Online Ripple Filter Capacitor Calculator: Find the Right Capacitance from Ripple Current and Frequency

When filtering a power supply, selecting the capacitor often trips me up: the datasheet only lists ripple current and switching frequency, so I have to work backward to figure out how large a capacitor to use. Change the circuit topology (rectification, Buck, sinusoidal ripple) and the coefficient in the formula changes too, so every time I have to dig through my notes to double-check. Later I settled on using an online ripple filter capacitor calculator: enter the parameters, and the capacitance plus several quantities to verify come out together. Here I record how I use it and a few real measurements for future reference.

Tool URL: https://www.dd-cat.cn/tools/ripple-cap

Enter four quantities — first pick the right ripple type

Open the page and first select the ripple type, which determines the waveform coefficient $K$ in the formula:

Type	Scenario	Waveform coefficient K
Line-frequency full-wave rectification (bridge)	Transformer + rectifier bridge	2
Line-frequency half-wave rectification	Half-wave rectification	1
Switching output (Buck triangular wave)	DC-DC output filtering	8
Sinusoidal ripple current	General sinusoidal ripple	2π

The remaining three inputs:

• Current I (A): for full-wave / half-wave, enter the load current; for Buck, enter the inductor ripple current $\Delta I_L$ (peak-to-peak); for sine, enter the RMS ripple current;
• Ripple frequency f (Hz): line-frequency full-wave = 2× line frequency (50 Hz → 100 Hz), half-wave = line frequency, switching supply = switching frequency;
• Allowable ripple ΔV (mV): peak-to-peak; commonly 1~2 V for rectification and 20~100 mV for switching supplies.

There is just one core formula:

$$C = \frac{I}{K \cdot f \cdot \Delta V}$$

Change any number and the result updates instantly. The current parameters are synced into the URL, so sending the link to a colleague opens the same set of values — no screenshots needed to compare parameters.

The output is more than a capacitance value

In the results, these are the ones I look at most often:

• Required filter capacitance C and recommended standard capacitance: the latter rounds up to the nearest E6-series standard value no smaller than the theoretical value (common electrolytic / ceramic values 1.0 / 1.5 / 2.2 / 3.3 / 4.7 / 6.8), so what you get is a part you can actually buy;
• Actual ripple at the standard value: how much ripple becomes after switching to the standard capacitance, with a direct verdict of “meets the ripple requirement” or “still high”;
• Ripple current (RMS): the ripple current the capacitor must withstand — this is the easiest thing to overlook during selection; get the capacitance right but exceed the rated ripple current, and the capacitor will heat up and bulge;
• Equivalent capacitive reactance |Zc| and ESR limit: once ESR exceeds this limit, ripple is no longer determined by capacitance but dominated by ESR — simply adding more capacitance won’t help.

One real use: 50 Hz full-wave rectification

A linear supply on hand with a 1 A load; I wanted to filter after 50 Hz line-frequency full-wave rectification with 1 V (1000 mV) of allowable ripple:

• Type set to “Line-frequency full-wave rectification”, $f$ entered as 100 Hz;
• Required C = 5000 µF (5 mF);
• Recommended standard value 6800 µF (E6 rounded up);
• Actual ripple at the standard value ≈ 735 mV, a bit more comfortable than the 1 V target;
• RMS ripple current ≈ 1 A, ESR limit ≈ 0.33 Ω.

If I had used 4700 µF instead (the E6 step below), the ripple would reach 1.06 V, just over the limit — this is exactly the benefit of giving a “recommended standard value” rather than only the theoretical value: tolerance and temperature drift are not yet accounted for, so it first lands on a safe step.

Switching to a Buck output

A 500 kHz Buck with inductor ripple $\Delta I_L = 0.3\ \text{A}$ and 50 mV of allowable output ripple:

• Type set to “Switching output (Buck triangular wave)”, $I$ entered as 0.3 A;
• Required C = 1.5 µF, recommended also 1.5 µF;
• Actual ripple at the standard value = 50 mV, just meets the requirement;
• RMS ripple current ≈ 0.087 A;
• ESR limit ≈ 0.17 Ω — ordinary ceramics are far below this value, so no worries; switching to a high-ESR electrolytic would require caution.

Buck’s coefficient is 8, much larger than rectification’s 2, so for the same current and ripple requirements, a switching supply actually needs less capacitance — this is part of where the size advantage of switching supplies comes from.

Half-wave and sinusoidal ripple, too

• Half-wave rectification: load 0.5 A, ripple frequency 50 Hz, allowable ripple 2 V → required 5000 µF, recommended 6800 µF, actual ripple ≈ 1471 mV, RMS ripple current ≈ 0.71 A. The half-wave coefficient is only 1, so at the same current you need twice the capacitance of full-wave to reach the same ripple — if full-wave is available, don’t use half-wave.
• Sinusoidal ripple: 1 A RMS, 100 Hz, allowable 1 V → recommended 2200 µF, actual ripple ≈ 723 mV. This comes up where the ripple source is modeled directly as a sine wave.

Quick reference for common combinations

Scenario	Type	I	f	ΔV	Recommended capacitance
50Hz full-wave rectification	full	1 A	100 Hz	1000 mV	6800 µF
50Hz half-wave rectification	half	0.5 A	50 Hz	2000 mV	6800 µF
Buck output	buck	0.3 A	500 kHz	50 mV	1.5 µF
Sinusoidal ripple	sine	1 A	100 Hz	1000 mV	2200 µF

A few notes

• Don’t pick the wrong waveform coefficient: full-wave 2, half-wave 1, Buck 8, sine 2π. Choosing the wrong type can make results differ severalfold, especially when rectification and switching are mixed.
• Rectification ripple frequency is 2× the line frequency: after full-wave rectification of 50 Hz mains it is 100 Hz, not 50 Hz — getting this wrong doubles the capacitance error.
• Check the ripple current: especially for large electrolytics — compare the RMS value from the tool against the capacitor’s rated ripple current, and leave temperature derating margin.
• Treat the ESR limit as a red line: beyond it, adding capacitance won’t solve the problem — use a low-ESR part or parallel multiple capacitors.
• Leave margin when selecting: use 1.3~2× the actual capacitance and 1.5~2× the working voltage for the voltage rating; if the required capacitance exceeds 10 mF (the tool will warn), consider a switching supply or active ripple suppression first instead of piling up electrolytics.

That is the process I usually follow when using it to select filter capacitors. It strings the whole chain — ripple current → capacitance → standard value → actual ripple / ESR verification — onto a single page, saving the time of looking up formulas and the E6 table each time, and greatly reducing the chance of mistakes.