Find the minimum PCB trace width for a given current with the IPC-2221 formula, or the maximum current a trace can carry. Choose copper weight, allowed temperature rise and internal or external layer, and get trace resistance, voltage drop and power loss for its length.
Last updated:
Calculate
A
°C
°C
For resistance, voltage drop and power loss
Enter your values to see the result.
Advertisement
Advertisement
How to use the PCB Trace Width Calculator
Choose the direction: width from current, or current from width.
Enter the current (or trace width) and pick the copper weight.
Set the allowed temperature rise; 10 °C is conservative, 20–30 °C is common for power traces.
Select external or internal layer; inner layers need much wider traces.
Optionally enter the length to get resistance, voltage drop and power loss.
IPC-2221 trace width formula
Step
Formula
Notes
Cross-section area
A = (I ÷ (k × ΔT^0.44))^(1/0.725)
A in mil², I in A, ΔT in °C
Constant k
0.048 external · 0.024 internal
From IPC-2221 curve fits
Copper thickness
t = oz × 1.378 mil
1 oz ≈ 35 µm
Trace width
W = A ÷ t
1 mil = 0.0254 mm
Resistance
R = ρL ÷ A
ρ = 1.724×10⁻⁸ Ω·m at 20 °C, +0.393%/°C
Quick reference: 1 oz copper, 10 °C rise
Current
External width
Internal width
0.5 A
0.12 mm (4.5 mil)
0.30 mm (11.8 mil)
1 A
0.30 mm (11.8 mil)
0.78 mm (30.8 mil)
2 A
0.78 mm (30.8 mil)
2.03 mm (80.0 mil)
3 A
1.37 mm (53.8 mil)
3.56 mm (140.0 mil)
5 A
2.77 mm (108.9 mil)
7.19 mm (283.2 mil)
These are minimum widths for heating only. Also consider voltage drop on long traces, fusing current during faults, manufacturing limits (often 0.1–0.15 mm minimum) and clearance between conductors at your working voltage.
Frequently asked questions
What formula does this PCB trace width calculator use?
The IPC-2221 (formerly IPC-D-275) formula: I = k × ΔT^0.44 × A^0.725, where I is current in amps, ΔT the temperature rise in °C, A the cross-section in square mils, and k = 0.048 for external layers or 0.024 for internal layers. Width = A ÷ (copper thickness in mils).
How wide should a trace be for 1 A?
With 1 oz copper on an outer layer and a 10 °C rise, IPC-2221 gives about 0.30 mm (11.8 mil). On an inner layer the same current needs roughly 0.78 mm (30.8 mil).
Why do internal traces need to be wider?
Inner-layer traces are surrounded by laminate, which conducts heat poorly, so they cannot shed heat to the air. IPC-2221 halves the k constant for internal layers, which makes them about 2.6 times wider for the same current and temperature rise.
What does copper weight (oz) mean?
It is the weight of copper spread over one square foot. 1 oz/ft² is about 35 µm (1.378 mil) thick, 2 oz is about 70 µm. Thicker copper carries more current at the same width.
What temperature rise should I choose?
10 °C is a common conservative default. Power traces are often designed for 20–30 °C if the board and parts tolerate it. The trace temperature is the ambient plus this rise, and it must stay well below the laminate's rating.
Is IPC-2221 still accurate?
It is a widely used, generally conservative rule of thumb based on old test data. The newer IPC-2152 standard accounts for board thickness, planes and other factors and often allows narrower traces for external layers. For critical designs, check IPC-2152 or thermal simulation.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.