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MT amperage calculator: coil-shot ampere-turns, head shots and prods

Estimate magnetizing current the way MT study notes teach it: coil-shot ampere-turns from the part's length-to-diameter ratio, head-shot current per inch of diameter, and prod current per inch of spacing. These are study rules of thumb. Your written procedure and the governing standard set the actual current.

Method checked against ASME BPVC Section V, Article 7, by paragraph number: T-753.2 (coil ampere-turn formulas), T-754.1 (direct contact, 300 to 800 A per inch of diameter) and T-752.2 and T-752.3 (prod current and spacing), with the fill-factor split and the L/D range of 2 to 15 from ASTM E1444/E1444M-16e1 Appendix X4. The values were confirmed in the US NRC NDE and Technology Codes Student Manual, Ch. 7, Sulzer test procedure TP-0002 and Portland Community College, WLD 204 Magnetic Testing on 20 Oct 2026. A calculation aid for study and estimating, not a substitute for your written procedure or the governing standard.

Results update as you type. Enter dimensions in inches. Every result is a study rule of thumb. The current your written procedure sets controls, along with any field check it requires.

Coil shot: ampere-turns from L/D

L/D = 4. 11,250 ampere-turns, or 2,250 A through 5 turns.

Head shot: amperes per inch of diameter

Rate 300 to 800 A per inch of outer diameter, direct or rectified current: 600 to 1,600 A.

Prods: amperes per inch of spacing

Rate 100 to 125 A per inch: 600 to 750 A.

The procedure controls, not these rules

The current used on a real part comes from the written procedure, which follows the standard your contract names, for example ASTM E1444/E1444M, ASTM E709 or ASME BPVC Section V, Article 7. Equipment makers' instructions also apply. This page cites those documents by paragraph number and does not reproduce their text. Code editions change, and many procedures confirm the field with a field indicator rather than trusting a formula.

Coil shot: ampere-turns from L/D

A coil makes a longitudinal field, which shows discontinuities running across the part. Divide the part length by its diameter to get L/D, then use the fill-factor rule your notes give:

  • Low fill factor (part lying near the wall of a much larger coil): NI = 45,000 / (L/D).
  • High fill factor (part nearly fills the coil): NI = 35,000 / (L/D + 2).

NI is ampere-turns. Divide by the number of coil turns N to get amperes. Both constants come from ASME BPVC Section V, Article 7, T-753.2. ASME picks the formula by L/D rather than by fill factor: T-753.2(a) uses 35,000 / (L/D + 2) at an L/D of 4 or more, T-753.2(b) uses 45,000 / (L/D) from 2 up to 4, and T-753.2(c) rules the coil out below 2. Exam study material sorts the same two formulas by fill factor instead, as ASTM E1444/E1444M-16e1 Appendix X4 does (X4.3.1.1 low fill, X4.3.2 high fill), so check which convention your exam or procedure uses. The NRC student manual and Sulzer's TP-0002 print the ASME version, and the PCC packet prints the 45,000 rule with an 8 in by 2 in example and asks for an L/D of 3 or greater.

ASTM E1444-16e1 X4.3 gives the fill-factor formulas for L/D from 2 to 15. ASME V has no such cap. When L/D is 15 or more, the calculator uses 15, a convention many study calculators follow, so a longer bar does not keep getting a smaller current; your procedure may instead have you shoot a long part in sections. Recent editions of ASME V T-753.2 separately ask for field adequacy to be shown wherever the area examined lies more than 9 in either side of the coil's center, so the calculator flags parts longer than 18 in. A short part has its poles too close together for a useful coil field, so the calculator flags L/D below 3 and below 2. Some study notes also give a formula for a part centered in a low-fill coil; it is not included.

Head shot: amperes per inch of diameter

Passing current through the part makes a circular field, which shows discontinuities running along it. ASME BPVC Section V, Article 7, T-754.1 gives a single range for direct contact with direct or rectified current: 300 to 800 A per inch of outer diameter, whatever the size of the part. Multiply the rate by the diameter. For a part that is not round, ASME uses the greatest cross-section diagonal at right angles to the current instead of the diameter. The NRC student manual and Atlantis NDT print the same range. Some older training notes give higher rates in diameter bands instead; if your notes do, check which document your exam or procedure follows.

Prods: amperes per inch of spacing

ASME BPVC Section V, Article 7, T-752.2 gives 90 to 110 A per inch of prod spacing for sections under 3/4 in thick and 100 to 125 A per inch at 3/4 in or thicker. T-752.3 caps the spacing at 8 in and treats spacing under 3 in as usually impractical. The PCC packet's example procedure and Atlantis NDT print the same rates. Multiply the rate by the spacing. Closer prods crowd particles around the contacts; wider prods leave a weak field between them.

Worked examples

These fictional parts were worked by hand and match the calculator. They are calculation examples, not exam items.

Hand-checked study examples (inches, amperes)
SetupWorkingResult
Coil, low fill, 8 in × 2 in bar, 5 turns45,000 / (8 / 2) = 11,250 ampere-turns; 11,250 / 52,250 A
Coil, high fill, 10 in × 2 in bar, 5 turns35,000 / (10 / 2 + 2) = 5,000 ampere-turns; 5,000 / 51,000 A
Coil, low fill, 40 in × 2 in bar, 5 turnsL/D = 20, worked at 15: 45,000 / 15 = 3,000 ampere-turns; 3,000 / 5600 A
Head shot, 2 in diameter, DC2 × 300 to 2 × 800600 to 1,600 A
Head shot, 8 in diameter, DC8 × 300 to 8 × 8002,400 to 6,400 A
Prods 6 in apart, 1 in plate6 × 100 to 6 × 125600 to 750 A
Prods 6 in apart, 1/2 in plate6 × 90 to 6 × 110540 to 660 A

Assumptions and use limits

Inputs are in inches and results in amperes; for half-wave current, read the meter the way your equipment instructions say. The coil rules assume a solid round part; for other shapes your procedure defines the effective diameter. The head-shot range is for direct or rectified current; this calculator does not scale it for AC. The rules do not account for material permeability, coil length or residual-method needs, and they say nothing about acceptance. A credential holder has not yet signed off on these worked methods, so treat them as unreviewed study notes until one does.

Related MT pages

Check which MT General outline applies to your exam date, then see the MT Level II practice page.