Confusing symbol in led labeling: How to Obtaining precision with hobby electronics?

Does anybody know what that "f" looking sign is next to the 5, left-hand bottom corner of the LED labeling?:

It Indicates the diameter of the LED package.

Summary of Conclusion: Don't seek precision unless you have the means to obtain it.

...I'm a young electronics hobbyist, who has the passion to become a robotics engineer at later stages of my maturity.

Lately I have been tinkering with my first circuit designs, simple stuff really. My first circuits were "plug-and-play", based off of rough, highly in-precise calculations, but it would get things lit like the image below:

Later, I decided to revise my scketch, adding in LED's and using a more scientific method to make more precise calculations about the circuits functionality...

Long story, but I got my circuit working. Thanks.

• See if they are 5mm diameter. – Eric S Sep 19 '17 at 1:34
• @EricShain They are. I can't get my circuit to work with KVL. If I add resistors, it just won't light up. – Iam Pyre Sep 19 '17 at 1:36
• I think the symbol refers to the LEDs diameter. As for the circuit, maybe a EE can help. – Eric S Sep 19 '17 at 1:38
• Have you fixed your multimeter problems in your earlier question? If not, treat your measurements as random numbers until you do! It could be anything - flat battery in the meter, defective power supply, worn-out contacts on the breadboard, etc, etc - or just "user errors". – alephzero Sep 19 '17 at 2:21
• @alephzero A lot to say, but I believe the multimeter problem was more error on my side. First, I didn't understand KVL and it's relationship to voltage drops, I get that now and the readings make sense. I applied KVL to a simple circuit, 9v battery and three resistors. I got the appropriate readings at each point in the circuit. But now, I can't get KVL to work with a series circuit of resistor and led, I can only get my leds to light with resistor if I put the LED's in parallel. A lot to talk about here, will post it with pictures later if I don't solve the problem. – Iam Pyre Sep 19 '17 at 2:27

• I checked, the ISO symbol is a $\phi$ ... – Carl Witthoft Sep 19 '17 at 13:26