Transformer Turns Ratio Calculator
Voltage, current and turns for a transformer.
Reviewed by the Calculation Machine team · Updated 24 October 2026 · How we check our maths
Your values
ⓘ Live results
- Turns ratio
- 20:1
- Secondary turns
- 40
- Primary current
- 0.15 A
- Power
- 36 VA
Quick answer
A transformer's turns ratio equals its voltage ratio: Np ÷ Ns = Vp ÷ Vs, while current goes the opposite way: Ip ÷ Is = Ns ÷ Np. A 240 V to 12 V transformer has a 20:1 ratio, so an 800-turn primary needs 40 secondary turns, and a 3 A secondary load draws about 0.15 A from the primary in an ideal transformer.
What the Transformer Turns Ratio calculator does
This calculator relates the primary and secondary sides of a transformer. From the two voltages it gives the turns ratio, then uses the primary turn count to find the secondary turns, and the secondary current to find the primary current and the load in volt-amperes (VA). It also serves anyone searching for a transformer winding ratio.
It is useful when rewinding or building a small transformer, checking whether a control or doorbell transformer is large enough for its load, or understanding how step-up and step-down ratios work in coursework.
The results assume an ideal transformer: no losses, perfect magnetic coupling and no voltage drop under load. Real units lose a few percent to winding resistance and core losses, so primary current is slightly higher than shown. For the cable feeding the transformer, the wire voltage drop calculator checks the run.
The formula
a = Vp ÷ Vs = Np ÷ Ns = Is ÷ Ip Ns = Np ÷ a Ip = Is ÷ a S = Vs × Is
| Symbol | Meaning | Unit |
|---|---|---|
| a | Turns ratio (shown as a:1) | — |
| Vp, Vs | Primary and secondary voltage (RMS) | V |
| Np, Ns | Number of turns on the primary and secondary windings | turns |
| Ip, Is | Primary and secondary current (RMS) | A |
| S | Apparent power delivered to the load | VA |
These are the ideal transformer equations from Faraday's law: each turn on a shared core carries the same volts per turn, so voltage scales with turns, and power in equals power out, so current scales inversely. A ratio below 1 (shown as, for example, 0.1:1) is a step-up transformer, equivalent to 1:10. Impedance transforms by the square of the ratio: a 20:1 transformer makes an 8 Ω load look like 3,200 Ω from the primary.
Worked examples
Step-down for a 12 V lighting circuit
A 240 V to 12 V transformer with an 800-turn primary supplies 3 A of low-voltage lighting.
Inputs
- Primary voltage240 V
- Secondary voltage12 V
- Primary turns800
- Secondary current3 A
Result
- Turns ratio20:1
- Secondary turns40
- Primary current0.15 A
- Power36 VA
The 20:1 ratio needs 40 secondary turns and the load is 36 VA. Choose a transformer rated comfortably above 36 VA, such as 50 VA, to allow for losses and inrush.
North American 24 V control transformer
A 120 V to 24 V HVAC control transformer with a 500-turn primary feeds a 2 A thermostat and valve load.
Inputs
- Primary voltage120 V
- Secondary voltage24 V
- Primary turns500
- Secondary current2 A
Result
- Turns ratio5:1
- Secondary turns100
- Primary current0.4 A
- Power48 VA
A 5:1 ratio, 100 secondary turns and a 48 VA load. Common furnace transformers are rated 40 VA, so this load would overload one; a 50 VA or larger unit is needed.
Step-up transformer
A 230 V supply is stepped up to 2,300 V for a test rig, with a 200-turn primary and 0.5 A on the secondary.
Inputs
- Primary voltage230 V
- Secondary voltage2300 V
- Primary turns200
- Secondary current0.5 A
Result
- Turns ratio0.1:1
- Secondary turns2,000
- Primary current5 A
- Power1150 VA
The ratio shows as 0.1:1 (a 1:10 step-up) with 2,000 secondary turns. The primary carries ten times the secondary current, 5 A, for a 1,150 VA load.
Typical transformer ratios
Turns ratios for some everyday transformers, from nominal voltages.
| Application | Primary → secondary | Turns ratio |
|---|---|---|
| UK/EU bell or lighting transformer | 230 V → 12 V | 19.2:1 |
| US HVAC control transformer | 120 V → 24 V | 5:1 |
| US distribution (pole) transformer | 7,200 V → 240 V | 30:1 |
| US commercial step-down | 480 V → 208 V | 2.31:1 |
| Isolation transformer | 230 V → 230 V | 1:1 |
| Audio output (example) | 20:1 to an 8 Ω speaker | 20:1 (3,200 Ω reflected) |
How real transformers differ from the ideal result
- Losses: small transformers are typically 80–95% efficient and large distribution units above 98%. Divide the ideal primary current by efficiency for a closer estimate.
- Regulation: a small transformer's secondary voltage is noticeably higher with no load than at rated load, so builders often add a few percent more secondary turns than the ideal ratio suggests.
- Magnetizing current: the primary draws some current even with nothing connected.
- Ratio tolerance: IEEE C57.12.00 requires the measured turns ratio of power and distribution transformers to be within 0.5% of the nameplate, which is what a turns-ratio (TTR) test checks.
Sizing a transformer by VA
Transformers are rated in volt-amperes because their heating depends on current, not on the load's power factor. Add up the VA of everything on the secondary — for resistive loads VA equals watts — and pick the next standard size up, with headroom for motors, solenoids or lamps with high inrush current. For three-phase units, total VA is √3 × line voltage × line current; the three-phase power calculator handles that case, and the wire gauge calculator helps size the secondary wiring for the current.
How to use the Transformer Turns Ratio Calculator
- 1Enter the primary (input) voltage from the supply or the transformer nameplate.
- 2Enter the secondary (output) voltage you need; enter a higher figure than the primary for a step-up transformer.
- 3Enter the number of primary turns if you are designing or rewinding; otherwise any value still gives the correct ratio.
- 4Enter the secondary load current to see the primary current and VA load, then size the transformer above that VA.
Frequently asked questions
How do you calculate a transformer turns ratio?
Divide primary voltage by secondary voltage, or primary turns by secondary turns — they are equal in an ideal transformer. 240 V ÷ 12 V gives 20:1.
How many secondary turns do I need?
Secondary turns = primary turns × secondary voltage ÷ primary voltage. For 800 primary turns, 240 V in and 12 V out, that is 40 turns, plus a few percent to cover load regulation.
Does a step-down transformer increase current?
Yes. Stepping voltage down by 20:1 lets the secondary supply up to 20 times the primary current, since power in roughly equals power out.
What does a 10:1 turns ratio mean?
The primary has 10 turns for every secondary turn, so 120 V in gives 12 V out and a 1 A secondary load draws about 0.1 A from the primary.
Can I run a transformer backwards to step up voltage?
Often yes for small isolation and control transformers, but the output voltage will be a little low because the turns were set for the original direction, and some designs are not intended for reverse use. Check the manufacturer's guidance first.
References
- IEEE C57.12.00: General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers — Institute of Electrical and Electronics Engineers (IEEE)
- IEEE C57.12.90: Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers — Institute of Electrical and Electronics Engineers (IEEE)
- Fundamentals of Electric Circuits, Alexander and Sadiku — McGraw-Hill
About this calculator
- Built and reviewed by the Calculation Machine team · How we build and test calculators
- Last reviewed: October 24, 2026
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