Amidon iron-powder toroid calculator (T series)

When you need this

The same as for the ferrite series, but with AL normalised differently — and with the colour code by which a core is recognised in the box.

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

Toroid material	Material #0Material #1Material #2Material #3Material #6Material #7Material #8Material #10Material #12Material #15Material #17Material #18Material #26Material #30Material #40Material #52
Toroid size	T-12T-16T-20T-25T-30T-37T-44T-50T-60T-68T-80T-94T-106T-130T-157T-184T-200T-200AT-225T-225AT-300T-300AT-400T-400AT-520
Required inductance L (µH)
Units used in the reference table	millimetresinches
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
Permeability μ
Colour code
Powder type

Reference table of T-series materials

Material	μ	Powder type	Colour code	Working frequencies	Sizes	What it is for
№0	1	Phenolic	tan	100 MHz - 300 MHz	15	Mostly above 100 MHz. The inductance from AL is imprecise and depends strongly on the winding technique.
№1	20	Carbonyl C	blue / no colour	0.5 MHz - 5 MHz	16	Very similar to #3, but with a higher volume resistivity and better stability.
№2	10	Carbonyl E	red / no colour	2 MHz - 30 MHz	25	High volume resistivity. High-Q coils over 2…20 MHz — the HF workhorse.
№3	35	Carbonyl HP	grey / no colour	0.05 MHz - 0.5 MHz	16	Excellent stability and Q at low frequencies, 50…500 kHz.
№6	8	Carbonyl SF	yellow / no colour	10 MHz - 50 MHz	24	High Q and temperature stability over 20…50 MHz.
№7	9	Carbonyl TH	white / no colour	3 MHz - 35 MHz	7	Similar to #2 and #6, but more stable with temperature.
№8	35	Carbonyl GQ4	orange / no colour	N/A	18	Low core loss and good linearity under a strong DC bias. The most expensive.
№10	6	Carbonyl W	black / no colour	30 MHz - 100 MHz	15	Good Q and high stability over 40…100 MHz.
№12	4	Synthetic Oxide	green / white	50 MHz - 200 MHz	13	Good Q and moderate stability over 50…200 MHz. If Q matters most this is a good choice; if stability does, #17 is better.
№15	25	Carbonyl GS6	red / white	0.1 MHz - 2.0 MHz	14	Excellent stability and good Q. For the broadcast bands, where both Q and stability matter.
№17	4	Carbonyl	blue / yellow	20 MHz - 200 MHz	15	Similar to #12, but more stable with temperature. Q drops about 10 % over 50…100 MHz and 20 % above 100 MHz.
№18	55	N/A	green / red	N/A	14	Low losses like #8, but with a higher permeability and a lower price. Holds up well under a DC bias.
№26	75	Special	yellow / white	LF filters, chokes	27	The highest permeability of the powdered cores. EMI filters and DC chokes.
№30	22	N/A	green / no colour	N/A	1	Good linearity, low price, relatively low permeability — popular for large power chokes in switching supplies.
№40	60	N/A	green / yellow	N/A	7	Inexpensive, with characteristics close to the very popular #26.
№52	75	N/A	green / blue	N/A	22	Lower high-frequency losses at the same permeability as #26. For new RF choke designs.

📐 Formulas

For iron powder the manufacturer normalises the inductance factor per hundred turns:

L [µH] = AL · (N/100)²        →        N = 100·√( L[µH] / AL )

That is the only difference in the formula from the ferrite series — but the two normalisations are very easy to confuse, and the error is a factor of a hundred. Which is why the series live on separate pages.

The reference tables hold 39 sizes, the AL matrix [µH/(100 turns)²] for 16 materials, and also the permeability, the colour code, the powder type and the recommended frequency range.

Notation:

• L — the inductance you need [µH]
• N — number of turns
• AL — inductance factor [µH/(100 turns)²]
• OD × ID × H — dimensions of the core

Limits of the calculator:

• AL is normalised per hundred turns — putting it into the ferrite-series formula means being wrong by a factor of a hundred
• AL is given for small signal; under a DC bias the permeability falls gradually — unlike ferrite, which saturates abruptly
• The batch spread of AL is usually ±5…10 %
• The colour code tells you the material only, not the size

🎯 Where it is used

• HF tuned-circuit coils on materials 2, 6, 7 — high Q and good stability.
• Chokes for switching supplies on materials 26, 40, 52 — gradual saturation.
• EMI filters on material 26.

Practical notes:

1. At the foot of the page is the table of materials with colour swatches, working frequencies and what each is for.
2. A T-50-2 has AL = 49 µH/(100 turns)². For 2 µH that comes to 20 turns.
3. Iron powder has a low permeability — you need more turns than on ferrite, but the linearity and stability are far better.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator

UR3PKI. «Amidon iron-powder toroid calculator (T series)». CyberDev.Space. https://cyberdev.space/en/radio/calculators/coils/amidon_iron_powder (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.