How do you calculate transformer turns ratio?

Turns Ratio = N_primary ÷ N_secondary = V_primary ÷ V_secondary = I_secondary ÷ I_primary. Example: 120V primary, 12V secondary: ratio = 120/12 = 10:1 (step-down). The turns ratio determines voltage transformation — a 10:1 transformer with 120V input produces 12V output.

How do you calculate secondary voltage?

V_secondary = V_primary × (N_secondary ÷ N_primary). Or: V_secondary = V_primary ÷ Turns Ratio. Example: 240V input with 2:1 ratio = 120V output. For step-up: V_secondary = V_primary × step-up ratio.

How does transformer current relate to voltage?

Current is inversely proportional to voltage: I_secondary = I_primary × (N_primary ÷ N_secondary). A step-down transformer that halves voltage doubles current (and vice versa). Power is conserved: V1 × I1 = V2 × I2 (ideal). Example: 120V/10A primary = 12V/100A secondary (1:10 step-down).

How do I size a transformer?

Transformer VA rating = V_secondary × I_secondary (for single phase) or √3 × V_line × I_line (for three phase). Add 20-25% safety margin. Example: 24V output at 10A = 240VA → use 300VA transformer. For inductive loads (motors), size for starting current (3-6× running current). Always size for the maximum expected load.

Transformer Calculator — Free Turns Ratio, Voltage & Current Tool | AllInOneTools

⚡ Electrical & Energy

Transformer Calculator

Calculate turns ratio, secondary voltage, secondary current, and power for step-up and step-down transformers. Includes impedance matching and efficiency calculations.

Primary

120V

⚡→

Secondary

12V

10A

Transformer Parameters

Primary Voltage

Secondary Voltage

Primary Current (or leave 0)

Efficiency

Turns (Optional)

Primary Turns (Np)

turns

Secondary Turns (Ns)

turns

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Transformer Properties

🔌

Power Analysis

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Related Electrical & Energy Tools

Transformer Calculator: How to Calculate Turns Ratio, Voltage, Current, and Power

Transformers are electromagnetic devices that transfer electrical energy between circuits through magnetic coupling. They are fundamental to the power grid (stepping voltage up for transmission, down for distribution), power supplies (converting mains voltage to device voltages), audio equipment (impedance matching), and countless other applications. Understanding transformer calculations is essential for electrical engineers, technicians, and hobbyists.

Core Transformer Formulas

Turns Ratio: a = Np/Ns = Vp/Vs = Is/Ip
Secondary Voltage: Vs = Vp × (Ns/Np)
Secondary Current: Is = Ip × (Np/Ns) × Efficiency
Power: Pp = Vp × Ip, Ps = Vs × Is
VA Rating: VA = Vs × Is (secondary)
Impedance: Zp = Zs × (Np/Ns)²

Worked Example — 120V to 12V Step-Down

Primary: 120V, 1A → Pp = 120W
Turns ratio: 120/12 = 10:1
Ideal Is = 1A × 10 = 10A
At 95% efficiency: Is = 10 × 0.95 = 9.5A
Output power: 12 × 9.5 = 114W
Losses: 120 − 114 = 6W (heat)

Transformer Types

Type	Ratio	Common Use	Efficiency
Step-Down	>1:1	Power supplies, chargers	90–98%
Step-Up	<1:1	Power transmission, inverters	95–99%
Isolation	1:1	Safety, noise isolation	95–98%
Auto	Variable	Voltage regulation, variacs	97–99%
Current (CT)	High ratio	Current measurement, protection	99%+
Audio Output	Various	Impedance matching, tube amps	85–95%

Impedance Matching

Transformers reflect impedance across their windings according to the square of the turns ratio: Z_reflected = Z_load × (Np/Ns)². This is critical in audio (matching speakers to amplifiers), RF circuits (antenna matching), and power systems. Example: matching an 8Ω speaker to a 5,000Ω tube output stage requires a ratio of √(5000/8) = 25:1.

Frequently Asked Questions

How do you calculate turns ratio?▼

Turns Ratio = Np/Ns = Vp/Vs. Example: 120V to 12V = 10:1 step-down. A 10:1 transformer divides voltage by 10 and multiplies current by 10 (minus losses). The same transformer works in reverse as 1:10 step-up.

How to calculate secondary voltage?▼

Vs = Vp × (Ns/Np) or Vs = Vp ÷ Turns Ratio. Example: 240V with 2:1 ratio = 120V out. For step-up: multiply primary voltage by the step-up factor.

How does current relate to voltage in transformers?▼

Inversely proportional: Is = Ip × (Np/Ns). Step-down halves voltage → doubles current. Power is conserved: Vp×Ip = Vs×Is (ideal). At 95% efficiency, output power = 0.95 × input power.

What is transformer efficiency?▼

η = (Pout/Pin) × 100%. Small transformers: 90-95%. Large power: 97-99%+. Losses: core losses (constant, hysteresis + eddy currents) and copper losses (I²R, load-dependent). Max efficiency when core loss = copper loss.

How to size a transformer?▼

VA = Vs × Is for single-phase. Add 20-25% margin. Example: 24V at 10A = 240VA → use 300VA. For inductive loads, size for starting current (3-6× running). Always size for maximum expected load.

What is impedance matching?▼

Z_reflected = Z_load × (Np/Ns)². Transfers maximum power when source and load impedances match. Audio example: 8Ω speaker to 5kΩ tube amp needs √(5000/8) = 25:1 ratio. Critical in RF, audio, and telecom.

Step-up vs step-down transformer?▼

Step-down: Vs < Vp (more primary turns). Power supplies, chargers. Step-up: Vs > Vp (more secondary turns). Power transmission, ignition coils. Same transformer works both ways — just swap which side is input.