Measure the specific gravity of the coolant with a hydrometer.
Measure the coolant temperature, and calculate the concentration from the relation between the specific gravity and temperature. Use the following table for reference.
RELATION BETWEEN COOLANT CONCENTRATION AND SPECIFIC GRAVITY
THE FOLLOWING TABLE IS APPLICABLE ONLY TO THE SPECIFIED HIGH QUALITY ETHYLENE GLYCOL (ANTIFREEZE) COOLANT
The safe operating temperature is -15°C (5°F) when the measured specific gravity is 1.058 at coolant temperature of 20°C (68°F)
If the concentration of the coolant is below 30%, the anti-corrosion property will be adversely affected. In addition, if the concentration is above 60%, both the anti-freeze and engine cooling properties will decrease, adversely affecting the engine. For these reasons, be sure to maintain the concentration level within the specified range. Do not mix brands of coolant.