PCB Trace Delay: Convert Length & Delay with Stackup Assumptions

Convert PCB trace length and delay with an explicit propagation model

For a uniform, nonmagnetic transmission line in a quasi-TEM approximation, one-way propagation delay is length multiplied by the square root of effective relative permittivity divided by the speed of light. The inverse converts a delay allocation into a corresponding uniform-line length. The calculation is useful only when the permittivity and measurement endpoints match the intended route.

The speed of light used here is 0.299792458 mm per picosecond. If length is in millimeters, multiplying it by sqrt(effective permittivity) and dividing by that constant gives delay in picoseconds. This is the same velocity relationship used in the existing impedance tool. This page adds the dedicated bidirectional conversion and an explicit sensitivity interval; it does not create another impedance solver.

The result excludes packages, connectors, vias and other nonuniform structures. It also does not extract frequency-dependent group delay from a channel. Use it to reconcile units, understand a planning assumption or check a uniform segment, then complete the channel accounting required by the device specification.

Keep the units and endpoints visible

One inch is exactly 25.4 mm, and one nanosecond is 1,000 picoseconds. Entering a value intended as nanoseconds into a picosecond field produces a thousandfold error. The tool labels the delay input in ps and includes units with exported results, so an estimate can be checked independently instead of copied as an unlabeled number.

Define where the length starts and ends. A CAD centerline measurement can omit a package contribution that an interface timing budget includes. A connector model may already include part of a launch represented elsewhere. List the included segments before adding their delays; otherwise, an apparently careful total can double-count a feature or miss it entirely.

Choose effective permittivity for the actual line

Microstrip and grounded coplanar structures place fields in more than one dielectric region. Their effective permittivity depends on the geometry and surrounding materials. A bulk laminate Dk is therefore not a universal substitute. For an ideal homogeneous stripline, using the surrounding dielectric's relative permittivity is a reasonable stated assumption, but real constructions may require another level of modelling.

The calculator asks for the input basis explicitly. Choose either an effective value from a relevant model or measurement, or a homogeneous-stripline dielectric assumption. The choice is recorded in the brief. It does not silently transform a material data-sheet number into a solved effective value, and it does not promise that a selected basis is valid for your route.

When a supplier or analysis tool gives a propagation value directly, retain its units, frequency range and method. If those data include dispersion, a single effective value may describe only a chosen condition. Consult the transmission-structure comparison when the reference geometry itself is still uncertain. Geometry selection should precede numerical confidence.

Work through a length-to-delay calculation

Assume an illustrative uniform route is 50 mm long and has effective relative permittivity 3.6. The square root of 3.6 is approximately 1.89737. Dividing by 0.299792458 gives approximately 6.329 ps/mm. Multiplying by 50 mm gives approximately 316.45 ps, or 0.31645 ns, of one-way propagation delay.

Now change only the assumed permittivity to 3.4 and 3.8. The corresponding delays are approximately 307.53 ps and 325.12 ps. This range tells you how sensitive this particular model is to the entered interval. It is not a measured spread of manufactured boards, and it does not include geometric variation, copper roughness, coating, temperature or frequency dependence unless those effects are already represented appropriately in the input.

The example deliberately uses a declared effective value rather than labeling it as “FR-4.” A material family name is not enough to define a propagation calculation. Save the material system, stackup and model conditions with the number if it will influence a design requirement or procurement decision.

Convert a timing allocation into length without confusing it with a limit

For delay-to-length conversion, divide the delay by the calculated delay per unit length. An illustrative 500 ps allocation at effective permittivity 3.6 corresponds to about 79.00 mm of uniform line. The tool provides the numerical result for the entered values and converts it to inches when that is the selected output unit.

This number is not automatically the maximum allowed PCB route. A device budget may reserve time for packages, connectors, setup/hold behavior or another signal relationship. The designer must identify the part of the allocation that belongs to the uniform board segment. Nor does staying below that length demonstrate acceptable attenuation, reflections or crosstalk.

Sensitivity runs in the inverse direction for this mode: a higher permittivity gives a shorter length for the same delay. The worksheet orders the reported minimum and maximum accordingly. That detail matters when selecting a conservative allocation; using the same multiplication logic for both conversion directions would give a misleading interpretation.

Add unlike route sections with their own assumptions

A route may contain a surface section, a buried section and several transitions. Do not assign the whole route one effective value merely because it uses one PCB. For the uniform segments, calculate each delay with its applicable propagation value and sum the delays. Account for nonuniform regions with suitable models or measurements instead of inserting an arbitrary extra millimeter per via.

For a simple independent example, take a 30 mm segment with effective permittivity 3.0 and a 20 mm segment with effective permittivity 4.0. Their ideal delays are about 173.33 ps and 133.43 ps, with an unrounded sum of about 306.75 ps. This segment total still excludes the transition between them. It is an accounting example, not a validated channel model.

The high-speed layout guide helps define the route and reference-transition record. When comparing two signals, subtract complete delays built with compatible endpoints and assumptions. Equal physical length on two different structures does not necessarily imply equal electrical delay.

Convert in either direction and examine sensitivity

Choose the conversion direction first. In length-to-delay mode, the numeric input is length in the selected unit. In delay-to-length mode, it is delay in picoseconds. Enter a nominal permittivity and a lower/upper interval containing that nominal value. Use equal bounds if you want only the nominal result and have no justified sensitivity range.

All project fields start empty. Loading the example labels it as illustrative, and editing any input invalidates the previous result and export. The tool rejects a reversed interval, an out-of-range permittivity or a nonnumeric input. A valid zero length or delay returns zero; it is not treated as a hidden default or a statement about a complete channel.

The downloaded brief records assumptions and units. Attach it manually if requesting engineering review; the Contact form does not automatically receive worksheet values. If your requirement depends on loss, dispersion or mode conversion, include that need explicitly instead of presenting this conversion as a complete answer.

Compare a calculation with evidence at matching reference planes

If you measure propagation with time-domain equipment, define the event and the reference planes used to determine delay. A reflected event may represent a round trip, whereas the calculator returns one-way time. Fixture delay and trigger alignment can influence the comparison. Preserve the setup and extraction method rather than reporting a time difference without context.

For frequency-domain data, group delay relates to phase variation with frequency. A single nondispersive velocity approximation is not the same as extracting that quantity from a measured channel. If the application needs phase matching over a band, compare the appropriate frequency-dependent behavior, not just one delay number derived from a nominal Dk.

After a material or stackup change, revisit the original input basis. Reusing a value from a previous board can conceal a different dielectric environment. Keep the route revision, model revision and fabrication release connected. A useful manufacturing inquiry identifies which assumptions are fixed, which may be proposed by engineering, and which verification is required for the final construction.

Questions about PCB propagation delay

Model references and further design work

Analog Devices MT-094 discusses propagation and transmission-line approximation context. The Rogers engineering resources support material-related analysis. Use relevant model or measurement data to select the input; neither reference makes a generic value valid for an unspecified construction.

For width and gap questions, return to controlled-impedance design. For a build discussion, send the actual stackup and the timing task with the drawing. The useful conversion is the one whose units, endpoints and assumptions remain understandable to the next engineer.