Buck and Boost Converter Inductor Calculator

Find the inductance, duty cycle and peak current for a switching (DC-DC) regulator.

Input voltageVOutput voltageV

Output currentASwitching frequencyHz

Ripple current ratio (typically 20–40%)%

Formulas and cautions

Buck: D = (V_out)/(V_in), L = ((V_in − V_out)·D)/(f·ΔI). Boost: D = 1 − (V_in)/(V_out), inductor current I_L = (I_out)/(1 − D), L = (V_in·D)/(f·ΔI). The converter is assumed lossless. Besides inductance, the inductor's saturation current rating must exceed the peak current, and the controller datasheet's recommendations take priority in a real design.

Try it: inductor current in a buck converter

The inductor current ramps up while the switch is on and down while it is off. Change the parts and the load and watch the ripple and the operating mode.

Input V_in12 V Output V_out5 V Inductance L10 µH Switching frequency f500 kHz Load current I_out2 A

Inductor current over three switching periods

Duty cycle D: 41.7%
Conduction mode: continuous (CCM)
Ripple ΔI (peak to peak): 583 mA
Peak current: 2.29 A
Valley current: 1.71 A
Load at the CCM/DCM boundary: 292 mA
L for CCM at this load: 1.46 µH

While the switch is on, the inductor sees V_in − V_out and its current rises in a straight line; while it is off, it sees −V_out and the current falls. The average of this triangle is the load current. The ripple depends only on the voltages, L and f, not on the load.

D = (V_out)/(V_in)

ΔI = ((V_in − V_out)·D)/(f·L)

I_pk = I_out + (ΔI)/(2)

When the load falls below half the ripple, the current reaches zero before the next cycle and rests there: discontinuous mode (DCM). The converter still works, but the duty cycle shrinks and the regulation loop behaves differently. A larger L or a higher f lowers the ripple and keeps the converter in continuous mode (CCM) down to lighter loads, and the inductor's saturation current must stay above the peak.

Worked example: a 12 V → 5 V, 2 A buck converter

Duty cycle: D = (V_out)/(V_in) = (5 V)/(12 V) = 41.7%
Ripple current: ΔI = 30% × 2 A = 0.6 A
Inductance: L = ((V_in − V_out)·D)/(f·ΔI) = ((12 − 5) V × 0.4167)/(500 kHz × 0.6 A) = 9.72 µH → standard 10 µH
Pick an inductor whose saturation current exceeds the 2.29 A peak.

Inductor values for common conversions

Type	Conversion	Output	D	Avg inductor current	L	E12	Peak
TypeBuck	Conversion12 → 5 V	Output2 A	D41.7%	Avg inductor current2 A	L9.72 µH	E1210 µH	Peak2.29 A
TypeBuck	Conversion12 → 3.3 V	Output1 A	D27.5%	Avg inductor current1 A	L15.9 µH	E1218 µH	Peak1.13 A
TypeBuck	Conversion5 → 3.3 V	Output1 A	D66.0%	Avg inductor current1 A	L7.48 µH	E128.2 µH	Peak1.14 A
TypeBuck	Conversion24 → 12 V	Output2 A	D50.0%	Avg inductor current2 A	L20 µH	E1222 µH	Peak2.27 A
TypeBoost	Conversion5 → 12 V	Output0.5 A	D58.3%	Avg inductor current1.2 A	L16.2 µH	E1218 µH	Peak1.36 A
TypeBoost	Conversion3.7 → 5 V	Output1 A	D26.0%	Avg inductor current1.35 A	L4.75 µH	E125.6 µH	Peak1.52 A
TypeBoost	Conversion12 → 24 V	Output1 A	D50.0%	Avg inductor current2 A	L20 µH	E1222 µH	Peak2.27 A

At 500 kHz with 30% ripple, assuming 100% efficiency; the real input current is higher by the efficiency loss. Running from a battery? Check runtime with the battery life calculator.

Using it in practice

Step by step

Choose buck (step-down) or boost (step-up) and enter input voltage, output voltage, load current and switching frequency.
Pick the ripple current as a fraction of the load; 20 to 40% is the usual range.
Read the inductance, duty cycle and peak current, then choose an inductor whose saturation current is above that peak with margin.

Common mistakes

Choosing an inductor by inductance only. Above its saturation current the inductance collapses and the current shoots up. The saturation and RMS current ratings matter as much as the microhenries.
Forgetting that a boost converter draws more input current. Power in is at least power out, so stepping 3.7 V up to 5 V at 1 A pulls well over 1.35 A from the battery once losses are included.
Ignoring the minimum input. A buck converter's duty cycle has a ceiling, so it cannot output a voltage close to its input. Use the lowest input you expect, such as a nearly empty battery, when checking.

Practical tips

Follow the converter IC's datasheet first; its recommended inductor range takes the control loop into account, which this calculator does not. Keep the high-current loop (input capacitor, switch, diode or synchronous switch) as small as possible on the board, and place the input capacitor right at the chip. A shielded inductor radiates less noise into nearby analog circuits.

Frequently asked questions

How much ripple current should I design for?

20 to 40% of the average inductor current is the usual range. Less ripple needs a larger, more expensive inductor; more ripple raises the peak current and output ripple. 30% is a common starting point.

Why does the inductor's saturation current matter?

Above its saturation current an inductor's inductance collapses, current spikes and the converter can fail. Choose a part whose saturation rating is above the calculated peak current, with some margin.