PCB Trace Current Calculator - IPC-2221 Chart Fit and Drop

A pcb trace current calculator returns the maximum steady-state current a copper trace can carry before its temperature rises more than the allowed budget above the surrounding air, using the IPC-2221 chart-fit regression. The calculator pairs that ampacity equation with Ohm's law so resistance, voltage drop, and power dissipation come out at the rated current.

• • Size a power trace for a known current budget: Set the trace width, copper weight, and target temperature rise to read the resulting ampacity.
• • Pick the trace width for a target current: Iterate the trace width until the calculator's maximum current matches the load current.
• • Compare internal versus outer layer traces: Flip between External and Internal trace location to see the 50 % derating the IPC-2221 chart fit applies to buried traces.
• • Pre-flight the DC voltage drop on a long power net: Enter the actual trace length to read the resistance, voltage drop, and power dissipation before fab.

Use this pcb trace current calculator on USB VBUS rails, motor drivers, LED strings, battery charging paths, and power-stage FET loops. The governing ampacity curve comes from IPC-2221, which kept the chart-based model originally published in IPC-D-275.

The trace is a heat sink as well as a conductor. Wider and thicker traces carry more current because they dissipate I²R heat into the surrounding copper and dielectric. Outer layer traces carry more current than inner layer traces at the same cross-section because the air above the trace helps carry heat away.

For the matching controlled-impedance view of the same copper trace, pcb impedance calculator takes the same width and substrate inputs and returns the characteristic impedance the high-speed signal will see.

The calculator evaluates the IPC-2221 chart-fit regression with the cross-sectional area and temperature rise you enter, then chains Ohm's law with copper's temperature-corrected resistivity to read resistance, voltage drop, and power dissipation at the rated current.

• Trace location: External (outer layer, k = 0.048) or Internal (inner layer, k = 0.024). Picks the IPC-2221 chart-fit coefficient.
• Trace width (W): Copper trace width in millimetres. 0.5 mm for a signal trace, 1 to 5 mm for power traces.
• Copper weight (oz): Foil weight in ounces per square foot. 1 oz ≈ 0.0348 mm thick.
• Temperature rise ΔT (°C): Allowed trace temperature above ambient. The IPC-2221 chart covers 10 °C to 100 °C; 10 °C is the conservative default.
• Ambient temperature (°C): Air or enclosure temperature around the PCB. Used to compute absolute trace temperature.
• Trace length (mm): Distance the trace runs from source to load. Drives R, V_drop, and P through R = ρ L / A.

The 0.048 and 0.024 coefficients come from a regression fit to the IPC-2221 chart data (which descended from IPC-D-275), with the 0.44 and 0.725 exponents adopted from that same fit.

According to AdvancedPCB's trace width calculator, the IPC-2221 chart-fit equation is I = k · ΔT^b · A^c with k = 0.048 for external layers and k = 0.024 for internal layers, b = 0.44, and c = 0.725, fitted to the published IPC-2221 curves with A in square mils.

According to HyperPhysics, the DC resistivity of copper at 20 °C is 1.68 × 10⁻⁸ Ω·m and the temperature coefficient of annealed copper at 20 °C is 0.00393 /°C, which give the temperature-corrected trace resistance used in this calculator.

When the trace feeds an antenna or RF front end, rf unit converter converts dBm, dBµV, V, and mW at the system impedance the trace was designed for so the receiver power is comparable.

Four ideas cover every trace sizing conversation; once you understand them, the calculator outputs land in context.

These four ideas match the design rules a fab house will quote against, so the calculator result compares directly to the trace width table in your board vendor's quote.

Once the trace ampacity and load current are locked, op amp gain calculator sizes the amplifier's gain and feedback network so the op-amp output stage stays inside the supply rails the trace will carry.

Six quick steps take you from a copper weight and a target current to a verifiable trace width that hits the IPC-2221 chart-fit ampacity on the layer you want.

1. 1 Pick the trace layer: Choose External for an outer layer or Internal for a buried layer. Internal traces start at roughly 50 % of external capacity at the same cross-section.
2. 2 Enter the trace width: Type the width in mm you are willing to commit on the layout. The default 0.5 mm works as a starting point.
3. 3 Enter the copper weight: Type the foil weight in oz. 1 oz is the default for two-layer boards; 2 oz is common on power boards; 3 oz is heavy copper.
4. 4 Enter the temperature rise budget: Type the allowed trace temperature rise above ambient in °C. 10 °C is conservative; 20 °C is common; 30 °C is the upper limit.
5. 5 Enter ambient and length: Type the air temperature and trace length so the calculator can read voltage drop and power dissipation.
6. 6 Read max current, drop, and power: Read Max Current as the ampacity at the requested rise. Read Voltage Drop and Power Dissipation to verify the trace is acceptable for your load.

A USB 2.0 VBUS trace on a 1 oz two-layer board wants to carry 0.5 A. Set External, W = 0.5 mm, copper = 1 oz, ΔT = 10 °C, ambient = 25 °C, length = 50 mm; the calculator returns 1.44 A, 74 mV drop, and 0.107 W dissipation, leaving three times the headroom for that load.

When the trace is fed from a battery, battery capacity calculator converts the cell's mAh and C-rating into the runtime the trace will see at the load current.

Six practical benefits of running the IPC-2221 chart-fit equation yourself instead of staring at the standard's chart.

• • Returns ampacity, drop, and dissipation together: Max current, voltage drop, and power dissipation derive from one set of inputs.
• • Matches the IPC-2221 chart-fit coefficients exactly: Uses k = 0.048 for external and k = 0.024 for internal traces, with the 0.44 / 0.725 exponents.
• • Handles internal and outer layer traces: Flip the layer location to see the 50 % derating inner traces carry at the same cross-section.
• • Reads absolute trace temperature: Computes the actual trace temperature by adding the temperature rise to the ambient temperature.
• • Mirrors Saturn PCB Toolkit and fab-house tables: Same equation form as Saturn PCB Toolkit and most fab-house trace width calculators.
• • Teaches the geometry trade-offs: Shows cross-section next to ampacity, so the next move is obvious from the output table.

The biggest benefit is iteration speed. A 30-second pcb trace current calculator run tells you whether the planned trace width meets the load current and voltage-drop budget.

For signal integrity on the same trace, baud rate calculator converts between baud, bit rate, and symbol rate so the trace's edge rate is compared against the calculator's ampacity and length.

Five factors decide whether the calculator number matches a thermal probe on a real board, and two limitations tell you when to reach for a thermal simulation.

• • The IPC-2221 chart-fit was derived from 1 oz and 2 oz traces between about 0.2 mm and 10 mm wide. For 0.5 oz flex or 4 oz heavy copper, expect 5 to 15 % error versus thermal probe data.
• • The calculator models a single straight trace on a uniform reference plane. Stripline, microstrip, or differential pairs need a thermal simulation instead.

Most disagreement between the calculator and a thermal probe comes from copper plating and soldermask. Trust the measured coupon above 5 % disagreement.

According to Omni Calculator, the calculator pairs the IPC chart-fit ampacity model with Ohm's law to derive the trace resistance, voltage drop, and power dissipation at the rated current.