Transformer Calculator (Turns Ratio, Voltage and Current)
Choose what to solve for, then enter the turns ratio (primary turns divided by secondary turns) and the known voltages and currents. Set the efficiency to 100 % for an ideal transformer, or lower to see losses. All voltages and currents are shown, with power in, power out and power lost.
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Formula
- Turns ratio:
n = Np / Ns = Vp / Vs - Secondary voltage:
Vs = Vp / n, and primary voltageVp = Vs × n - Currents:
Is = efficiency × n × Ip(ideal transformer: efficiency = 1), soIp = Is / (efficiency × n) - Power:
Pin = Vp × Ip,Pout = Vs × Is,loss = Pin − Pout
Voltage and current are linked to the turns ratio in opposite ways, so a step-down transformer (n > 1) raises the current.
Worked example
n = 10, Vp = 230 V, Ip = 0.5 A, ideal: Vs = 230 / 10 = 23 V, Is = 10 × 0.5 = 5 A, Pin = Pout = 115 W.
At 95 % efficiency: Is = 0.95 × 10 × 0.5 = 4.75 A, Pout = 23 × 4.75 = 109.25 W, loss 5.75 W.
Frequently asked questions
How do you calculate the secondary voltage of a transformer?
Divide the primary voltage by the turns ratio Np : Ns. A 230 V primary on a 10 : 1 transformer gives 230 ÷ 10 = 23 V on the secondary.
How does current change across a transformer?
Voltage and current move in opposite directions. An ideal transformer keeps power constant, so the secondary current is the turns ratio times the primary current: 10 × 0.5 A = 5 A on a 10 : 1 step-down, at 23 V and 115 W either side.
How do I find the turns ratio from the voltages?
Divide the primary voltage by the secondary voltage. A 240 V to 12 V transformer has a ratio of 240 ÷ 12 = 20, so 20 primary turns for every secondary turn.
What does the efficiency setting do?
Real transformers lose some power as heat in the windings and core, so output power is the efficiency times input power. At 95 %, 115 W in gives 109.25 W out (4.75 A at 23 V) and 5.75 W is lost.
Can I use this for safety or final design sign-off?
No. This calculator is for learning and design checks, not safety certification. It ignores regulation, saturation, inrush current and frequency limits, so check the transformer's datasheet and rated VA, and follow local electrical codes.