Amidon ferrite toroid calculator (FT series)

When you need this

For a catalogue core there is no need to work out the geometry: the manufacturer has already measured and published the AL factor for every «material + size» pair. That is more accurate than calculating it from the permeability.

The colour of the core tells you the material; the size code gives OD × ID × H

Toroid material
Toroid size
Required inductance L (µH)
Units used in the reference table

Result
Number of turns N
Exact (unrounded) number of turns
Inductance after rounding
Data for the chosen toroid
Symbols
Dimensions OD × ID × H
Inductance factor AL
Initial permeability μi
Saturation flux density Bs
Curie temperature

Reference table of FT-series materials

Material	μi	Bs, G	Br, G	Hc, Oe	Curie, °C	Sizes	What it is for
43	850	2950	1310	0.45	135	19	Suppression of pulse and radio interference over 20…250 MHz.
52	250	4200	3300	0.6	250	1	Ni-Zn ferrite: combines a high saturation flux density with a high Curie temperature.
61	125	2400	1000	1.9	350	16	LC circuits up to 25 MHz, or interference suppression over 200…1000 MHz.
67	40	2300	800	4	450	13	High-Q coils in LC circuits up to 50 MHz. Interference suppression up to 1000 MHz.
68	20	2500	700	7	500	1	Non-standard, few sizes available. High-Q coils up to 100 MHz.
75	5000	4700	1200	0.1	175	10	Interference suppression over 0.5…20 MHz.
77	2000	4900	1500	0.2	190	16	General purpose: LC circuit coils, transformers, interference suppression.
F	3000	4700	N/A	N/A	210	8	High saturation flux density at high temperatures. Power transformers, interference suppression over 0.5…20 MHz.
H	N/A	N/A	N/A	N/A	N/A	1	Has a high permeability.
J	5000	4300	N/A	N/A	145	13	Low resistivity with low losses over 1 kHz…1 MHz. Pulse and low-level broadband transformers.
K	N/A	N/A	N/A	N/A	N/A	2	Mostly in baluns and broadband transformers over 1…50 MHz.
W	N/A	N/A	N/A	N/A	N/A	9	High permeability. Chokes for suppressing radio and pulse interference over 100 kHz…1 MHz.

📐 Formulas

L [nH] = AL · N²        →        N = √( L[nH] / AL )

That is all. The whole job of the calculator is to pull the right AL out of the tables: each material has its own list of sizes, because not every combination is made.

The reference tables hold:

28 sizes with OD × ID × H in inches, as the manufacturer gives them;
the AL matrix [nH/N²] for 12 materials;
initial permeability μi, saturation flux density Bs and remanence Br [G], coercive force Hc [Oe], Curie temperature.

Notation:

L — the inductance you need [µH]
N — number of turns
AL — inductance factor [nH/N²]
OD × ID × H — dimensions of the core

Limits of the calculator:

Amidon ferrite toroids carry no colour code — they are glossy black or dull grey. The colour code belongs to the iron-powder cores, the T series
AL is given for small signal and low frequency: at radio frequencies the inductance changes noticeably
The batch spread of AL is usually ±20…25 %
For power chokes it is core saturation that decides, not AL

🎯 Where it is used

Broadband transformers and baluns on materials 43, 61, 77.
Interference-suppression chokes on materials 31, 43, 75.
Tuned-circuit coils up to 50 MHz on material 67.

Practical notes:

At the foot of the page is the full table of materials with what each is for. The row of the selected material is highlighted.
An FT-50-43 has AL = 440 nH/N². For 2 µH that comes to 2 turns — check you have not slipped an order of magnitude in the inductance.
The list of sizes narrows itself: only those actually made in the selected material are shown.

How to cite this calculator

UR3PKI. «Amidon ferrite toroid calculator (FT series)». CyberDev.Space. https://cyberdev.space/en/radio/calculators/coils/amidon_ferrite (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.

Reference table of FT-series materials

📐 Formulas​

Notation:​

🎯 Where it is used​

📐 Formulas

Notation:

🎯 Where it is used