This transformer calculator applies the turns-ratio law Vs/Vp = Ns/Np (and its current mirror Is/Ip = Np/Ns) to find secondary voltage, current or turns — for ideal transformers stepping AC up or down.
| Turns ratio Ns:Np | – |
| Secondary voltage Vs | – |
| Secondary current Is (ideal) | – |
| Power in → out | – |
| Type | – |
Turns Are the Exchange Rate Between Voltage and Current
The power being exchanged is our Electric Power Calculator‘s subject. Part of our Physics Calculators collection.
Worked Example (Verified)
- Doorbell/phone-charger classic (defaults): Np 1200, Ns 100, Vp 120 V → Vs = 120 × 100/1200 = 10 V, and Ip 2 A → Is = 24 A ideal — voltage ÷12, current ×12, power conserved (240 W both sides before losses)
- Grid arithmetic: the pole transformer outside a house does ~7,200 V → 240 V (30:1); the substation before it and the generator step-up before that form a chain of ratios multiplying to millions-to-one from turbine to toaster
- Why DC won’t do: transformers need CHANGING flux (Faraday) — steady DC makes a one-time blip, then the primary is just a low-resistance short. AC won the grid partly on this one equation
Transformer Discipline
- Ideal math conserves power: Vp·Ip = Vs·Is. Real units run 95–99% (large) down to ~80% (small wall adapters) — the efficiency field adjusts output power, not the ratio law.
- Current capability is wire-gauge business: the high-current side needs thick windings; swapping primary/secondary roles on a real unit can overheat the thin side.
- Turns must be integers on real cores, and volts-per-turn is set by core flux limits (saturation) — design starts from the core, not the ratio alone.
- Isolation transformers (1:1) change nothing numerically and everything electrically: safety through galvanic separation.
Related Tools
- Electric Power Calculator — P = VI in depth
- Magnetic Force Calculator — the field at work
- Physics Calculators — the full collection
Frequently Asked Questions
How do you calculate transformer secondary voltage?
Vs = Vp × (Ns/Np). With 120 V in and a 100:1200 turns count: 120 × 100/1200 = 10 V out. The same ratio inverts for current: Is = Ip × (Np/Ns). This calculator reports both plus power at your efficiency.
What is the turns ratio?
Ns:Np — secondary turns over primary turns. Below 1 (like 100:1200 = 0.083) steps voltage down; above 1 steps it up. Every other transformer quantity is this one ratio wearing different clothes.
Why can’t transformers create power?
Conservation of energy: stepping voltage up by the turns ratio steps current down by the same ratio, so V × I is invariant (minus real losses). A “step-up” transformer trades current for voltage — the grid’s reason for existing: high voltage = low current = low I²R line losses.
Why do transformers need AC?
Induction requires changing magnetic flux (Faraday’s law: EMF ∝ dΦ/dt). AC’s flux oscillates continuously; DC’s flux is static after switch-on, inducing nothing — and the primary winding’s low DC resistance then draws destructive current. (Switch-mode supplies chop DC into high-frequency AC first, then transform.)
How efficient are real transformers?
Very: ~95–99% for distribution and power units, 80–95% for small adapters. Losses are copper I²R heating and core losses (eddy currents — fought with laminated cores — and hysteresis). The calculator’s efficiency field applies your figure to output power.
Is this transformer calculator free?
Yes — free, instant, no sign-up.