FR4 Dielectric Constant Calculator and PCB Design Guide

What Is the Dielectric Constant of FR4?

The FR4 dielectric constant (Dk, or relative permittivity εr) is a measure of how the PCB's insulating laminate responds to an electric field. For preliminary designs, engineers often use Dk ≈ 4.0–4.5, but that is not a guaranteed value for all FR4. FR4 refers to a family of glass-reinforced epoxy materials with different resin systems, fabric constructions, and electrical characteristics.

The correct FR4 Dk depends on the specified laminate, glass style, resin content, operating frequency, and test method. A single “FR4 = 4.4” assumption is especially risky for controlled-impedance and high-speed layouts.

FR4 Dielectric Constant vs. Frequency: Real Manufacturer Data

FR4 exhibits dielectric dispersion: its Dk can change with frequency. The following Isola 370HR examples are construction-specific published figures—not universal values for every FR4 PCB.

Source: Isola 370HR official construction-specific Dk/Df tables. Values are for the stated constructions and test conditions.

The table reveals two important facts: the same FR4 material family can have different Dk values because its construction differs, and a Dk measured at 1 GHz should not automatically be applied at 10 GHz. In the calculator, frequency is used for guided wavelength and phase; it does not invent a Dk-versus-frequency curve.

How the Calculator Works: The Actual Formulas

The calculator treats the signal path as an ideal, lossless, nonmagnetic transmission line. It first determines the effective relative permittivity and then calculates propagation velocity and delay:

Here, c = 299.792458 mm/ns. The effective relative permittivity (εeff) depends on the transmission-line cross-section:

Stripline: Fully Embedded Trace

For the ideal homogeneous stripline model, εeff = Dk. This makes the delay calculation particularly simple, but it does not calculate stripline impedance. Actual stripline impedance needs the trace dimensions, copper thickness, plane spacing, and stackup details.

Microstrip: Surface Trace

A microstrip trace has fields in both air and the PCB substrate. The calculator uses a common quasi-static approximation for εeff based on the trace width W, dielectric height h, and bulk Dk. The result is lower than bulk Dk for a conventional air-exposed microstrip.

For microstrip only, the tool also shows a preliminary characteristic impedance Z₀ using a zero-conductor-thickness closed-form approximation. This does not include solder mask, finite copper thickness, copper roughness, frequency dispersion, or manufacturing tolerances; it cannot replace your board shop's impedance field solver.

Worked Example: Dk 4.2 vs. 3.8

For an ideal embedded 50 mm transmission line, Dk = 4.2 produces approximately 341.80 ps of one-way delay, while Dk = 3.8 produces approximately 325.12 ps. That is a 16.68 ps difference over the same physical length. Change the line length or switch to Microstrip in the calculator above to see the corresponding results for your geometry.

FR4 Dk vs. Df: Why Both Matter

Dk influences electromagnetic propagation, phase velocity, and transmission-line impedance. Df (dissipation factor, or loss tangent) describes dielectric energy loss and contributes to signal attenuation. A PCB material with lower Dk does not automatically have lower Df or lower total insertion loss. High-speed and RF material selection must consider both, alongside copper loss, roughness, and circuit geometry.

How Dk Affects 50-Ohm and 100-Ohm PCB Routing

Dielectric properties affect the characteristic impedance of 50 Ω single-ended traces and 90/100 Ω differential pairs, but Dk alone is insufficient to calculate impedance. A reliable controlled-impedance calculation requires stackup geometry, reference-plane arrangement, finished copper thickness, line width, and—in differential pairs—trace spacing and coupling.

Use the microstrip Z₀ result above for early concept checks only. For final fabrication, provide Highleap Electronics with controlled-impedance targets and a proposed stackup so appropriate geometry and material assumptions can be reviewed.

What to Send a PCB Manufacturer

For an FR4 PCB with controlled impedance or demanding high-speed signals, include the laminate grade (or approved alternatives), layer stackup, core/prepreg details if available, finished copper requirements, controlled nets and target impedances, tolerances, fabrication files, and any loss or timing constraints. For complete PCB assembly projects, also provide the BOM, component placement data, and assembly requirements.

Highleap Electronics offers PCB fabrication and PCB assembly services. A manufacturable stackup should consider material availability, dielectric thickness, fabrication tolerances, assembly heating, and long-term reliability—not just a nominal Dk number.