PCB Via Current Calculator

The PCB Via Current Calculator works out how much current one plated through-hole via can carry before its barrel heats up past the temperature rise you allow, and how many vias you need in parallel to move your actual load. It starts from the three things you control in the fabrication notes — finished hole diameter, copper plating thickness, and board thickness — and reports ampacity, barrel resistance, voltage drop, power loss, and the real temperature rise at your operating current.

How a Via Carries Current

A via is not a solid copper post. It is a drilled hole whose wall is electroplated with a thin layer of copper, so in cross-section the conductor is a ring, not a disc. The air in the middle carries nothing. That is why plating thickness matters so much: on a 0.3 mm hole, going from 20 µm to 25 µm of plating adds a quarter more copper and a fifth more current, while the drill size on the fabrication drawing does not change at all.

where \(d\) is the finished hole diameter, \(t\) is the plating thickness, and \(D = d + 2t\) is the outer barrel diameter. Convert the result to square mils (multiply mm² by 1550) before using the IPC formula.

with \(k = 0.024\) for internal conductors and \(k = 0.048\) for external conductors, \(\Delta T\) in °C, and \(A\) in mil². The exponents are the important part: current scales with area to the power 0.725, so doubling the copper buys about 65% more current, and it scales with temperature rise to the power 0.44, so doubling the allowed rise buys only about 36% more.

with \(\rho = 1.72 \times 10^{-8}\) Ω·m for copper at 20 °C, \(\alpha = 0.00393\) per °C, \(L\) the board thickness the barrel spans, and \(T\) the barrel temperature. A typical 0.3 mm via through a 1.6 mm board lands near 1 mΩ.

Internal or External? Why Via Numbers Differ Between Calculators

IPC-2221 describes traces, not vias, and the standard offers two constants: one for conductors buried inside the stack-up and one for conductors on an outer layer with air above them. A via barrel is genuinely in between. It is surrounded by laminate, which argues for the internal constant, but it is hollow, air-filled, and tied to a pad on each surface, which argues for the external one. Different tools pick different sides, and that is the whole reason two reputable calculators can disagree by a factor of two on the same via.

This calculator always shows both. Use the conservative internal number for design reviews and anything safety-related, and treat the external number as an upper bound — the current at which the via is definitely not fine.

Typical Via Current Capacity

The table below assumes 25 µm (1 mil) of plating and a 10 °C rise — the most common combination in practice.

Notice how flat that column is: a hole nearly seven times wider carries barely three times the current. Copper area grows only linearly with hole diameter for a fixed plating thickness, and ampacity then grows more slowly still. Several small vias beat one big one almost every time — and they are cheaper to drill.

How Much Plating Will You Actually Get?

If you do not know what your fabricator supplies, enter 20 µm. It is the floor for Class 2 and it keeps the answer on the safe side of whatever you actually receive.

Design Tips for High-Current Vias

How to Use This Calculator

1. Enter the finished hole diameter — the hole after plating, in mm or mil.
2. Enter the copper plating thickness in µm, mil, or ounces. 25 µm is the usual default.
3. Enter the board thickness the via passes through; a blind or buried via spans less than the full stack.
4. Set the allowed temperature rise (10 °C is standard) and your target current.
5. Pick the IPC-2221 model, then click Calculate Via Current.
6. Read the ampacity of one via, the number of vias needed, and the resistance, drop, and power loss.

Limitations You Should Know About

• IPC-2221 is a trace standard. Applying it to a via barrel is standard industry practice but it is an approximation, not a measurement.
• Steady-state DC only. Short pulses can exceed these numbers by a wide margin; high-frequency current crowds toward the barrel wall surface.
• No neighbouring-copper credit. A via dropping into a large plane runs cooler than this predicts; a via in an isolated island runs hotter.
• Plating varies. Real plating is thinner in the middle of a deep hole than at the ends. Use the minimum your fabricator guarantees, not the average.
• Not a substitute for thermal simulation on anything dense, high-power, or safety-critical.

One number worth keeping in perspective: a single 0.3 mm via through a 1.6 mm board is only about 1 mΩ, which is negligible next to the trace it connects. Via resistance starts to matter on high-current rails and switching-regulator return paths, where several milliohms in series add up — and it always matters for the power lost as heat, which is what raises the barrel temperature in the first place.

Frequently Asked Questions

How much current can a PCB via carry?

It depends almost entirely on how much copper is in the barrel wall. A standard 0.3 mm finished hole with 25 µm of plating has about 40 mil² of copper, which carries roughly 1 A at a 10 °C rise on the conservative IPC-2221 internal model, or about 1.9 A on the external model. A 0.5 mm hole with the same plating carries roughly 1.5 to 3 A. Doubling the hole size does not double the current, because ampacity scales with cross-section to the power 0.725.

What is the formula for via current capacity?

The via barrel is treated as a conductor with an annular cross-section, so A = π t (d + t), where d is the finished hole diameter and t is the plating thickness. That area, in mil², goes into the IPC-2221 ampacity formula I = k ΔT^0.44 A^0.725, with k = 0.024 for internal conductors and k = 0.048 for external conductors.

Should I use the internal or external IPC-2221 constant for a via?

IPC-2221 was written for traces, not vias, so neither constant is exact. The internal constant k = 0.024 is the conservative choice and is what most design reviews expect, because the barrel is surrounded by laminate. Some calculators use the external constant k = 0.048 on the grounds that the hollow barrel is cooled by air and by the pads at both ends. This calculator shows both so you can see the spread and decide how much margin you want.

How many vias do I need for 5 amps?

Divide the target current by the capacity of one via and round up. With 0.3 mm vias plated to 25 µm at a 10 °C rise, one via carries about 1 A on the conservative model, so 5 A needs at least 5 vias, and 6 to 8 is a safer choice once uneven current sharing is allowed for. Larger 0.5 mm vias cut that to 3 or 4.

How thick is via plating?

IPC-6012 Class 2 requires an average of 20 µm of copper in the hole wall with an 18 µm minimum, and Class 3 requires an average of 25 µm with a 20 µm minimum. Most fabricators quote 25 µm, which is 1 mil or about 0.7 oz. Heavy copper processes can reach 50 to 75 µm at extra cost. If you do not know what your fabricator gives you, entering 20 µm keeps the answer on the safe side.

Do thermal vias carry current or heat?

Both, and the same geometry drives each. A via array under a power pad conducts heat into an inner plane or the bottom copper, and the copper barrel that carries that heat also carries current. Thermal via arrays are usually 0.3 mm vias on a 1.0 to 1.2 mm grid, so the current capacity this calculator reports for one via, multiplied by the number of vias in the array, tells you how much current the same array can pass.

Related Calculations

Size the copper on the layer as well as through it with the PCB Trace Width Calculator, check the loss along the run with the Voltage Drop Calculator, and work out conductor resistance directly with the Wire Resistance Calculator.