Transformer Turns Ratio CalculatorFree online other calculator — instant results, no sign-up
Estimate ideal secondary voltage from winding turns.
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About the Transformer Turns Ratio Calculator
The Transformer Turns Ratio Calculator estimates ideal secondary voltage from the primary voltage and the number of turns in each winding. It uses the secondary-to-primary turns ratio to show how a transformer steps voltage up or down. With a primary voltage of 120 V, 1000 primary turns, and 100 secondary turns, the estimated secondary voltage is 12 V.
- Primary voltage (V)
- The voltage applied across the primary winding, measured in volts (V), with a default of 120 V.
- Primary turns
- The number of wire turns in the primary winding, a dimensionless count with a default of 1000.
- Secondary turns
- The number of wire turns in the secondary winding, a dimensionless count with a default of 100.
Formula and method
V_s = V_p × (N_s / N_p) = 120 V × (100 / 1000) = 12 V
- V_sEstimated ideal secondary voltage, in volts (V).
- V_pApplied primary voltage, in volts (V).
- N_sNumber of turns in the secondary winding.
- N_pNumber of turns in the primary winding.
Divide the secondary turns by the primary turns to obtain the voltage multiplier, then multiply the primary voltage by that ratio. For the default inputs, 100 / 1000 = 0.1, so 120 V × 0.1 = 12 V. Primary turns must be greater than zero.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Primary voltage120 V
- Primary turns1000
- Secondary turns100
Results
- Result12 V
How to use the Transformer Turns Ratio Calculator
- 1Enter the primary voltage in volts; the default is 120 V.
- 2Enter the primary winding turn count; the default is 1000.
- 3Enter the secondary winding turn count; the default is 100.
- 4Calculate the ideal secondary voltage using V_s = V_p × (N_s / N_p).
- 5Read the result in volts: the default inputs give 12 V, indicating a step-down transformer.
Reading your result
The Result row reports the estimated ideal voltage across the secondary winding in volts. With the default inputs, Result = 12 V means the secondary voltage is one-tenth of the 120 V primary voltage. This is a voltage estimate, not a current or power rating. If the primary voltage is entered as an RMS value, interpret the secondary result as RMS as well.
Limitations
The calculation assumes an ideal transformer with perfect magnetic coupling and no winding resistance, leakage flux, or core losses. Actual secondary voltage can vary with load and transformer design. Use positive whole-number turn counts for physical windings; a zero primary turn count makes the formula undefined. The relation applies to suitable alternating-voltage operation, not steady DC, and does not assess frequency suitability, core saturation, insulation, temperature rise, or electrical safety. The result alone does not establish whether a transformer can safely supply a particular load.
Common questions
Why do the default inputs produce 12 V?+
The secondary has 100 turns compared with 1000 primary turns, giving a secondary-to-primary ratio of 0.1. Multiplying the 120 V primary voltage by 0.1 gives 12 V.
Which way around is the turns ratio used?+
This calculation uses secondary turns divided by primary turns, N_s / N_p. For the defaults, that multiplier is 0.1; the same windings can also be described as having a primary-to-secondary turns ratio of 10:1.
Can this calculator estimate a step-up transformer's voltage?+
Yes. When the secondary has more turns than the primary, N_s / N_p is greater than one, so the estimated secondary voltage exceeds the primary voltage.
Does a 12 V result tell me how much current the secondary can deliver?+
No. The result estimates voltage only. Available current depends on the transformer's power rating, winding construction, cooling, and operating conditions, none of which are inputs to this calculator.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.