USB-C Power Delivery Calculator

Introduction: what USB-C Power Delivery actually negotiates

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). The source advertises a list of power options called Power Data Objects (PDOs) in a Source Capabilities message. The sink picks exactly one of them and answers with a Request message. Only after the source replies Accept, ramps the rail and sends PS_RDY does the higher voltage actually appear on VBUS. Until then the port sits at vSafe5V.

Three independent things have to agree before any of that succeeds: what the charger can offer, what the device is willing to request, and what the cable is certified to carry. This calculator models all three. You give it a target wattage, the charger's rated power and the class of USB-C cable you are using, and it works out which Fixed Supply PDO can actually deliver that power, how much current it needs, how much of that voltage a worst-case compliant cable will lose, and whether a Programmable Power Supply (PPS) window could cover the same job.

The important thing the tool refuses to do is hand you an impossible profile. A 3 A cable cannot reach 100 W no matter which charger it is plugged into, Extended Power Range voltages cannot be negotiated over a cable without an EPR e-marker, and no compliant PD source of any kind exceeds 240 W. Those ceilings are enforced rather than glossed over.

How to use the PD profile calculator: three inputs, one verdict

1. Enter Desired Power (W): the wattage your device wants at full load (for example 18, 45, 65, 100, 140, 240).
2. Enter Charger Rated Power (W): the number printed on the adapter. This gates which PDOs a compliant source is even allowed to advertise - a 100 W charger has no 28 V PDO, and a 140 W charger has no 48 V PDO.
3. Choose the USB-C Cable Class. USB-C cables advertise exactly one of two current ratings in their e-marker, 3 A or 5 A, so the calculator offers three real-world classes rather than a free-form ampere box: a plain 3 A cable, a 5 A e-marked cable certified to 100 W, and a 5 A EPR cable certified to 240 W.
4. Select Calculate. The result line gives the lowest standard PD voltage that can carry the requested power, the current it needs, a lower-current alternative where one exists, the worst-case cable drop at that operating point, and whether PPS could serve the same target.
5. Review the generated table and the chart to compare every tier, then use Copy Result to paste the summary into a build note, a purchase list or a support ticket.

The power formula and the USB-PD voltage ladder

Everything starts from the basic electrical power relationship:

Formula: P = V × I

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

Formula: I = P / V

A voltage is only viable if the power actually available at that tier reaches your target. Available power is not simply V × I_max; it is the smallest of three separate ceilings - what the cable can carry, what the charger is rated for, and the ceiling the specification puts on that power range:

Formula: P_avail = min(V × I_cable, P_charger, P_range)

P_range is 100 W for every Standard Power Range (SPR) tier and 240 W for the Extended Power Range (EPR) tiers. I_cable is 3 A or 5 A and never anything else, because those are the only two current capabilities a USB-C cable e-marker can report. Among the viable tiers the calculator names the lowest voltage that works, because that is the conservative choice, and separately names the highest viable voltage as the lower-current, cooler-running alternative.

The voltage ladder itself

The Fixed Supply PDO voltages are not arbitrary. USB PD defines 5 V, 9 V, 15 V and 20 V as the normative SPR rails, and USB PD 3.1 added 28 V, 36 V and 48 V as the EPR rails. 12 V is extremely common in the field but is an optional voltage rather than a required one, so a perfectly compliant charger may go straight from 9 V to 15 V. The calculator includes 12 V and labels it optional so you do not design around a rail that might not be there.

Above 100 W the ladder is also gated by charger size. A source may only advertise 28 V once it exceeds 100 W, may only advertise 36 V at 140 W or more, and may only advertise 48 V at 180 W or more. That is why entering a 140 W charger does not unlock the 48 V row in the table.

Cable resistance and the voltage drop budget

The USB Type-C Cable and Connector Specification does not publish a single "cable resistance" number; it publishes a voltage drop budget measured at the cable's rated current, and a compliant assembly - wire plus both sets of contacts - must stay inside it. The budget is 500 mV on VBUS and 250 mV on GND, so 750 mV around the loop. Turning that into a worst-case loop resistance is one division:

Formula: R_loop ≤ (0.500 V + 0.250 V) / I_rated

That gives 250 mΩ for a barely-compliant 3 A cable and 150 mΩ for a barely-compliant 5 A cable. The drop and the heat dissipated in the cable at an operating current I then follow directly:

Formula: V_sink = V − I R_loop, P_loss = I^2 R_loop

This is deliberately a pessimistic figure: it is the worst cable that could still pass certification. A good short cable is often three or four times better. It is useful precisely because it bounds the problem - if the worst legal cable still only loses 2 % of the rail, cable drop is not your issue, and if it loses 12 %, it very well might be. It also shows why the specification prefers higher voltages: because loss scales with the square of current, moving 100 W from 20 V to 48 V cuts the current from 5 A to about 2.08 A and the cable loss by roughly a factor of six.

PPS and AVS: the programmable rails

Alongside the Fixed Supply PDOs, a source may advertise Augmented PDOs. The best known is Programmable Power Supply (PPS), which advertises a continuous window instead of a single rail. The sink then walks the output in 20 mV voltage steps and 50 mA current steps, which is what allows a phone to drive its battery almost directly and skip a conversion stage. Typical advertised windows are 3.3-5.9 V, 3.3-11 V, 3.3-16 V and 3.3-21 V. PPS lives inside the Standard Power Range, so it is bounded by 5 A and by 100 W.

The step size is the reason the calculator reports a PPS voltage rounded up to a 20 mV boundary: a sink cannot ask for 13.013 V, only for 13.02 V. Above SPR, EPR replaces PPS with Adjustable Voltage Supply (AVS), which spans 15 V up to the source's top EPR rail in coarser 100 mV steps and has no current-limit mode.

Worked example: charging a 65 W laptop over a 3 A cable

Target 65 W, a 100 W charger, and an ordinary USB-C cable with no 5 A e-marker. Required current at each SPR tier:

• At 9 V: 65 ÷ 9 ≈ 7.22 A - far past any cable.
• At 15 V: 65 ÷ 15 ≈ 4.33 A - above the 3 A cable limit.
• At 20 V: 65 ÷ 20 = 3.25 A - still above 3 A, and 20 V is the top of SPR.
• At 28 V (EPR): the arithmetic would fit at 2.32 A, but EPR requires an EPR e-marked cable and a charger above 100 W, so the row is locked regardless.

So with a plain 3 A cable no PD profile reaches 65 W. The ceiling is 60 W at 20 V × 3 A, and the calculator says exactly that rather than inventing a profile. Swap in a 5 A e-marked cable and 15 V becomes viable at 4.33 A, so the recommendation becomes 15 V, with 20 V × 3.25 A offered as the lower-current alternative and 100 W of headroom at 20 V.

Worst-case cable drop makes the difference concrete. At 20 V and 3.25 A through a barely-compliant 5 A cable (150 mΩ), the loop drop is 0.49 V - the sink sees about 19.51 V - and 1.6 W is burned in the cable. Choosing 15 V instead means 4.33 A, a 0.65 V drop and 2.8 W of cable loss. Same power to the laptop, nearly twice the heat in the cable. That is the trade the "lower-current alternative" line exists to surface. In practice a real 65 W laptop charger advertises 20 V at 3.25 A and the laptop requests it, which is why the alternative line usually matches what your hardware actually does.

Limitations and assumptions behind these numbers

The tool models the three hard constraints - cable class, charger rating and specification ceilings - and it models them correctly. It does not model everything.

• Advertised PDOs are assumed, not known: the calculator assumes a compliant charger of the rating you entered offers the standard ladder up to its capability. A real charger may skip rails; many stop at 20 V, and 12 V is optional throughout.
• Per-PDO current is idealised: real Fixed PDOs advertise a specific maximum current, often below the cable limit (a 65 W charger typically offers 20 V at 3.25 A, not 20 V at 5 A). The calculator uses the charger's total rating as the cap.
• Device policy is not modelled: a sink may decline EPR, prefer PPS, or request less power when warm or nearly charged.
• The cable drop figure is a worst case: it assumes an assembly sitting exactly on the 500 mV / 250 mV compliance limit. Good cables are much better; non-compliant cables can be much worse.
• No thermal or battery modelling: sustained delivery depends on adapter thermals and battery state of charge, neither of which appears here.
• Spec ceilings are hard: 100 W for SPR, 240 W for EPR (48 V × 5 A). Targets above 240 W are outside USB-PD entirely and the calculator refuses them instead of extrapolating.

Sources: the normative Fixed Supply voltages (5/9/15/20 V SPR, 28/36/48 V EPR), the 100 W SPR ceiling, the 240 W EPR ceiling and the PPS 20 mV / 50 mA step sizes are defined in the USB Power Delivery Specification (Revision 3.1 and later) and summarised in the USB Promoter Group's USB PD 3.1 developer update. The 3 A default versus 5 A e-marked cable current capability, the EPR cable e-marker requirement and the 500 mV VBUS / 250 mV GND cable voltage-drop budget come from the USB Type-C Cable and Connector Specification. General USB-IF charger guidance is at usb.org/usb-charger-pd. The relationship P = V × I is Ohm's-law power arithmetic. Last reviewed August 2026.

Common USB-PD voltage levels (reference)

The table below is a quick reference for the PD voltage ladder and the ceilings that apply to each rail. The calculator generates a second, input-specific table after you press Calculate.

SPR power is additionally capped at 100 W regardless of voltage; EPR is capped at 240 W. EPR rails were introduced with USB PD 3.1 and always require an EPR-certified e-marked cable.

Troubleshooting and buying guidance

If charging feels slow, work the three constraints in order. First the cable: a "100 W" charger paired with a cable that has no 5 A e-marker is limited to 60 W, and no firmware update will change that. Second the charger: check the printed PDO list on the case, because a 65 W charger advertising 20 V at 3.25 A cannot feed a 100 W laptop even over a perfect cable. Third the device: confirm it requests the profile at all, since some laptops only pull their full rating when the lid is open or the battery is low.

When comparing accessories it helps to separate the same three questions: what the device will request, what the charger can offer, and what the cable can safely carry. The cable is the one most often overlooked and the one most likely to silently cap you, which is why it is a first-class input here rather than a footnote.

A practical way to use the output is as a checklist. If the calculator recommends 20 V at 4.35 A for an 87 W laptop, you need (a) a charger whose rating covers 87 W and which actually offers a 20 V PDO at that current, (b) a 5 A e-marked cable, and (c) a device that requests that profile. Miss any one and the contract falls back to the next best option, usually 20 V at 3 A.

Safety notes (why the limits matter)

USB-PD is safe by default: a USB-C port starts at vSafe5V and only raises the rail after a successful negotiation, and it drops back to 5 V the moment the contract is lost. Still, heat is real. Cable loss scales with I²R, so a small increase in current produces a much larger increase in heating. A 3 A cable is not "almost" a 5 A cable; it is a different class of product with larger conductors and a mandatory e-marker chip that tells the source it may exceed 3 A.

The cable class you select should reflect what the cable is certified for, not what the charger's box claims. If you are unsure, pick the 3 A class. For EPR the ecosystem is stricter still: a 240 W cable must report 50 V and 5 A capability in its e-marker, and USB-IF stopped certifying 100 W USB-C to USB-C cables in favour of the 240 W class. The presence of "240 W" on a charger says nothing about whether your cable or your device can use EPR.

Design notes for engineers and educators

For product design the full table is usually more useful than the single recommendation, because it shows how close each tier sits to its ceiling. If your design needs 60 W and you expect most users to bring 3 A cables, then 20 V is the only SPR rail that can reach it at all, and it reaches it with exactly zero margin - 15 V × 3 A is only 45 W. That single fact can decide whether you ship a cable in the box, whether you add a barrel jack, or whether you implement graceful performance scaling when the negotiated contract comes in under the design point.

For teaching, the calculator is a concrete demonstration of constraint intersection and proportional reasoning. Students can see that doubling the voltage halves the current for the same power, then connect that to I²R losses and to why the specification climbed to 48 V rather than simply allowing more amps. Try 18 W phone charging, 45 W ultrabook, 100 W workstation and 240 W dock, and watch which constraint binds first in each case - it is rarely the same one twice.

Quick checklist before you buy a charger or cable

• Know your target wattage: read the device's supplied adapter rating or its USB-PD input specification.
• Confirm the cable class: 3 A unless the cable is explicitly e-marked for 5 A; EPR needs a 240 W-certified cable.
• Check the charger's PDO list: a "65 W" charger is usually 20 V × 3.25 A, and a real 100 W charger needs 5 A support.
• Consider EPR only when needed: 28/36/48 V is for 140-240 W devices and requires compatible gear on both ends.
• Watch PPS if you charge phones: 20 mV steps matter far more than peak wattage for battery-direct charging.
• Use the table for margin: sitting right on a tier's ceiling means warm cables and throttling in real use.

With those basics in mind, use the calculator below to estimate a viable PD profile and to document the reasoning behind the choice. The goal is not to replace the USB-PD specification but to make the everyday question - "will this cable and this charger actually deliver the power I need?" - answerable in a few seconds.

Questions people ask when charging feels slow