Transformer Turns Ratio Calculator

Step-by-Step Guide

1. 1Enter Known Voltage Values: Input primary voltage, secondary voltage, or both, depending on what you know.
2. 2Enter Known Turns Values: Input primary turns, secondary turns, or both, if known from a specification or nameplate.
3. 3Provide at Least Two Known Values: The calculator needs both voltages, both turns counts, or one voltage plus both turns counts.
4. 4Read the Ratios: View turns ratio, voltage ratio, and current ratio in both decimal and x:1 format.
5. 5Check Calculated Missing Values: See any derived voltage or turns values the calculator computed from your inputs.

What This Calculator Provides

• ✓Real-time calculation as you type
• ✓Solves from five different combinations of known values
• ✓Turns ratio, voltage ratio, and current ratio
• ✓Ratios displayed as decimal and x:1 format
• ✓Calculates missing voltages or turns automatically
• ✓Full step-by-step derivation
• ✓Six built-in presets (step-up, step-down, isolation, more)
• ✓Calculation history
• ✓Export calculation as a text file
• ✓No signup required

Pro Tips

• 💡Use the voltage ratio shortcut (Vp ÷ Vs) when you only need turns ratio and don't care about the actual turn counts — this avoids needing to know or measure the physical windings at all.
• 💡Remember that current ratio is the inverse of voltage ratio — a step-down transformer that halves voltage will roughly double current, which matters for downstream cable and fuse sizing on the secondary side.
• 💡For replacement or repair winding calculations, use the original transformer's known voltage and turns relationship to derive the turns-per-volt figure, then apply that ratio to any new winding you need to calculate.
• 💡Check the calculator's ratio display format — it shows both decimal (like 2.5) and x:1 notation (like 2.5:1), which is the conventional way transformer ratios are documented on nameplates and in datasheets.
• 💡For isolation transformers, verify the turns ratio comes out to exactly 1:1 as a sanity check — any deviation suggests either a winding count error or that the transformer isn't a true isolation type.

Common Mistakes to Avoid

• ✕Confusing which side is primary and which is secondary. The primary winding connects to the power source; the secondary delivers output. Swapping them inverts every ratio in the calculation.
• ✕Assuming current ratio equals voltage ratio rather than its inverse. A 2:1 step-down voltage ratio corresponds to a 1:2 current ratio (secondary current is double primary current), not another 2:1.
• ✕Ignoring transformer losses when working with real (non-ideal) transformers. This calculator assumes an ideal transformer with 100% efficiency — real transformers have small losses that make actual secondary voltage and current slightly different from the calculated ideal values.
• ✕Treating turns ratio as fixed for all load conditions. The ratio itself is a physical property of the winding and doesn't change, but secondary voltage can sag under heavy load due to winding resistance and leakage reactance, which this ideal-transformer calculation doesn't model.
• ✕Mixing up which known values to enter when you have a partial data set. If you know one voltage and both turns counts, enter exactly those three fields — entering an incorrect fourth value (like guessing the other voltage) will produce an inconsistent result.