Engineering
Transformer Calculator
Calculate turns ratio, required secondary turns, secondary current, input apparent power, primary current, and modeled loss. The custom two-coil diagram keeps winding, voltage, current, load, and loss quantities attached to the correct side.
Secondary full-load current (A)-
Primary-to-secondary turns ratio-
Required secondary turns-
Estimated input apparent power (VA)-
Estimated primary current (A)-
Modeled conversion loss (VA)-
Decision view
Transformer winding and load diagram
Transformer winding and load diagramPrimary and secondary coils carry their own voltage, current, turns, and apparent-power labels.
Exact scenario comparisonSecondary voltage (V) changes while all other entered assumptions remain constant.
| Secondary voltage (V) | Secondary full-load current (A) | Primary-to-secondary turns ratio | Required secondary turns | Estimated input apparent power (VA) | Estimated primary current (A) | Modeled conversion loss (VA) |
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How to use Transformer Calculator
- Enter RMS voltages, primary turns, and the load's apparent-power requirement using compatible AC assumptions.
- Check insulation, core flux, frequency, conductor ampacity, regulation, duty, and temperature rise outside this worksheet.
- Select a real transformer with appropriate ratings and approvals rather than sizing from arithmetic alone.
Calculator guide
Understanding Transformer Calculator
An ideal transformer relates voltage to winding turns, while load current is governed by apparent power. The efficiency input then connects secondary load to primary input and exposes modeled loss.
Calculation method
How the calculation works
Apply the ideal voltage-to-turns relationship, then adjust input apparent power and current using the entered efficiency. Set voltage ratio equal to turns ratio, divide load VA by secondary voltage for load current, divide load VA by efficiency for input VA, and divide input VA by primary voltage for primary current.
Electrical review
Checks required beyond the ideal winding ratio
A correct ratio is necessary but not sufficient for a safe transformer.
Worked situations
Practical examples
- 230 V primary and 24 V secondary produce a turns ratio of about 0.104 secondary-to-primary.
- With 1,000 primary turns, the proportional secondary winding is about 104 turns.
- A 240 VA secondary load at 24 V draws 10 A, before considering regulation and device-specific behavior.
Better inputs
Useful tips
- Use VA rather than watts when load power factor is not unity.
- Keep primary and secondary current labels distinct; a step-down transformer raises current capability.
- Allow engineering margin and check manufacturer curves for inrush and regulation.
Before relying on the result
Limitations and common mistakes
- This steady-state model does not design a magnetic core or winding.
- Frequency, waveform, saturation, leakage reactance, copper temperature, regulation, harmonics, inrush, insulation, grounding, protection, and certification are excluded.
- Efficiency is entered as one constant value.
Reference
Key terms
- Turns ratio
- Secondary winding turns divided by primary winding turns.
- Apparent power
- RMS voltage multiplied by RMS current, expressed in VA.
- Regulation
- Change in secondary voltage between load conditions.
- Modeled loss
- Difference between input VA and delivered load VA under the entered efficiency.
Important note
Calculated from the entered values using the displayed engineering relationship. Confirm design values, load cases, safety factors, standards, and field conditions with a qualified professional.
Frequently asked questions
Why is secondary current larger in a step-down transformer?
Approximately conserved apparent power means current rises as voltage falls.
Can watts be entered as VA?
Only when the relevant load power factor makes that appropriate.
Does the turn count include regulation compensation?
No. It follows the ideal voltage ratio.
Can this be used to wind a mains transformer?
Not by itself; mains equipment requires qualified magnetic, thermal, insulation, protection, and compliance design.