For those of you who would like to "guess" the ice thickness, here is a simple spreadsheet table. I apologize for the English units, (temperature is degrees F and ice thickness is inches) but I am from the US, so there you go. If you want to setup your own calculations, this is taken from:
http://lakeice.squarespace.com/ice-growth You can also use the attached chart from that same reference to predict ice thickness using metric units. Note that it is important to start at the current ice thickness if there is existing ice.
Some notes of caution. This is semi-theoretical and does NOT include all possible factors (like wind, current, etc.). Do not assume this will work for you. Always check the ice, take safety equipment, go with a friend, etc. Also, this chart assumes you start with a very thin layer of ice. It will NOT work with open water unless it is completely still and already at 32F. There are more complexities to this calculation that I won't go into here.
Ice Thickness Calculator
Number of Days
Temp (F) 1 2 3 4 5 6 7
32 0.0 0.0 0.0 0.0 0.0 0.0 0.0
30 0.2 0.4 0.6 0.8 1.0 1.2 1.4
28 0.4 0.8 1.2 1.6 1.9 2.2 2.5
26 0.6 1.2 1.7 2.2 2.6 3.0 3.4
24 0.8 1.6 2.2 2.8 3.3 3.8 4.3
22 1.0 1.9 2.6 3.3 3.9 4.5 5.0
20 1.2 2.2 3.0 3.8 4.5 5.1 5.7
18 1.4 2.5 3.4 4.3 5.0 5.7 6.4
16 1.6 2.8 3.8 4.7 5.5 6.3 7.0
14 1.7 3.0 4.2 5.1 6.0 6.8 7.6
12 1.9 3.3 4.5 5.5 6.5 7.3 8.1
10 2.0 3.6 4.8 5.9 6.9 7.8 8.7
8 2.2 3.8 5.1 6.3 7.3 8.3 9.2
6 2.3 4.0 5.4 6.7 7.8 8.8 9.7
4 2.5 4.3 5.7 7.0 8.1 9.2 10.2
2 2.6 4.5 6.0 7.3 8.5 9.6 10.6
0 2.8 4.7 6.3 7.7 8.9 10.0 11.1