RESONANCE · CAPACITANCE · TOLERANCE
RFID & NFC Tuning Calculator
Find the capacitor your coil needs, or work back to a target inductance. Include chip and stray capacitance, compare tolerance limits, and use measured AC resistance to estimate winding Q.
Plan your resonance target
Results update as you editFROM INPUT TO SPECIFICATION
How to use this calculator
Start with a known design requirement, check the assumptions, and keep a record of the values you used.
Choose the value to solve
Start with your target frequency and measured inductance, or solve for inductance or frequency from the capacitance you already have.
Include every parallel capacitor
Enter external, chip-input and stray capacitance for the equivalent circuit. Add component tolerances to view the possible resonance range.
Validate the assembled antenna
Use the result as a tuning starting point. Measure with the intended chip, ferrite, enclosure and nearby materials, then update the inputs and save the revised design.
METHOD & SCOPE
What the calculation means.
One defined circuit. Three ways to solve it.
The ideal inductor is in parallel with the sum of external, chip-input and entered stray capacitance. All calculations use this same lumped circuit.
The lower frequency bound combines maximum inductance and maximum capacitance; the upper bound uses their minimum values. Each capacitance uses its own tolerance. These are worst-case limits, not a probability distribution or operating bandwidth.
Separate coil Q from loaded tag Q
Optional winding Q is ωL/Rac, using series winding resistance entered for the displayed frequency. It excludes chip loading, capacitor losses and matching-network losses. Leaving Rac blank keeps Q unknown.
If the chip and stray capacitance already exceed the target total, a parallel external capacitor cannot reach that target. The result identifies this constraint. Complete reader matching, read range and automatic parasitic extraction are outside this tool.
DESIGN QUESTIONS
Before you build.
Can I use this for 125 kHz RFID and 13.56 MHz NFC?
Yes, for the stated ideal LC circuit. Enter 125 kHz or 13,560 kHz respectively. Use the inductance, chip capacitance and stray capacitance that apply to your specific device and assembly. The same resonance equation does not make different chip circuits interchangeable.
Why is the required external capacitance negative?
The entered chip and stray capacitance already exceed the total needed for the target frequency and inductance. Adding a positive parallel capacitor cannot achieve that target. Reduce inductance or inherent capacitance, or select a lower target frequency.
Can I use DC resistance to calculate NFC coil Q?
Use series AC resistance measured at the displayed frequency. DC resistance does not include frequency-dependent winding loss. This tool reports winding-only Q and excludes chip loading and capacitor losses, so it is not the loaded tag Q.
Does this design a complete NFC reader matching circuit?
No. It solves a single ideal parallel LC circuit for resonance. A complete reader network also depends on the reader IC, target impedance, damping, filter components, coupling and circuit topology. Follow the IC supplier’s matching guidance and measure the assembled design.
CONTINUE YOUR DESIGN
The next step for your coil.
Keep the calculation with your drawing and target specifications when discussing a coil design.