If you look in the datasheet for your 1N4148 diodes you will see in the electrical characteristics section, a rating called "Forward Voltage". The number listed here is the average voltage that the family of diodes will drop at a given current (the voltage drop can change with current, depending on the diode). For example the 1N4148 family will drop maximum of 1V when there is 10mA flowing through it. Since this is the maximum, your diode will probably drop less at that current. The actual value will vary slightly from diode to diode. Sometimes there will also be a graph showing the relationship between forward voltage and forward current. The easiest way to find out for sure would probably be to use a diode test function on a multimeter to determine the drop of your specific diode. As Mike said, the 1n400x family of diodes usually drop about .6V or .7V, you can get them at RadioShack.
