m\) = mass, units are in \(kg\), or \(Ib\), or \(g\)
\(V\) = Volume, units are in \(m^3\), or \(ft^3\), or \(cm^3\)
Specific Weight of Water under Various Temperatures
The density of water if affected by temperature at a constant pressure. As the temperature increases, the density decreases. However, it should be noted that below 4°C, the density of water decreases when the temperature goes below zero.
The table below indicates that when the temperature decreases below 4°C, the density of water decreases. When the temperature increases to 100°C, the density decreases as well.
When water reaches a temperature below 0°C, the water turns to ice, and is less dense than water causing it to float on water. This is due to the decrease in density and this is attributed to atomic formation of ice.
| Temperature (°F) | Temperature (°C) | Density (grams/cm3) | Density (kg/m3) | Specific Weight of Water (kN/m3) |
| -4°F | -20°C | 0.99355 | 993.55 | 9.747 |
| 14°F | -10°C | 0.99817 | 998.17 | 9.792 |
| 39.2°F | 4.0°C | 1 | 1000 | 9.810 |
| 40°F | 4.4°C | 0.99999 | 999.99 | 9.81 |
| 50°F | 10°C | 0.99975 | 999.75 | 9.808 |
| 60°F | 15.6°C | 0.99907 | 999.07 | 9.801 |
| 70°F | 21°C | 0.99802 | 998.02 | 9.791 |
| 80°F | 26.7°C | 0.99669 | 996.69 | 9.778 |
| 90°F | 32.2°C | 0.9951 | 995.1 | 9.762 |
| 100°F | 37.8°C | 0.99318 | 993.18 | 9.743 |
| 120°F | 48.9°C | 0.9887 | 988.7 | 9.699 |
| 140°F | 60°C | 0.98338 | 983.38 | 9.647 |
| 160°F | 71.1°C | 0.97729 | 977.29 | 9.587 |
| 180°F | 82.2°C | 0.97056 | 970.56 | 9.521 |
| 200°F | 93.3°C | 0.96333 | 963.33 | 9.450 |
| 212°F | 100°C | 0.95865 | 958.65 | 9.404 |
Example 1 : Calculate specific weight of water
The density of water at 4°C is 1000 kg/m3. The equation for the specific weight of water is \(\gamma = \rho g\) [kN/m3] = 1000 x 9.81 = 9810 N (9.81 kN).
Example 2 : Calculate weight of water in water tank
A water tank is at a plan of 5m x 5m and is 3m high. The total volume = 75 m3
The total weight of the water is 75 m3 x 1000 kg/m2 = 75000 kg
Weight of water in kN = 75000N x 9.81 m/s2= 735750 N (735.75 kN)