Having a way of obtaining quick estimates for some design parameter is very useful in engineering, especially during the early stages of a project/design. These estimates help you make quick design decision and adjust your design if your estimate indicates that a certain direction is too risky or will cost too much.

I often hear these quick estimates being called Rule of thumbs. For example:

We can create a rule of thumbs for that! Let's say we have the following information about Edinburgh, UK:

From this, we can create the following rule of thumbs:

Therefore, on average, the annual rent value is 4.7% of the property value. We can then say that a rule of thumb is to ask for approximately 5% of your property value (per year).

Now, let’s apply this rule. If your property is worth £200,000 pounds, an average rent value to ask per month would be (4.71% of £200,000)/12 = £785. You can then start with this value as a reference and adjust it as you get more information and progresses with your plan of renting the house.

Now, let’s say you are in the middle of a design meeting, and you want to know quickly if you could use a FIR filter in your data path to solve your problem. The key question is:

When I was reading Richard Lyons book (Understanding Digital Signal Processing), I came across the following quick estimate formula for the number of taps of a FIR filter:

Where:

From this equation, given the specs, you can check very quickly if a FIR filter could be used or if you need to look for alternatives (if the number of taps is too high!).