PCB Calculator

A PCB trace calculator applies IPC-2221 and standard transmission-line formulas to four related design problems: characteristic impedance, current-carrying capacity, trace resistance, and the minimum trace width needed for a target current. Four modes, Impedance, Current Capacity, Resistance, and Trace Width, each take a distinct set of board parameters and return the value needed to size copper before a board goes to fabrication.

Impedance mode finds the characteristic impedance of a microstrip or stripline trace from trace width, trace thickness, dielectric height, and the substrate's dielectric constant (Er). Microstrip traces run on an outer layer over a single reference plane; stripline traces run on an inner layer between two reference planes, which changes the field geometry and the formula that applies. The microstrip formula is Z0 = (87 / sqrt(Er + 1.41)) x ln(5.98 x H / (0.8 x W + T)); the stripline formula is Z0 = (60 / sqrt(Er)) x ln(1.9 x (2 x H) / (0.8 x W + T)), where H is dielectric height, W is trace width, and T is trace thickness, all expressed in the same units.

Current Capacity mode finds the maximum current a trace can carry per IPC-2221, from trace width, copper weight in ounces per square foot, allowable temperature rise, and whether the trace sits on an external or internal layer. Cross-sectional area is trace width multiplied by copper thickness, where 1 oz/ft² of copper equals 1.378 mils of thickness. The IPC-2221 formula is I = k x dT^0.44 x A^0.725, with k = 0.048 for external layers and k = 0.024 for internal layers; a buried internal trace has less path to ambient air than an external one, so it carries less current for the same allowed temperature rise.

Resistance mode computes trace resistance from trace width, trace length, copper weight, and operating temperature. Copper resistivity rises with temperature and follows rho = 1.724e-6 x (1 + 0.00393 x (T - 25)) ohm-cm; resistance equals rho x length divided by cross-sectional area, once every dimension is converted to centimeters.

Trace Width mode inverts the current-capacity relationship to find the minimum width for a required current: cross-sectional area A = (I / (k x dT^0.44))^(1/0.725), and width follows by dividing that area by the copper thickness for the selected copper weight.

All four modes accept fractions such as 3/4, mixed numbers such as 1 1/2, decimals, and scientific notation such as 1.5e3. Settings holds a step-by-step formula derivation, useful for checking a result against a hand calculation, and adjustable decimal precision.

PCB trace calculations are fundamental to reliable board design across industries ranging from consumer electronics to aerospace. Proper impedance control prevents signal integrity issues, while correct current and width sizing avoids thermal failures and trace damage.

• High-Speed Digital Design: a 50-ohm microstrip trace on FR-4 substrate (Er = 4.5) with a 10-mil dielectric height needs approximately 18 mils of trace width at 1.4 mils thickness, the controlled impedance DDR4 memory routing requires.
• Power Supply Layout: a 5-amp power rail on a 2 oz copper external layer with a 20 degree Celsius allowable temperature rise needs a minimum trace width of approximately 25 mils, avoiding overheating on voltage regulator output traces.
• RF Circuit Design: stripline impedance for a 75-ohm transmission line embedded between ground planes in a multilayer stackup confirms trace geometry before fabrication, minimizing return loss.
• Automotive Electronics: traces for high-current motor drivers operating at elevated ambient temperatures need wider margins, since current capacity drops at narrower allowable temperature rise on internal layers.
• Prototype Verification: trace resistance for a 6-inch run of 8-mil trace on 0.5 oz copper at 85 degrees Celsius operating temperature flags voltage drop concerns before a board revision is committed.
• LED Driver Boards: required trace width for constant-current LED strings drawing 350 milliamps each across multiple channels balances board density against thermal performance on single-sided aluminum substrates.
• Signal Integrity Analysis: microstrip and stripline impedance for the same trace geometry compare layer assignment trade-offs during PCB stackup planning.
• Educational Reference: stepping through the IPC-2221 current capacity formula with specific inputs shows how copper cross-section, temperature rise, and layer placement interact.