Enter values and press Calculate.
A winding's wire must be thick enough to carry its RMS current without overheating, and thin enough to fit the available window. The design target is a current density J (amps per mm² of copper) that keeps resistive heating manageable: A = Irms/J. From that cross-sectional area you can look up the nearest standard AWG (or SWG/metric) wire size. Once the wire is chosen, its resistance and the resulting copper loss can be estimated from the total wire length (turns × mean length per turn).
| Quantity | Formula |
|---|---|
| Required copper area | A = Irms / J |
| Wire diameter (round) | d = √(4A/π) |
| Total wire length | L = N × MLT |
| Winding resistance | R = ρ × L / A |
| Copper loss | Pcu = Irms² × R |
Copper resistivity ρ is about 1.72×10⁻⁸ Ω·m at 20 °C and rises roughly 0.4%/°C, which this calculator accounts for. Typical current densities range from about 2–3 A/mm² for large, natural-convection transformers to 5–8 A/mm² for small, well-cooled high-frequency windings.
Divide the RMS winding current by your target current density: A = Irms/J. This gives the required copper cross-sectional area, which you then match to the nearest standard AWG, SWG or metric wire gauge.
Typical values are 2–3 A/mm² for large, naturally-cooled transformers, and 4–8 A/mm² for small, well-ventilated or forced-air-cooled windings. Higher density means thinner wire but more heating.
For round wire, d = √(4A/π). A 0.5 mm² area corresponds to about 0.80 mm bare copper diameter, before adding enamel insulation thickness.
R = ρ×L/A, where ρ is copper resistivity (about 1.72×10⁻⁸ Ω·m at 20 °C), L is the total wire length (turns × mean length per turn), and A is the cross-sectional area.
It is the average length of wire used for one turn around the core, depending on the core's window dimensions and bobbin shape. It is usually taken from the core/bobbin datasheet or measured on an existing sample winding.
Yes. Copper resistance rises about 0.39–0.40% per °C above 20 °C, so a winding running hot has noticeably higher resistance (and copper loss) than at room temperature.
Copper loss is the I²R heating in the winding. It directly determines the winding's temperature rise, so keeping it within budget (via wire size and current density) is essential to avoid insulation damage.
At high frequency, skin and proximity effects make current crowd toward the wire surface, so several thinner, insulated strands (litz wire) in parallel can carry the same current with less effective resistance than one thick wire.
Compare your calculated area (in mm² or circular mils) to a standard AWG table and pick the nearest gauge with equal or greater area. Common transformer wires range from about AWG 40 (very fine) to AWG 10 (heavy).
Yes. The bare conductor area sets the resistance and current capacity, but the insulated (enamelled) outer diameter is what determines how many turns fit in the core window — always check the insulated OD in the wire's data sheet.
Either choose a larger core (more window area, see the Core Selection calculator), increase the allowed current density, or split the winding into parallel strands to fit the available space.
It is a good DC-resistance estimate for lower frequencies. At high switching frequencies, skin and proximity effects raise the AC resistance above this DC value, so add margin or use an AC-resistance correction for SMPS designs.
Core Selection (Area Product) • Turns (Faraday's Law) • Wire Gauge / Ampacity • All Calculators