RC Rise Time Calculator

The famous "BW × tr = 0.35" rule-of-thumb for single-pole systems has been part of oscilloscope engineering since the 1950s Tektronix manuals. Its derivation: for a Gaussian-shape step response (which approximates RC low-pass behavior), 10-90% rise time = 0.35 / BW_3dB. The constant 0.35 is actually 2.2/(2π), since tr = 2.2·RC and BW = 1/(2πRC).

Oscilloscope probe compensation capacitors, PCB transmission-line termination design, digital-logic edge rate specifications, and every serial-link eye-diagram spec trace back to this simple Gaussian-approximation relationship. It's the reason a 100 MHz scope can resolve edges no faster than ~3.5 ns, and why 1 GHz processors need 350 ps edges (consuming a disproportionate share of PCB signal-integrity budget).

The root-sum-square (RSS) combination of cascaded rise times — tr_total = √(Σtr²) — similarly descends from the Gaussian assumption. A 1 ns signal observed through a 1 ns scope shows √(1² + 1²) = 1.41 ns rise, from which the real signal can be de-embedded. Professionals at Keysight, Tektronix, and LeCroy still teach this as the first rule of time-domain measurement.

Enter R and C. The tool returns 10-90% rise time tr = 2.2·RC, RC time constant τ = R·C, equivalent 3 dB bandwidth BW = 0.35/tr, and 5τ settling time (99.3% of final value). For step-response viewing on a scope, a good rule: scope bandwidth should be at least 3× the signal's fundamental frequency, giving enough margin that observed rise time is dominated by the signal, not the instrument.

Fall time for a symmetric RC network equals rise time. For asymmetric topologies (e.g., push-pull output with different pull-up and pull-down impedances), fall time differs and requires separate calculation. Everything runs client-side; no values leave your browser.