EMI Shielding Effectiveness Calculator — Material SE in dB

Calculate

Shielding Effectiveness: evaluate a barrier of known thickness. Required Thickness: size a barrier to reach a target SE.

Plane wave applies beyond the far-field boundary λ/2π. Electric near field for high-impedance sources (rod antennas, fast-switching nodes). Magnetic near field for low-impedance sources (loops, transformers, motors, power conductors).

Select a preset metal or enter custom σr and μr. Values shown are relative to annealed copper. Preset μr values are low-frequency figures — a warning appears when used above their valid frequency range.

Frequency of the interference source. Supported range: 1 Hz to 40 GHz.

Solid continuous barrier thickness. Range: 0.1 µm to 50 mm (0.004 to 1969 mils).

In Shielding Effectiveness mode: optional, enables MEETS TARGET / BELOW TARGET verdict. In Required Thickness mode: required. Range: 1 to 300 dB.

Overview

This calculator applies the classical Schelkunoff method to compute the theoretical shielding effectiveness of a solid metal barrier, covering plane-wave, electric near-field, and magnetic near-field sources, with a reverse solver for required thickness.

What to Look at First

The headline is the total shielding effectiveness in dB (forward mode) or the minimum thickness in your selected unit (reverse mode)

The badge below it states the target screen: MEETS TARGET with the margin in dB, BELOW TARGET with the shortfall in dB, or an informational label when no target was entered

The breakdown shows where the attenuation comes from — absorption, reflection, and the multiple-reflection correction

Advisory notes appear when the model is near its limits: permeability used above its valid frequency range, reflection loss floored at zero, a thin-barrier regime, or a figure beyond practical measurement

Read the notes before acting on the number; they tell you how much to trust it

How to Use the EMI Shielding Effectiveness Calculator

  1. Pick the mode — Shielding Effectiveness evaluates a barrier you have; Required Thickness sizes one for a target.

  2. Choose the field type. Use Plane wave for distant sources (far field), Electric near field for high-impedance sources such as rod antennas or fast-switching nodes, and Magnetic near field for low-impedance sources such as transformers, motors, or power conductors.

  3. For a near-field type, enter the distance from the noise source to the shield surface — not the cable length, enclosure size, or where you stand.

  4. Enter the frequency (Hz to GHz).

  5. Select a material, or pick Custom and enter relative conductivity σr (copper = 1) and relative permeability μr.

  6. In Shielding Effectiveness mode, enter the barrier thickness (mm, mils, or µm). A target SE in dB is optional — leave it blank for an informational result.

  7. In Required Thickness mode, enter the target SE; the minimum thickness appears in your selected unit.

  8. Read the headline, then the breakdown and any advisory notes about model limits.

This tool models a continuous homogeneous solid barrier only. Apertures, seams, gaskets, cable penetrations, and conductive coatings are out of scope. For final design, verify by measurement per IEEE 299 or ASTM D4935.

Inputs & Outputs

Inputs

  • •Mode — Options: Shielding Effectiveness — evaluate a barrier, Required Thickness — size a barrier for a target
  • •Field Type — Options: Plane wave — far field, Electric near field — high-impedance source, Magnetic near field — low-impedance source
  • •Material — Options: Copper — σr = 1.00, μr = 1, Aluminum — σr = 0.61, μr = 1, Steel (SAE 1045) — σr = 0.10, μr = 1000, Stainless 430 — σr = 0.02, μr = 500, Mu-metal — σr = 0.03, μr = 20,000, Zinc — σr = 0.29, μr = 1, Brass — σr = 0.26, μr = 1, Nickel — σr = 0.20, μr = 100, Tin — σr = 0.15, μr = 1, Silver — σr = 1.05, μr = 1, Custom material…
  • •Frequency
  • •Frequency Unit — Options: Hz, kHz, MHz, GHz
  • •Barrier Thickness
  • •Thickness Unit — Options: mm, mils, µm
  • •Distance from Source
  • •Distance Unit — Options: m, cm, in, ft
  • •Target SE (dB)
  • •Relative Conductivity σr
  • •Relative Permeability μr

Outputs

  • •Total Shielding Effectiveness (dB)
  • •Absorption Loss A (dB)
  • •Reflection Loss R (dB)
  • •Multiple-Reflection Correction B (dB)
  • •Status

Formula

Schelkunoff Shielding Effectiveness Decomposition

Total SE adds three terms (all in dB):

SE = A + R + B

Skin depth — sets the absorption scale:

δ = 1 / √(π · f · μ · σ)    [m]
μ = μr · 4π×10⁻⁷ H/m,  σ = σr · 5.8×10⁷ S/m

Absorption loss — linear with thickness:

A = 8.686 · t / δ    [dB]
Closed form: A = 131.4 · t · √(f · μr · σr)  (t in m, f in Hz)

Barrier impedance:

|Zs| = √2 / (σ · δ)    [Ω]

Wave impedance Zw by field type:

Plane wave:       Zw = Z0 = 376.73 Ω
Electric near:    Zw = Z0 · c / (2π · f · r)
Magnetic near:    Zw = Z0 · 2π · f · r / c

Reflection loss (floored at 0):

R = max(0, 20 · log10(Zw / (4 · |Zs|)))    [dB]

Multiple-reflection correction (≤ 0 always):

B = min(0, 10 · log10((1 − x·cosθ)² + (x·sinθ)²))    [dB]
where  x = 10^(−A/10),  θ = 0.23026 · A  radians

Far-field boundary:

r_t = λ/(2π) = c / (2π · f)

Required Thickness (reverse): bisection solve SE(t) = target over t ∈ [10⁻⁸, 1] m, ceiled to 3 significant figures in the selected unit.

What Is EMI Shielding Effectiveness?

Shielding effectiveness is the ratio, expressed in decibels, between the field strength arriving at a barrier and the field strength that gets through it: SE = 20·log10(E_incident / E_transmitted). A value of 20 dB means the field is reduced ten-fold, 60 dB a thousand-fold, 120 dB a million-fold.

The Schelkunoff treatment splits that attenuation into three mechanisms that simply add in dB: reflection at the surfaces, where the impedance mismatch between the wave and the metal turns energy back; absorption inside the metal, where the field decays exponentially with depth; and a multiple-reflection correction for barriers too thin to absorb the energy bouncing between their two faces. Which mechanism carries the load depends on the source type, the frequency, and the material — the sections below take each in turn.

EMI Shielding Effectiveness Formula

The total is the sum of three loss terms, each in dB:

SE = A + R + B

Skin depth sets the scale for absorption: δ = 1 / √(π · f · μ · σ), where μ = μr · 4π×10⁻⁷ H/m and σ = σr · 5.8×10⁷ S/m.

Absorption loss grows linearly with thickness: A = 8.686 · t / δ (t and δ in the same unit). Metric closed form: A = 131.4 · t · √(f · μr · σr), with t in meters and f in Hz. Imperial closed form: A = 3.34 · t · √(f · μr · σr), with t in inches and f in Hz.

Reflection loss depends on the wave impedance Zw of the incident field and the barrier impedance |Zs| = √2 / (σ·δ): R = 20 · log10( Zw / (4 · |Zs|) ), floored at 0 dB. Zw equals Z0 = 376.73 Ω for a plane wave, Z0 · c / (2π·f·r) for an electric near field, and Z0 · 2π·f·r / c for a magnetic near field, where r is the source distance in meters.

The multiple-reflection correction matters when the barrier is thin relative to the skin depth: B = min( 0, 10·log10( (1 − x·cosθ)² + (x·sinθ)² ) ), where x = 10^(−A/10) and θ = 0.23026 · A in radians. B is treated strictly as a correction loss — the phase-aware expression can produce a small positive ripple near A ≈ 10 dB, which is discarded as a conservative choice.

The far-field boundary separating near-field and plane-wave regimes: r_t = λ / 2π = c / (2π · f).

Skin Depth and Absorption Loss

Skin depth is the distance into a conductor at which the field falls to about 37% of its surface value, and each skin depth of travel costs about 8.7 dB of absorption loss. That single relationship drives most of the thickness math in shielding: a barrier two skin depths thick contributes roughly 17 dB of absorption, three skin depths roughly 26 dB, and so on linearly.

Skin depth shrinks with the square root of frequency and with the square root of the μr·σr product. In copper it is about 66 µm (2.6 mils) at 1 MHz and about 2.1 µm (0.08 mils) at 1 GHz — which is why at high frequency almost any solid metal absorbs strongly, while at low frequency even thick sheets of a non-magnetic metal may contribute only a few dB. Magnetic materials shorten the skin depth through permeability: steel at line and audio frequencies absorbs far more per millimeter than copper, despite its lower conductivity.

Plane Wave vs Near-Field Shielding

The boundary between near field and far field lies at λ/2π from the source: about 48 m at 1 MHz, 48 cm at 100 MHz, 4.8 cm at 1 GHz. Beyond that distance the wave impedance settles at 377 Ω and the plane-wave model applies regardless of what generated the field.

Electric near-field sources — rod antennas, sparks, fast voltage edges — present a high wave impedance, so the mismatch against a metal barrier is enormous and reflection loss runs even higher than the plane-wave figure. Magnetic near-field sources — loops, transformers, motors, high-current conductors — present a low wave impedance, the mismatch shrinks, and reflection loss can fall toward zero.

Material SE vs Enclosure SE

The figure this page produces is a material property, not a box rating. A continuous sheet of metal can compute to hundreds of dB, but a finished enclosure leaks through apertures, seams, gasket joints, cable penetrations, and imperfect grounding and bonding — paths the flat-sheet model does not see. In practice those paths set the ceiling: a single unfiltered cable through the wall can dominate the entire result.

Values above roughly 100–120 dB should be read as theoretical. They exceed the repeatability of enclosure measurements and the leakage limits of real hardware, so differences between two designs in that range carry no practical meaning. Use the estimate to compare materials and size thicknesses, then verify the finished assembly by measurement: IEEE 299 covers enclosures, ASTM D4935 covers planar material samples.

Low-Frequency Magnetic Shielding

Low-frequency magnetic fields are the hardest shielding case because both standard mechanisms weaken at once: the wave impedance is low, so reflection loss is small and often floors at zero, and skin depths in non-magnetic metals are large, so absorption per millimeter is modest. What remains is absorption through high permeability — which shortens the skin depth — and flux diversion, where a high-μ alloy gives the field an easier path around the protected volume.

High-permeability materials come with their own fine print. Published mu-metal permeability can begin falling around the kHz range, depending on alloy, anneal, and geometry; steel and nickel figures are likewise low-frequency values. Forming, mechanical stress, saturation in strong fields, and installation gaps all reduce effective permeability, and flux-diversion performance depends heavily on shape, which a flat-sheet model cannot capture.

Required Thickness for a Target SE

Required Thickness mode answers the sizing question directly: given a target in dB, a frequency, a material, and a field type, it solves for the thickness at which the computed SE meets the target. Shielding effectiveness rises monotonically with thickness, so the solution is unique, and the displayed answer is rounded up in the unit you selected — the value on screen always delivers at least the requested attenuation.

Key Facts

  • One skin depth of material provides roughly 8.7 dB of absorption loss; doubling thickness doubles A in dB.
  • Skin depth shrinks with the square root of frequency: in copper, about 66 µm (2.6 mils) at 1 MHz and about 2.1 µm (0.08 mils) at 1 GHz.
  • Reflection dominates at low frequency for electric and plane-wave sources; absorption dominates at high frequency for every source type.
  • Low-frequency magnetic fields are the most difficult to shield — reflection loss can fall to zero, leaving absorption and flux diversion as the only mechanisms.
  • The near-field/far-field boundary sits at λ/2π: about 48 m at 1 MHz, 48 cm at 100 MHz, 4.8 cm at 1 GHz.
  • The multiple-reflection correction is significant when absorption is below roughly 10–15 dB and negligible beyond about two skin depths.
  • Published permeability figures for steel, nickel, stainless, and mu-metal are low-frequency values; mu-metal permeability can begin falling around the kHz range.
  • Computed values above roughly 100–120 dB exceed what enclosure measurements can resolve and what leakage paths permit in practice.

Applications

  • Material selection: comparing how a given thickness of aluminum, steel, or copper performs at the frequencies of a known interference source before committing to a design.
  • Sizing to a goal: turning a 60 dB or 80 dB design requirement into a concrete minimum thickness at the EMI frequency in play.
  • Board-level shield cans evaluated against nearby clock or switching noise.
  • Screening sensitive analog or measurement circuits from a transformer or motor identified as a magnetic near-field source.
  • Foil and thin-sheet comparison, where the multiple-reflection penalty decides the outcome.
  • First-pass numbers for shielded room or cabinet walls ahead of an IEEE 299 measurement campaign.
  • Pre-compliance planning: the loss breakdown shows whether a failing margin is a reflection problem (material choice) or an absorption problem (thickness or permeability).

Example Calculation

Example 1 — Aluminum Foil at 1 MHz

Barrier: aluminum, 25 µm (0.98 mils) thick, plane wave. At 1 MHz the skin depth of aluminum is 84.6 µm (3.33 mils), so the foil is only 0.30 skin depths thick. Absorption loss A = 8.686 × 0.30 = 2.57 dB. Reflection loss for a plane wave on aluminum at this frequency is R = 105.99 dB. Because the foil is thin, the multiple-reflection correction is substantial: B = −4.12 dB. Total: SE = 2.57 + 105.99 − 4.12 = 104.4 dB.

Against a 100 dB design target the screen reads MEETS TARGET with 4.4 dB of margin — MODERATE headroom.

Example 2 — Required Steel Thickness for 80 dB

Target: 80 dB, plane wave at 10 MHz, steel (σr = 0.10, μr = 1000). The skin depth at 10 MHz is 2.09 µm, and reflection contributes R = 58.14 dB, leaving about 21.9 dB for absorption. Solving and rounding up gives a minimum thickness of 5.27 µm (0.208 mils) — at that thickness A = 21.90 dB, B = −0.02 dB, and the delivered total is 80.03 dB ≥ 80 dB.

Standards & References

Units

Shielding effectiveness, losses, margins — decibels (dB) throughout; margins and shortfalls are reported in dB, not percent. Frequency accepts Hz, kHz, MHz, or GHz. Thickness uses millimeters, micrometers, or mils (1 mil = 0.001 in = 25.4 µm = 0.0254 mm; 1 mm = 39.37 mils). Source distance accepts meters, centimeters, inches, or feet (1 in = 2.54 cm; 1 ft = 0.3048 m). Skin depth is shown in µm below 1 mm, otherwise in mm. Material properties — σr is conductivity relative to annealed copper (5.8×10⁷ S/m); μr is relative permeability (non-magnetic metals = 1). Required thickness is rounded up in the unit you selected, so the displayed value always meets the target.

Limitations

  • Single homogeneous metallic barrier only — no apertures, slots, seams, or cable penetrations.
  • No gaskets (MIL-DTL-83528 territory), multilayer laminates, conductive coatings, or thin-film materials.
  • Does not model magnetic saturation, anisotropic, perforated, or composite materials.
  • Near-field expressions are dipole-source approximations; when reflection is clamped to 0 dB the multiple-reflection correction is also approximate.
  • Low-frequency magnetic shielding with high-μ materials is strongly geometry-dependent — flat-sheet estimates can overstate enclosure performance.
  • μr values are low-frequency figures that fall with frequency, flux level, mechanical stress, and anneal state.
  • Results above roughly 100–120 dB are theoretical: they exceed measurement repeatability and practical leakage limits.
  • Does not model shield grounding, bonding, or cable shield termination.

Common Mistakes to Avoid

  • Treating the material SE as an enclosure rating — a single seam, or a cable passing through the shield without filtering or proper shield termination, can dominate leakage and erase most of the computed attenuation.
  • Entering the observation distance, cable length, or enclosure size instead of the source-to-shield distance in near-field modes.
  • Applying room-temperature, low-frequency permeability (steel μr = 1000, mu-metal μr = 20,000) at RF, which overstates absorption and undersizes the barrier.
  • Choosing a near-field type when the source sits beyond λ/2π — the plane-wave model applies there.
  • Assuming a shield can works without good contact to the ground or reference structure: poor bonding or a floating shield can make the real system perform far below the material SE.
  • Expecting bulk-metal formulas to predict conductive paint, plating, or foil-laminate performance; thin coatings follow a different model.
  • Rounding a required thickness down to the nearest stock gauge and assuming the target still holds.
  • Comparing designs by values above 120 dB, where differences are not measurable and leakage governs.

Frequently Asked Questions

Does this calculator predict enclosure shielding effectiveness?
No. It predicts the theoretical material SE of a continuous solid barrier. Finished enclosure SE is usually limited by seams, apertures, gaskets, cable penetrations, and grounding and bonding quality — paths the flat-sheet model does not see. Use the estimate for material selection and thickness sizing, then verify the assembly by measurement per IEEE 299.
What counts as a good shielding effectiveness value?
It depends on the problem. 20–40 dB resolves many board-level coupling issues, 60–90 dB is a common range for commercial enclosures and shielded compartments, and 100 dB and above belongs to shielded rooms and specialized facilities. Above roughly 120 dB the numbers stop being measurable in practice, so chasing them on paper has no payoff.
Why does reflection loss show 0 dB?
For low-frequency magnetic fields the wave impedance can drop below the barrier impedance, and the standard reflection approximation returns a negative number that simply means it is outside its useful range. The screen floors it at zero and notes that absorption governs — which is exactly why low-frequency magnetic shielding relies on thickness and high-permeability material rather than reflection.
Which material works best against low-frequency magnetic fields?
High-permeability alloys such as mu-metal, or thick steel. Conductivity barely helps because reflection is negligible; permeability shortens the skin depth and diverts flux. Mind the caveats: permeability falls with frequency, and forming, stress, or saturation can cut it further — manufacturer data for the actual part beats any table value.
How do I choose between plane wave, electric near field, and magnetic near field?
First check the distance: beyond λ/2π from the source, use plane wave regardless of source type. Closer in, pick by source impedance — rod antennas, sparks, and fast voltage edges are electric (high-impedance) sources; loops, transformers, motors, and high-current conductors are magnetic (low-impedance) sources. The breakdown reports the λ/2π transition distance and warns if your distance lands in the far field.
Does thickness matter more than material choice?
At high frequency, barely — skin depths are tiny, absorption is huge, and almost any solid metal works, so conductivity and build quality decide. At low frequency the roles flip: reflection fades, and the product of thickness, conductivity, and permeability sets the result. The A/R/B breakdown shows which lever applies to your case.
Can I use this for conductive coatings, paints, or metallized fabric?
No. Layers much thinner than a skin depth follow a sheet-resistance model rather than the bulk-metal equations used here, and the estimate would mislead in both directions. The page covers solid metal sheet and foil; for coatings, rely on manufacturer attenuation data measured per ASTM D4935.
How accurate are the built-in permeability values for steel and mu-metal?
They are standard low-frequency engineering figures — useful for comparison, optimistic at higher frequencies. Ferromagnetic permeability falls as frequency rises, so the screen warns whenever a magnetic material is used above its validity range and, in reverse mode, notes that the computed minimum thickness may understate the real need. For final design, confirm μr at your frequency from the material datasheet.

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