Transformer Calculator
Calculate turns ratio, voltage, and current for transformers.
Reviewed by the ToolNestr Editorial Team — July 2026
Enter any 3 values to solve for the 4th.
How the transformer formula works
The fundamental relationship governing an ideal transformer is expressed by the equation Np / Ns = Vp / Vs = Is / Ip. This means the turns ratio directly determines both the voltage ratio and the inverse current ratio. If you know any three of these four quantities, you can solve for the remaining one.
For an ideal transformer, power is conserved: Vp × Ip = Vs × Is. A step-down transformer reduces voltage and increases current by the same factor. A step-up transformer does the opposite.
Worked example
Transformer Design
Engineers use the turns ratio to design transformers for power distribution, ensuring the correct voltage step-up or step-down for each stage of the electrical network.
Power Supply Repair
Technicians can verify transformer windings by measuring voltages and comparing the calculated ratio against the rated specification during power supply troubleshooting.
Audio Impedance Matching
Audio transformers match impedance between amplifier outputs and speakers or microphones, using the turns ratio to transform impedance by the square of the ratio.
Power Conversion
Switch-mode power supplies and flyback converters rely on the turns ratio to regulate output voltage across varying input and load conditions in modern electronics.
Tips for transformer calculations
Ideal transformer model
This calculator uses the ideal transformer model, which assumes 100% efficiency, no core losses, no copper losses, and perfect magnetic coupling. The ideal model is accurate enough for most design and verification purposes, especially at low power levels where losses are relatively small compared to the throughput.
Real-world efficiency
Real transformers achieve 95–99% efficiency depending on size, core material, and design. Large power distribution transformers operate at 98–99%, while small signal transformers may be only 70–80% efficient. Losses include copper loss (I²R in windings), core loss (hysteresis and eddy currents), and stray leakage losses.
Power must balance
For an ideal transformer, the apparent power on the primary side equals the apparent power on the secondary side: Vp × Ip = Vs × Is. This power balance is the foundation of all transformer calculations and provides a useful sanity check for your results.
Check your ratio direction
A ratio greater than 1 (Np > Ns) means step-down: voltage decreases and current increases. A ratio less than 1 means step-up: voltage increases and current decreases. Always confirm the direction matches your application before specifying a transformer.
What is a transformer?
A transformer is a passive electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. It consists of two or more coils of wire (windings) wrapped around a common magnetic core. When alternating current flows through the primary winding, it creates a time-varying magnetic field in the core, which induces a voltage in the secondary winding. The ratio of input to output voltage is determined solely by the ratio of turns in the windings.
Transformers are essential to modern power systems. They allow electricity to be transmitted at high voltages (reducing line losses over long distances) and then stepped down to safe usable voltages for homes and businesses. Without transformers, long-distance electrical power distribution as we know it would not be feasible. They are also used extensively in electronics for power conversion, signal coupling, and impedance matching.
The physical size of a transformer is determined by its power rating (in VA or kVA) and operating frequency. Power transformers operating at 50/60 Hz require large iron cores, while high-frequency transformers in switch-mode supplies can be much smaller using ferrite cores. Audio transformers span the range from tiny microphone matching transformers to large output transformers in tube amplifiers.
Step-up vs step-down transformers
A step-down transformer has more primary turns than secondary turns (Np > Ns), producing a lower secondary voltage. These are commonly used in power distribution to reduce transmission voltages to safe levels for end users. A step-up transformer has fewer primary turns than secondary turns (Np < Ns), producing a higher secondary voltage. Step-up transformers are used in power generation to raise voltage for efficient transmission and in applications like neon sign transformers and ignition systems.
The current direction is always opposite to the voltage direction. When voltage steps down, current steps up by the same factor, keeping power constant. For example, a transformer with a 10:1 step-down ratio reduces voltage by a factor of 10 but increases available current by a factor of 10. This is why high-voltage transmission lines carry relatively low current: the same power requires less current at higher voltage, reducing I²R losses in the conductors.
Core construction and materials
Transformer cores are constructed from materials with high magnetic permeability to confine and guide the magnetic flux. The most common core types are: laminated silicon steel (for mains-frequency power transformers, typically 0.35–0.5 mm thick laminations insulated from each other to reduce eddy current losses), ferrite (for high-frequency applications such as switch-mode power supplies and RF transformers), amorphous metal (for ultra-efficient distribution transformers with very low core losses), and powdered iron (for intermediate frequency ranges).
The three main core geometries are core-type (windings surround the core legs), shell-type (the core surrounds the windings), and toroidal (a donut-shaped core with windings wound through the centre). Toroidal cores produce less external magnetic field and are more compact, making them ideal for audio equipment, medical devices, and sensitive instrumentation where stray magnetic fields must be minimised.
Impedance transformation
An important property of transformers is impedance transformation. The impedance seen at the primary of a transformer is related to the impedance connected to the secondary by the square of the turns ratio: Zp = (Np/Ns)² × Zs. This allows transformers to match a high-impedance source to a low-impedance load, or vice versa, maximising power transfer. Audio output transformers, for example, match the high impedance of a vacuum tube amplifier to the low impedance of a loudspeaker.
This relationship also applies to transmission line transformers, baluns (balanced-to-unbalanced converters), and impedance-matching networks in RF systems. The ability to transform impedance is one of the most versatile features of transformers and extends their use far beyond simple voltage conversion.
Real transformer losses
While this calculator assumes an ideal transformer, real transformers experience several types of losses: copper losses (I²R losses in the winding resistance, proportional to the square of the load current), core losses (hysteresis losses from the magnetic domains reversing direction each cycle, plus eddy current losses induced in the core material), leakage flux (magnetic field lines that do not link both windings, causing inductive voltage drop), and stray losses (eddy currents induced in the tank walls and structural parts of large transformers).
Efficiency varies with load. Most transformers achieve peak efficiency near full load. At light loads, core losses dominate; at heavy loads, copper losses dominate. Power transformers are typically designed to have minimum total loss at around 75–100% of rated load. The efficiency of a well-designed large power transformer can exceed 99%, making transformers among the most efficient electrical machines ever built.
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Frequently asked questions
What is turns ratio?
The ratio of primary winding turns to secondary winding turns. Np/Ns = Vp/Vs = Is/Ip.
Does this work for step-up and step-down?
Yes — enter the values you know and it solves for the unknown, regardless of direction.
What about real-world losses?
This calculator assumes an ideal transformer. Real transformers have core losses and copper losses.
What is a center-tapped transformer?
A transformer with a tap at the centre of the secondary winding, providing two equal voltages.
Sources & references
This tool uses standard formulas and reference values from:
- • NFPA 70 — National Electrical Code (NEC), conduit fill, box fill and conductor ampacity. nfpa.org
- • IPC-2221, Generic Standard on Printed Board Design (trace width / current). ipc.org
- • NIST reference constants and unit definitions; IEEE standards where applicable. nist.gov
For educational and preliminary use. Verify against full engineering calculations and the governing standard before any real design.