Calculators / 05
Electrical Length and Critical Length Calculator
A trace becomes a transmission line when a reflection from its far end can no longer hide inside the edge that launched it. This calculator applies the rule the topic page derives — the round trip against a third of the rise time — so the two can never disagree.
Reading the numbers
- In air, light covers an inch in 84.7 ps; a dielectric slows it by √Dk.
- Below the critical length the reflection returns while the edge is still rising and merges into it. Above it the reflection arrives as a separate step, and the line needs a controlled impedance and a termination.
- It is the rise time, not the clock rate, that decides: a 10 MHz clock with a 100 ps edge is already electrically long on any trace longer than about 0.10 in at Dk 3.8.
Every value is in the address bar as you change it, so a link reproduces the calculation exactly.
Type values with SI prefixes — 100n, 2.2p, 16G — or
drag the sliders; a bare number keeps the prefix already shown.
The formulas
Go deeper — assumptions, how it is checked, and sources
Assumptions. One driver, one receiver, a uniform line. Microstrip has an effective Dk below the laminate's because part of its field is in air; stripline's equals the laminate's. Use the line impedance calculator to get εeff.
How it is checked. At Dk = 1 the delay is the vacuum value; delay scales linearly with length and with √Dk; and the critical length is exactly where the round trip equals tr/3.
Source. E. Bogatin, Signal and Power Integrity — Simplified, 3rd ed., Pearson, 2018, ch. 7.