USB-C Power Delivery Calculator

Introduction

USB‑C is the connector; USB Power Delivery (USB‑PD) is the negotiation protocol that decides what voltage and current a charger (the source) will provide to a device (the sink). Chargers advertise a set of supported power options called Power Data Objects (PDOs). The device chooses one option that it can accept, and the charger only enables higher power after the negotiation succeeds.

This calculator helps you estimate which common PD voltage level (5 V, 9 V, 12 V, 15 V, 20 V, and the newer 28/36/48 V levels) can meet a target wattage without exceeding your cable’s current rating. It also generates a comparison table showing the required current at each voltage and the maximum power your cable could carry at that voltage.

How to use the calculator

1. Enter Desired Power (W): the approximate wattage your device needs (for example, 45 W, 65 W, 87 W, 100 W).
2. Enter Cable Current Limit (A): typically 3 A for many passive USB‑C cables, or 5 A for e‑marked cables designed for higher power.
3. Select Calculate. The result line recommends the lowest standard PD voltage that can deliver the requested power while staying at or below the cable current limit.
4. Review the table to compare all PD voltage options and see how much headroom you have at each level.
5. Use Copy Result to copy the recommendation for a build note, purchase list, or troubleshooting message.

Formula and assumptions

The calculator uses the basic electrical power relationship:

Formula: P = V × I

$P=V\times I$

For each candidate PD voltage V, it computes the current required to deliver your target power P:

Formula: I = P / V

$I=\frac{P}{V}$

A voltage option is considered viable when the required current is less than or equal to your cable limit I_max. Among viable options, the calculator selects the lowest voltage that works. This mirrors a common design preference: avoid unnecessarily high voltage when a lower PD level can meet the power target.

The comparison table also shows maximum cable power at each voltage, computed as V × I_max. This is a cable‑limit estimate; real PD behavior also depends on what the charger and device actually support.

Worked example

Suppose you want 65 W and your cable is rated for 3 A. The required current at each voltage is:

• At 15 V: 65 ÷ 15 ≈ 4.33 A (too high for a 3 A cable)
• At 20 V: 65 ÷ 20 = 3.25 A (still above 3 A)
• At 28 V (EPR): 65 ÷ 28 ≈ 2.32 A (within 3 A, but only if both charger and device support EPR)

With a strict 3 A cable limit, the calculator will only recommend a profile that stays at or below 3 A. If none of the standard voltages meet that constraint, it will report that no standard PD voltage can provide the power. If you switch to a 5 A cable, then 15 V becomes viable because 4.33 A is below 5 A.

Limitations and practical notes

This tool is intentionally conservative and simplified. USB‑PD negotiation depends on the intersection of charger capabilities, device requests, and cable identification. The calculator assumes the voltage list is available and focuses on the cable current limit you enter.

• Charger/device support: Not every charger supports every voltage (for example, many stop at 20 V). Likewise, some devices will not request EPR (28/36/48 V) even if the charger can provide it.
• PPS behavior: Many modern chargers support Programmable Power Supply (PPS), which can negotiate intermediate voltages. This calculator uses fixed common PD voltage steps for clarity.
• Real power draw varies: Devices may draw less than the adapter rating depending on battery state, temperature, and firmware policies.
• Cable markings matter: A 5 A capability typically requires an e‑marked cable. If you are unsure, treat the cable as 3 A.
• Efficiency and heat: Higher voltage can reduce current for the same power, which can reduce cable heating (losses scale roughly with current squared). However, the “best” profile in practice may be chosen by the device for thermal or conversion‑efficiency reasons.

Common USB‑PD voltage levels (reference)

The table below is a quick reference for typical PD voltage tiers. The calculator will generate a separate table after you calculate, based on your inputs.

* Extended Power Range (EPR) levels introduced with USB‑PD 3.1.

Troubleshooting and buying guidance

If you are troubleshooting slow charging, a common failure mode is a mismatch between the charger’s advertised wattage and the cable’s current rating. For example, a “100 W” charger paired with a 3 A cable may be limited to 60 W at 20 V. Conversely, a 5 A e‑marked cable can unlock higher‑power PDOs when the device supports them.

When comparing accessories, it helps to separate three questions: (1) what the device will request, (2) what the charger can offer, and (3) what the cable can safely carry. This calculator focuses on the cable constraint because it is the easiest to overlook and the most likely to silently limit power.

A practical way to use the output is to treat it as a checklist. If the calculator recommends 20 V at 4.35 A for a 87 W laptop, then you know you need (a) a charger that offers a 20 V PDO at 5 A or at least enough current, (b) a 5 A e‑marked cable, and (c) a device that actually requests that profile. If any one of those is missing, the negotiated power will drop to the next best option.

Safety notes (why the limits matter)

USB‑PD is designed to be safe by default: a USB‑C port starts at 5 V and only increases voltage after a successful negotiation. Still, heat is real. Cable losses scale roughly with I²R, meaning that a small increase in current can create a much larger increase in heating. That is why a 3 A cable is not “almost” a 5 A cable; it is a different class of product with different conductor size and identification requirements.

The current limit you enter should reflect the cable’s rating, not the charger’s marketing. If you are unsure, assume 3 A. Many inexpensive USB‑C to USB‑C cables are 3 A even if they can physically plug into a 100 W charger. For EPR (28/36/48 V), the ecosystem is stricter: EPR requires appropriate e‑markers and compliant hardware on both ends. In other words, the presence of “240 W” on a charger does not guarantee your cable or device can use EPR.

FAQ

Does the calculator guarantee my device will charge at the recommended profile?

No. The result is a feasibility check based on the power equation and your cable limit. Real negotiation depends on the specific PDOs the charger advertises and what the device chooses to request. Some devices prefer certain voltages for efficiency, and some chargers omit intermediate steps like 12 V.

Why does the calculator pick the lowest voltage that works?

It is a simple, conservative rule that avoids unnecessarily high voltage when a lower tier can meet the target wattage. In practice, a device may choose a higher voltage to reduce current and cable heating, or it may choose PPS to optimize its internal conversion. The comparison table helps you see those trade‑offs.

What about USB‑A chargers or legacy fast charging?

This page is specifically about USB‑C Power Delivery voltage tiers. USB‑A ports and proprietary protocols can behave differently and may not offer the same negotiated voltage steps. If you are using USB‑A to USB‑C, the available power is often limited and may not follow the PD profiles shown here.

Design notes for engineers and educators

For product design, the most useful output is often the full table rather than the single recommendation. The table shows how close you are to the cable limit at each voltage and how much margin you gain by moving to a higher tier. For example, if your design needs 60 W and you expect many users to have 3 A cables, then 20 V is the only SPR tier that can possibly meet 60 W (because 15 V × 3 A is only 45 W). That kind of constraint can influence whether you include a barrel jack, whether you ship a 5 A cable, or whether you implement performance scaling.

For teaching, the calculator is a concrete demonstration of constraints and proportional reasoning. Students can see that doubling voltage halves current for the same power, and they can connect that to resistive losses and thermal limits. Encourage learners to try a few values (for example, 18 W phone charging, 45 W ultrabook, 100 W workstation) and observe how the recommended tier changes as the cable limit changes.

Quick checklist before you buy a charger or cable

• Know your target wattage: look at the device’s adapter rating or USB‑PD input spec.
• Confirm cable rating: 3 A for many basic cables; 5 A requires e‑marker for high power.
• Check charger PDOs: a “65 W” charger might be 20 V × 3.25 A; a “100 W” charger often needs 5 A support.
• Consider EPR only when needed: 28/36/48 V is great for 140–240 W devices, but requires compatible gear.
• Use the table for margin: if you are right at the limit, expect throttling or warm cables in real use.

With those basics in mind, use the calculator below to estimate a viable PD tier and to document the reasoning behind your choice. The goal is not to replace the USB‑PD specification, but to make the everyday “will this cable and charger actually deliver the power I need?” question easier to answer.