Propagation delay calculator

Convert signal length to propagation delay — or a target delay to the length that produces it — across PCB microstrip, stripline, coax and twisted pair.

Line & medium

Medium

εe ≈ 0.475·εr + 0.67 ≈ 2.6 on FR4 (fields partly in air)

Line length

mm — fill to get delay

Target delay

ns — fill to get required length

Velocity factor: 62.0%
Propagation delay: 5.38ns/m
Delay for entered length: 0.538ns
Length for entered delay: --mm
Length for entered delay: --in

Rules of thumb

1 ns of signal delay ≈ 300 mm in air, ≈ 186 mm on an FR4 microstrip, ≈ 146 mm on an FR4 stripline. To match two routes in a harness or a board (for example equalising a clock or data bus), the delay is length × delay-per-metre, so the tool gives you both directions: from a known run to its delay, and from a target delay to the length that produces it.

Related parts

Browse related parts:

coaxial cable flat ribbon cable high-speed connectors

Frequently asked questions

How is propagation delay calculated?

Signal velocity is v = c/√εe with c ≈ 299.8 mm/ns, so the delay per millimetre is √εe / c. On FR4, a microstrip outer layer has εe ≈ 2.6 (≈ 186 mm/ns) and a buried stripline εe ≈ 4.2 (≈ 146 mm/ns). The tool returns ns/m for your medium and converts length↔time both ways.

What is the propagation delay of 100 mm of FR4 microstrip?

About 0.54 ns. With εe ≈ 2.6 the delay is √2.6/299.8 ≈ 5.4 ps/mm, so 100 mm ≈ 0.54 ns. As a rule of thumb 1 ns of delay equals about 300 mm in air, 186 mm on an FR4 microstrip and 146 mm on an FR4 stripline.

Why do designers match trace lengths in a bus?

In parallel buses and differential pairs the propagation delay must be equal between signals so the bits arrive at the receiver within the setup window. Matching lengths to within the delay tolerance (for example 25 ps at higher speeds) is equivalent to matching propagation delay — the two are the same quantity in different units.

How it works

The effective dielectric εe depends on the medium: a microstrip couples through air, so εe ≈ 0.475·εr + 0.67 sits well below εr, while a stripline is fully embedded and εe ≈ εr. The velocity factor vf = 1/√εe converts directly to delay, and length-to-time uses the same constant in reverse. IC Source Direct provides this tool for reference only.

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