Feb 022015

Tire size calculator

The results of this wheel and tire size calculator are based on the mathematical equations of the sizes
entered, not the actual tire specs provided by the tire manufacturers. Please refer to the guides
supplied by manufacturers for exact specifications.Results within +3% of stock are considered acceptable.


Please enter actual Tread width – Aspect Ratio and Wheel / Rim diameter
(e.g. 205/65-R16)

Original Alternate
 /  –   /  – 

Graph: Actual speed vs. speedometer reading (km/h)

More about tires

Tire sizes are expressed by the manufacturers with three sets of numbers:

Tread width – Aspect Ratio – Wheel diameter (e.g. 215 55 R16)
Automobile tires are described by an alphanumeric tire code (in American English)
or tyre code (in British English, Australian English and others), which is
generally molded into the sidewall of the tire. This code specifies the
dimensions of the tire, and some of its key limitations,
such as load-bearing ability, and maximum speed.
Sometimes the inner sidewall contains information not
included on the outer sidewall, and vice versa.
For more in dept information about tire size and tyre codes see Wikipedia


1 Star2 Stars3 Stars4 Stars5 Stars (No Ratings Yet)

Apr 202012


The formula used in this
calculation is from the famous Wheelers approximations
which is accurate to <1% if the cross section is near
square shaped.

L (uH) =31.6*N^2* r1^2 / 6*r1+ 9*L + 10*(r2-r1)


  • L(uH)= Inductance in microHenries
  • N = Total Number of turns
  • r1 = Radius of the inside of the coil in meters
  • r2 = Radius of the outside of the coil in meters
  • L = Length of the coil in meters
Multilayer air cor inductors

NOTEThis formula applies at ‘low’ frequencies (<3MHz) using
enameled copper wire tightly wound.

Multilayer air coils Please note that the diameter is measured from center of wire trough
center of the coil and to center of the wire on the opposite side.

Inductance (L):
Coil Inner Diameter (d=2*r1):
Coil Length (l):
Wire Gauge: AWG
Number of Turns (N): turns
Turns per Layer: turns/layer
Number of Layers: layers
Coil Outer Diameter (D):
Wire Diameter:
Wire Length:
DC Resistance (R): Ω (at 20°C)


If it may happen that you find this multilayer aircoil calculator interesting for others, please consider sharing it.
Please rate this article: 1 Star2 Stars3 Stars4 Stars5 Stars (6 votes, average: 3.33 out of 5)

Apr 192012

info Winding the wire in a single layer produces an inductor with minimal parasitic capacitance, and hence gives the highest possible self-resonant frequency (SRF). Striving to obtain a high SRF and low losses is the key to producing coils which have radio-frequency properties bearing some useful resemblance to pure inductance.

The calculation is based on Wheeler’s 1928 formula for a single-layer solenoid which is given in its original form as:
L = a² N² / (9a + 10b) [microHenries] , b > 0.8a
Where b is the coil length in inches, and a is the radius in inches.

To convert this formula to SI units, we will use the symbols r = radius, D = 2r = diameter, l = solenoid length.
Factoring b from the denominator gives:
L = 10-6 a² N² / [ b (10 + 9a/b)] [Henrys] The quantity a/b is dimensionless, and so we can immediately substitute in the denominator:
L = 10-6 a² N² / [ b (10 + 9r/l)] = 10-6 a² N² / [ b (10 + 4.5 D/l)] Factoring 10 from the denominator gives:
L = 10-7 N² ( a² / b ) / (1 + 0.45 D/l) [Henrys]


  • L is the inductance in Henry
  • D is the coil diameter in meters
  • r is the radius in meters (or D/2)
  • l is the lenght of the coil in meters
  • N is the number of turns
Please note that the accuracy of this formula is ±0.33% if the ratio of D/l>0.4. so this formula fits best for long solenoids.

noteThis formula applies at ‘low’ frequencies (<3MHz) using enameled copper wire (magnet wire) close wound.

Tip 1Small reductions in the inductance obtained can be achieved by pulling the turns apart slightly. This will also reduce self-resonance. Other combinations of wire and coil diameter may be tried but best results are usually obtained when the length of the coil is the same as its diameter.

Tip 2 If you need good induction stability in the presence of vibration then wind the coil on a support made from a suitable non magnetic plastic or ceramic former and lock the windings using epoxy glue or other suitable adhesive.

Please note that the diameter is measured from center of wire trough
center of the coil and to center of the wire on the opposite side.


Required Inductance (L):
Coil Diameter (D):
Wire Diameter (d):
Coil Length (l):
Number of Turns (N):


Apr 182012

Info This is an popular coil geometri used in todays wireless charger circuits.
The formula used in this calculation is based on the
Harold A. Wheeler approximations
for air core flat spiral coil inductor.


  • L = inductance in μH
  • Di = inner diameter in inches.
  • s = distance between windings in inches
  • w = wire diameter in inches
  • N = number of turns

1 inch = 0,0254m=2,54cm = 25,4mm.
This formula applies at ‘low’ frequencies (<30MHz)
using enameled copper wire. Some people call it “magnet

Click on image to enlargeFlat spiral coil inductor example

Please note that the outer and inner diameter is measured from the center of the wire.

Flat spiral coil dimensions drawing


Coil inner diameter (Di):
Number of turns (N):
Wire Diameter (w):
Spacing between turns (s):
Inductance (L):
Outer diameter (Do):
Wire lenght (Wl):


More about flat spiral coils

A flat spiral coil is a type of an air core inductor
usually incorporated in the primary of a tesla generator,
RFID tag, and proximity detectors. In the same category
as the flat spiral coils we have planar spiral coils,
planar square spiral coils, planar rectangular spiral
coils, planar hexagonal spiral coils and octagonal spiral
coil. Planar coils are mostly used in high frequency
applications and designed as tracks on a circuit board.

A flat spiral coil belongs to the category of air core inductors

An “air core inductor” is an inductor that does not depend upon a ferromagnetic material to achieve its specified inductance. Some inductors are wound without a bobbin and just air as the core. Some others are wound on a bobbin made of bakelite, platsic, ceramic etc.

Advantages of an air core coil:

Its inductance is unaffected by the current it carries.
This contrasts with the situation with coils using ferromagnetic cores whose inductance tends to reach a peak at moderate field strengths before dropping towards zero as saturation approaches. Sometimes non-linearity in the magnetization curve can be tolerated; for example in switching power supplies and in some switching topologies this is an advantage.
In circuits such as audio cross over filters in hi-fi speaker systems you must avoid distortion; then an air coil is a good choice. Most radio transmitters rely on air coils to prevent the production of harmonics.
Air coils are also free of the “iron losses” which a problem with ferromagnetic cores. As frequency is increased this advantage becomes progressively more important. You obtain better Q-factor, greater efficiency, greater power handling, and less distortion.
Lastly, air coils can be designed to perform at frequencies as high as 1 Ghz. Most ferromagnetic cores tend to be rather lossy above 100 MHz.

And the “downside”:

Without a high permeability core you must have more and/or larger turns to achieve a given inductance value. More turns means larger coils, lower self-resonance dur to higher interwinding capacitance and higher copper loss. At higher frequencies you generally don’t need high inductance, so this is then less of a problem.
Greater stray field radiation and pickup:
With the closed magnetic paths used in cored inductors radiation is much less serious. As the diameter increases towards a wavelength (lambda = c / f), loss due to electromagnetic radiation will become significant. You may be able to reduce this problem by enclosing the coil in a screen, or by mounting it at right angles to other coils it may be coupling with.
You may be using an air cored coil not because you require a circuit element with a specific inductance per se but because your coil is used as a proximity sensor, loop antenna, induction heater, Tesla coil, electromagnet, magnetometer head, or deflection yoke etc. Then an external radiated field may be what you want.

Please consider sharing this page if you find it useful
Please rate this article: 1 Star2 Stars3 Stars4 Stars5 Stars (1 votes, average: 4.00 out of 5)

Apr 172012

Push image to enlarge
Square planar spiral coil Hexagon spiral coil inductor Octagon spiral coil inductor Sircular spiral coil inductor
InfoThe first approximation is based on a modification of an expression developed by Wheeler; the second is derived from electromagnetic principles by approximating the sides of the spirals as current-sheets; and the third is a monomial expression derived from fitting to a large database of inductors (and the exact inductance values).
All three expressions are accurate, with typical errors of 2 – 3%, and very simple, and are therefore excellent candidates for use in design and synthesis. The thickness of the inductor has only a very small effect on inductance and will therefore be ignored.
Fill in appropriate values in the white fields.Then push “calculate”.
1μm =0.001mm
1μm =0.00003937007874015748 inch
Number of turns (n): turns
Spacing between turns (s): μm
Turn width (w): μm
Outer Diameter (dout): μm
Calculated Inner diameter (Din) μm
Fill factor p=(Dout-Din)/(Dout+Din)
  Square Hexagonal Octagonal Circular
Modified Wheeler nH nH nH nH
Current Sheet nH nH nH nH
Monomial Fit nH nH nH nH


S.S. Mohan, M. Hershenson, S.P. Boyd and T.H. Lee
Simple Accurate Expressions for Planar Spiral Inductances
IEEE Journal of Solid-State Circuits, Oct. 1999, pp. 1419-24.
For multilayer spiral pcb coils see here:
A new calculation for designing multilayer planar spiral inductors

If you find this page useful for others, please consider sharing it.

Please rate this article: 1 Star2 Stars3 Stars4 Stars5 Stars (2 votes, average: 5.00 out of 5)