Capacitors combine the opposite way to resistors: in parallel their values add, in series they shrink. On a laptop board, almost every capacitor you meet is in parallel with others — a rail is a long row of decoupling and bulk capacitors all connected between the same two nets. This calculator adds them up, or works out a series combination when you need one.
How to use it
- Choose Parallel or Series.
- Choose the default unit for plain numbers (µF, nF or pF).
- Type the values in one list. A prefix written with a value overrides the default:
10, 4u7, 100n, 22p. - Read the total capacitance.
The formulas
- Parallel: C = C1 + C2 + C3…
- Series: 1/C = 1/C1 + 1/C2 + 1/C3… For two capacitors, C1 × C2 ÷ (C1 + C2).
In series, the total is smaller than the smallest capacitor, and the voltage divides between them in inverse proportion to their values — the smallest capacitor takes the largest share.
Worked example: a rail's total capacitance
The output of a 1.05 V regulator has two 22 µF bulk capacitors, eight 1 µF and twelve 0.1 µF decoupling capacitors. In parallel that is 44 + 8 + 1.2 = 53.2 µF. That number explains two things you see at the bench: the resistance reading on that rail climbs slowly as your meter charges more than 50 µF, and a single shorted 0.1 µF capacitor among twenty takes the whole rail down. Finding it is covered in How to Test a Capacitor with a Multimeter.
When series capacitors appear
Series connections are rare on laptop boards. You may see them in AC-coupled signal paths (one capacitor per line, not in series with each other) and in some high-voltage filter designs where two capacitors share a voltage. If you combine capacitors in series as a temporary repair, check each one's voltage rating against its share of the voltage.
Where it matters on a laptop board
- Understanding rail capacitance and why rails take time to charge or discharge.
- Replacing a large capacitor with two smaller ones in parallel when the exact part is unavailable.
- Estimating RC timing with RC Time Constant Calculator.
The component itself is described under capacitor.
FAQ
Can I replace one 22 µF capacitor with two 10 µF in parallel?
Electrically close, if the voltage rating and dielectric match. The pads must take two parts, which often means stacking or careful placement.
Why does a ceramic capacitor measure less than its marking?
Ceramic capacitors lose capacitance under DC bias and with temperature, especially high values in small packages.
Do parallel capacitors need the same voltage rating?
Each must be rated for the full voltage, since all of them see it.