# Online Coil Inductance Calculators yeild different Results

#### Fran3

Joined Mar 28, 2019
38
Subject: Online Coil Inductance Calculators yeild different Results

Designing an open air coil wound on PVC pipe with the following specs ...

Wire = 12 gauged insulated electrical wire from Lowes
- Wire Diameter = .125 including the insulation
- Turns per inch = 8
- Total Turns = 212
- Coil Diameter = 2 inches (5.08 cm)
- Coil Radius = 1 inch (2.54 cm)
- Coil Length = 26.5 inches (67.31 cm)
- Relative Permeability = 1

I get different results depending on which online calculator I use... (see 4 examples at the end of this post)

Calculator 1, 2, 3, & 5 shown below are at least between 5% & 10% the same ...
But Calculator # 4 is way different

I've also run the numbers myself using the equations offered on each site and get the same results as the sites calculator ...

Question 1: Which Calculator is most accurate
Question 2: Which Equation for an open air inductor is most accurate?

(Understanding that in practice results may differ)

Calculator links, inputs, and outputs listed below.

Thanks for any help.

-------------------------------
Calculator # 1:
All About Circuits Coil Inductance Calculator

Input:
Coil Radius = 2.54 cm
Solenoid Length = 67.31 cm
Nummber of turns = 212
Relative Permeability = 1
Output:
170.07 uH

-------------------------------
Calculator # 2:
Telsa Scientific Coil Inductance Calculator
At: https://www.teslascientific.com/products/coil-inductance-calculator/

Input:
Coil Diameter = 2 inches
Coil Length = 26.5
Number of turns 212
Output:
164.029 uH

-------------------------------
Calculator # 3:
TranslatorCafe.com Single-Layer Coil Inductance Calculator
At: https://www.translatorscafe.com/uni...tor/coil-inductance/?D=2&Du=cm&l=1&lu=cm&N=10

Input:
Coil Diameter = 2 inches
Number of turns = 212
Length of Coil = 26.5 inches
Output:
179.7369 uH

-------------------------------
Calculator # 4:

EEWeb Coil Inductance Calculator
At: https://www.eeweb.com/tools/coil-inductance/

On the right of the Inputs ... Select "Coil Inductance Calculator"
Inputs:
Number of Turns = 212
Loop Diameter = 2 inches
Wire Diameter = .125 inch (diameter of insulation surrounding 12 guage wire)
Relative Permeability = 1
Output:
0.00409 H or 4.09 mH

-------------------------------
Calculator # 5:
Rimstar.org Inductance Calculator
At: https://rimstar.org/science_electronics_projects/coil_design_inductance.htm

Input:
Relative Permeability = 1
Number of Turns = 212
Length = 26.5 inches
Output:
170.5218 uH

#### Papabravo

Joined Feb 24, 2006
18,964
Are you really expecting an answer to this question? It is not as if there is a universally agreed upon standard method of estimating an inductance. I find it comical that anybody would report values with 5-7 significant figures. Three should be the upper limit. What a hoot!

#### Fran3

Joined Mar 28, 2019
38
Papabravo, Thank you for your kind response to my post.

The various inductance values in the post were simply copied from the calculators...

However... Ignoring the decimal places...
There is a big difference in Calculator #4's answer of ~5 mH
And the average of the other 4 Calculators answers ~171 uH

And, I would have expected the delta of the other 4 Calculators answers 164 uH to 179 uH ...
To be less.

#### Tesla23

Joined May 10, 2009
514
I think there is a new rule of engineering - be skeptical of online tools! I suspect that, like much of the other stuff on the web, the motivation is clicks and eyeballs rather than technical accuracy. Often the folk that are in charge of some of these sites don't even understand the issue when you report a problem.

There are some very good methods of estimating inductance, all of which have limited range of validity. For your problem, probably the simplest approximation is Wheeler's  a and b are in inches.

from:
H. A. Wheeler, "Simple Inductance Formulas for Radio Coils," in Proceedings of the Institute of Radio Engineers, vol. 16, no. 10, pp. 1398-1400, Oct. 1928 (yes - it's been known for a long time)

Any online calculator should tell you the range of validity, and preferably the source of the approximation. And as Papa said, 5-7 significant figures are usually nonsense.

#### Papabravo

Joined Feb 24, 2006
18,964
Papabravo, Thank you for your kind response to my post.

The various inductance values in the post were simply copied from the calculators...

However... Ignoring the decimal places...
There is a big difference in Calculator #4's answer of ~5 mH
And the average of the other 4 Calculators answers ~171 uH

And, I would have expected the delta of the other 4 Calculators answers 164 uH to 179 uH ...
To be less.
I know precisely what you did, and I would suggest that you question their methods. I certainly would. Your expectation would be reasonable if they had all used the same methods, which clearly, they did not.

#### Papabravo

Joined Feb 24, 2006
18,964
I think there is a new rule of engineering - be skeptical of online tools! I suspect that, like much of the other stuff on the web, the motivation is clicks and eyeballs rather than technical accuracy. Often the folk that are in charge of some of these sites don't even understand the issue when you report a problem.

There are some very good methods of estimating inductance, all of which have limited range of validity. For your problem, probably the simplest approximation is Wheeler's

View attachment 273922 View attachment 273923

a and b are in inches.

from:
H. A. Wheeler, "Simple Inductance Formulas for Radio Coils," in Proceedings of the Institute of Radio Engineers, vol. 16, no. 10, pp. 1398-1400, Oct. 1928 (yes - it's been known for a long time)

Any online calculator should tell you the range of validity, and preferably the source of the approximation. And as Papa said, 5-7 significant figures are usually nonsense.
I am familiar with Wheeler as that is the one in the ARRL Handbook for Radio Amateurs.

#### DickCappels

Joined Aug 21, 2008
9,062
These methods, as @Papabravo implied are only estimates or approximations. Chose the one you believe to be best and just be prepared to have to make adjustments to your real life circuit.

#### Fran3

Joined Mar 28, 2019
38
Thanks everyone.

Telsa23, yes, I've use the equation you referenced...

And yes, online tools are to be used with caution ... that is why I checked my calculations against 5 online tools...
4 of which were close enough to the calculated inductance...
but one was way out... probably because I didn't quite understand their instructions... or my keyboard error

I'll be building a prototype next to check the design and tweak it in.

Thanks again to all for your comments.

• DickCappels