Via Size Calculator

Via failure happens suddenly and without warning. A via that works perfectly at 2A can fail catastrophically at 2.1A because the copper plating inside the via barrel overheats and melts. Unlike traces that show visible damage when overloaded, via failures are internal and invisible until the circuit stops working.

This calculator uses the IPC-2221 standard to size vias based on current density and thermal limits. The copper barrel inside each via acts as a tiny resistor - when current flows through it, the resistance creates heat. If the heat cannot dissipate fast enough, the temperature rises until the copper-to-substrate bond fails. The standard assumes standard FR4 PCB material with typical thermal properties.

Via current capacity scales with the cross-sectional area of the copper barrel, not the diameter. Doubling the via diameter increases capacity by roughly four times, but manufacturing constraints limit practical via sizes. Most PCB shops can reliably drill 0.1mm holes, while anything smaller than 0.08mm requires specialized equipment and costs significantly more.

Use this calculator for any PCB trace carrying more than 0.5A continuous current. Digital signals and low-power analog circuits rarely need via calculations since their currents are typically under 0.1A. Power supply rails, motor drivers, LED strips, and battery charging circuits always need proper via sizing.

Calculate vias during the schematic design phase, not during PCB layout. Via requirements influence component placement and routing strategy. If your power section needs large vias or many parallel vias, plan the layout accordingly rather than discovering the requirement after routing is complete.

Recalculate via sizes when changing copper weight, operating temperature, or current requirements. A design that works with 2oz copper may fail if manufactured with 1oz copper. Similarly, a via sized for 25°C operation may fail in a 50°C enclosure even at the same current level.

The biggest mistake is assuming trace width determines via requirements. A 2mm wide trace can carry 5A safely, but a single 0.2mm via on that trace can only handle 1.5A. The via becomes the bottleneck, overheating while the trace stays cool. Always calculate via requirements separately from trace width.

Another common error is using via calculators that ignore copper thickness. A 1oz copper PCB needs larger vias than a 2oz copper PCB for the same current. Many online calculators assume standard 1oz copper, leading to undersized vias on thin copper boards and oversized vias on thick copper boards.

Do not rely on PCB design software default via sizes. Most CAD tools default to 0.2mm vias regardless of current requirements. This works for low-current signals but fails catastrophically on power nets. Always calculate via sizes for each current level in your design, and use different via sizes for different current levels rather than one size for everything.

The IPC-2221 formula calculates via cross-sectional area from current and temperature rise: A = (I / (k × ΔT^b))^(1/c), where k=0.024, b=0.44, and c=0.725 for internal conductors. The area converts to diameter using A = π × (d/2)², so d = √(4A/π). These constants come from empirical testing of copper heating in FR4 substrates.

For a 2A current with 10°C temperature rise: A = (2 / (0.024 × 10^0.44))^(1/0.725) = 0.034 mm². This gives a via diameter of √(4 × 0.034 / π) = 0.21mm. The drill size is typically 0.05mm smaller than the finished via to account for copper plating thickness during manufacturing.

The formula breaks down at very low currents (under 0.1A) where manufacturing minimums dominate, and at very high currents (over 10A) where thermal modeling becomes more complex. Most PCB manufacturers have minimum drill sizes around 0.05-0.08mm regardless of electrical requirements, and maximum practical drill sizes around 1-2mm before mechanical drilling becomes unreliable.

IPC-2221 assumes steady-state current, but many circuits have pulsed loads that create different failure modes. A via that handles 3A continuous may fail at 6A for 100ms pulses due to I²R heating in the copper barrel. The thermal time constant of a typical via is 1-10 milliseconds, so short pulses heat the copper faster than heat can conduct away.