Free Transformer calculator
Enter the primary voltage, the secondary voltage or turns ratio, and the load (current or VA), and this calculator returns the turns ratio, secondary and primary current, and the VA rating of an ideal transformer — n = Vp/Vs, Vp·Ip = Vs·Is — updated live, as you type.
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Ideal-transformer estimates from the values you enter. Real units lose 2–5% to core and winding losses.
Results are estimates. Consult a professional.
How the transformer calculator works
A transformer is two coils wound on a shared magnetic core. An alternating current in the primary coil makes a changing magnetic field, and by Faraday's law of induction that changing field induces a voltage in the secondary coil. Because both coils link the same field, the voltage each one produces is set by how many turns it has. This calculator takes your primary voltage, a way of describing the secondary (its voltage or the turns ratio), and the load on the secondary (a current or a power), then returns the turns ratio, the secondary voltage, both currents, and the apparent power.
The turns-ratio relationships explained
Everything an ideal transformer does follows from a single number: the turns ratio. Read it once and you can predict every voltage and current in the circuit.
Voltage follows the turns directly
The secondary voltage is the primary voltage divided by the turns ratio: Vs = Vp / n. A transformer with more turns on the primary than the secondary (n greater than 1) lowers the voltage — a step-down transformer. More turns on the secondary (n less than 1) raises it — a step-up transformer. A 1:1 transformer leaves the voltage unchanged and is used purely for isolation.
Current runs the opposite way
Current transforms inversely: Is / Ip = n, so a step-down transformer that halves the voltage doubles the available current, and a step-up transformer that doubles the voltage halves the current. The side with the higher voltage always carries the lower current.
Apparent power is the same on both sides
Multiply voltage by current on either winding and you get the same apparent power: Vp × Ip = Vs × Is. That product, measured in volt-amperes (VA), is how transformers are rated. A "60 VA" transformer can supply 60 VA whether that is 12 V at 5 A or 24 V at 2.5 A.
A worked example using the transformer calculator
Sam is powering a 12 V lighting circuit from a 240 V mains supply with an ideal transformer. The lights draw 60 VA. Sam wants the turns ratio, the secondary and primary currents, and a check that the transformer is sized correctly.
Step 1 — Find the turns ratio
n = Vp / Vs = 240 ÷ 12 = 20. That is a 20:1 step-down transformer: 20 primary turns for every 1 secondary turn.
Step 2 — Find the secondary current
The load's apparent power and the secondary voltage give the secondary current: Is = S / Vs = 60 ÷ 12 = 5 A.
Step 3 — Find the primary current
Power is conserved, so the primary draws the same 60 VA at the higher voltage: Ip = S / Vp = 60 ÷ 240 = 0.25 A. Check: Vp × Ip = 240 × 0.25 = 60 VA = 12 × 5 = Vs × Is. ✓
Why current is inversely proportional to voltage
The most useful fact about a transformer is that voltage and current move in opposite directions. Step the voltage up and the current must come down; step it down and the current goes up. The reason is conservation of energy.
An ideal transformer creates no energy — it only moves it from one coil to the other. The power flowing in must equal the power flowing out, so Vp × Ip = Vs × Is. If Vs is smaller than Vp, then Is must be larger than Ip by exactly the same factor to keep the product equal. That factor is the turns ratio.
Step-up vs step-down quick-reference table
These common transformer ratios show how the same load power produces very different currents on each side. Every value here comes straight from n = Vp/Vs and Vp·Ip = Vs·Is.
| Primary Vp | Secondary Vs | Turns ratio n | Type | Load | Is | Ip |
|---|---|---|---|---|---|---|
| 240 V | 12 V | 20 : 1 | Step-down | 60 VA | 5 A | 0.25 A |
| 240 V | 24 V | 10 : 1 | Step-down | 120 VA | 5 A | 0.5 A |
| 120 V | 5 V | 24 : 1 | Step-down | 10 VA | 2 A | 0.083 A |
| 120 V | 1200 V | 1 : 10 | Step-up | 50 VA | 0.042 A | 0.417 A |
| 12 V | 12 V | 1 : 1 | Isolation | 36 VA | 3 A | 3 A |
n = Vp/Vs; Is = S/Vs; Ip = S/Vp. A ratio above 1 steps voltage down (and current up); below 1 steps voltage up (and current down). A 1:1 transformer changes neither — it isolates the two circuits.
Where transformers and the turns ratio are used
Transformers are everywhere AC power needs its voltage changed or two circuits need to be electrically isolated while still passing power.
- Power distribution — step-up transformers raise generator voltage to hundreds of kilovolts for low-loss transmission; step-down transformers bring it back to 120/240 V at your home.
- Power adapters and chargers — a small step-down transformer (or its switch-mode equivalent) drops mains voltage to the 5–24 V that electronics need.
- Audio and signal coupling — transformers match impedances and pass a signal while blocking DC, using the turns ratio to set the impedance ratio (n²).
- Isolation — a 1:1 transformer passes power but breaks the direct electrical connection, protecting people and equipment.
- Microwaves, welders and neon signs — high-ratio step-up transformers generate the thousands of volts these appliances need.
Once you know a winding's voltage and current, Ohm's law gives the resistance and power, and the voltage divider calculator covers scaling voltages without a transformer.
Real transformers vs the ideal model
This calculator models an ideal transformer: no losses, perfect coupling, power in exactly equal to power out. Real transformers come close but never quite reach it. Typical mains and power transformers run at about 95–98% efficiency, and small or cheap units can be lower.
- Copper losses (I²R) — the windings have real resistance, so some power is lost as heat that rises with the square of the current.
- Core losses — hysteresis and eddy currents in the iron core dissipate power even with no load, which is why a plugged-in transformer is slightly warm.
- Leakage and magnetizing current — not all the magnetic field links both coils, and the core itself draws a small magnetizing current, so the real current ratio is not perfectly inverse.
- Voltage regulation — under load the secondary voltage sags a few percent below the no-load value because of the winding resistance.
Common mistakes with transformer calculations
- Flipping the ratio — n = Np/Ns = Vp/Vs. A 20:1 transformer has the larger number on the primary. Writing it upside down turns a step-down into a step-up.
- Assuming the current ratio matches the voltage ratio — current goes the other way. The high-voltage side carries the low current, not the high one.
- Confusing VA with watts — transformers are rated in apparent power (VA). Only with a purely resistive load (power factor 1) does VA equal watts; with reactive loads the real power in watts is lower.
- Expecting a transformer to change DC — transformers need a changing magnetic field. They work on AC only; a steady DC voltage induces nothing in the secondary.
- Ignoring efficiency and headroom — sizing a transformer for exactly the load VA leaves no margin for losses or surge. Choose a rating above the load.
How accurate is this transformer calculator
The arithmetic is exact for an ideal transformer. For the values you enter, n = Vp/Vs, Is = S/Vs and Ip = S/Vp are computed to full floating-point precision, and the apparent power is identical on both sides by construction (Vp·Ip = Vs·Is).
Real transformers deviate for physical reasons, not arithmetic ones. Copper and core losses make a real unit 95–98% efficient, so the true primary current is a few percent higher and the loaded secondary voltage a few percent lower than the ideal figures. Treat the results as the design target, then add a VA margin and confirm against the transformer's datasheet. For the underlying theory, see the Wikipedia "Transformer" article and any standard electrical-engineering text.
Frequently asked questions about the free Transformer calculator
About this Transformer calculator
This calculator runs entirely in your browser — nothing you enter is sent anywhere. It applies the standard ideal-transformer equations n = Vp/Vs = Is/Ip and Vp·Ip = Vs·Is, working from your primary voltage, a secondary definition (voltage or turns ratio), and a load (current or VA) to return the turns ratio, both currents, and the apparent power.
Built for students, hobbyists and electricians sizing transformers and power supplies. Browse more tools in electronics calculators or see the full calculator library.