The same company sells the same battery as 20V in America and 18V in Britain. Same cells, different disclosure rules.

Eighteen volts and twenty volts MAX are the same battery

The clearest way to stop overpaying for a number. Pro Tool Reviews say it twice deliberately: 18V batteries and 20V MAX batteries produce exactly the same voltage. Five lithium cells in series give roughly 18 volts nominal, which is the working voltage, and roughly 20 to 21 volts maximum off the charger, which drops fast the moment the tool is under load. American labeling may quote the peak; the asterisk on a DeWalt box says so in three languages, noting the maximum initial battery voltage measured without a workload is twenty volts. Which brands use which convention is worth knowing too: Bosch, Milwaukee, Ridgid, Ryobi and Makita run on 18V platforms, while DeWalt, Craftsman and Porter-Cable use 20V MAX.

The proof is that one company uses both labels for one product

This is the fact that ends the argument, because it is not an interpretation of marketing but a manufacturer doing both things at once. DeWalt’s 20V MAX in the United States and 18V XR in the United Kingdom use the same internal lithium-ion cells with no hardware difference. The reason is regulatory: American rules permit advertising the peak voltage measured with no load, while British and European rules require the nominal voltage. So the same battery, from the same company, carries a different number depending on which market it is sold in. If voltage labels described a physical property, that could not happen.

The practice is less cynical than it looks, and the history explains it

Worth knowing so that this does not read as one brand behaving badly, because the convention was industry-wide before DeWalt adopted it. Milwaukee avoided the problem entirely by never quoting voltage at all, branding their lines M12 and M18, which one publication calls a clever way to brand a 12-volt-class line without having to describe it as 10.8V. Bosch moved from 10.8V to 12V Max in mid-2008 and Makita followed. DeWalt then adopted the 12V Max convention, and when their lithium 18V line launched they used 20V Max. Their stated reason was to create a distinction between their older, largely NiCad 18V system and the new slide-style battery line, which is a genuine problem: two incompatible systems that would otherwise both have been called 18V.

You are choosing a platform, and the batteries are what lock it in

The actual decision, and the one the voltage comparison distracts from. Batteries are not interchangeable between brands, so the first purchase commits you to an ecosystem. The scale of that commitment is worth quantifying before making it: one platform is described as covering more than two hundred and seventy-five solutions, another as letting batteries be shared among more than a hundred tools. So the question to ask is not which tool wins a review this month but which platform will still be selling the tool you want in four years, because at that point you will own several batteries and a charger and the switching cost is most of the value of the collection.

Amp-hours decide how long you work. Voltage does not

Once the voltage question is settled, this is the number that matters and it is set by different wiring. Cells are always arranged in groups of five wired in series, which is what sets the voltage, and those groups are then wired in parallel to increase amp-hours and total capacity in watt-hours. So series determines the voltage label and parallel determines the runtime, which is why comparing amp-hours across two batteries at the same nominal voltage is a real comparison while comparing 18 against 20 is not. The related purchase point: a second battery is usually worth more than a more powerful tool, and two batteries with one slow charger is functionally one battery.

The official adapter runs the opposite way from what people assume

A genuinely useful practical fact. Within DeWalt, there is no official adapter that lets older 18V NiCad batteries run newer 20V MAX tools. The migration path works the other way: an adapter lets new 20V MAX batteries run in most older 18V tools, so you can upgrade batteries without replacing an entire tool collection. That is worth understanding as a general shape rather than a single part number, because it means the supported upgrade path protects the tools you already own rather than the batteries you already own. Batteries are consumable and tools are not, and the adapters follow that logic.

Cross-brand adapters exist, and notice who is telling you about them

One guide in circulation states that batteries from different brands are often cross-compatible with the right adapter, and that guide is published by a company that sells adapters. That is worth naming rather than repeating as neutral advice. Manufacturers do not support cross-brand adapters, warranty positions are unlikely to survive one, and a battery management system communicating with a tool it was not designed for is not a small thing to improvise. If cross-brand compatibility matters to you, the reliable way to get it is to choose one platform, which is the same conclusion from the other direction.

The equivalence is the label, not the tools

Worth stating so that nobody takes the wrong lesson from all of this. Establishing that 18V and 20V MAX are the same voltage does not establish that the tools are the same. Performance variation is real and comes from tool design, motor efficiency including brushless versus brushed, and cell quality, which one publication notes differs between brands and even within a single brand. So the voltage number can be ignored and the rest of the specification cannot. And a caution about the reading you will do: this subject has a lot of poor content, including one guide in the search results claiming a 20V Max battery has five cells while an 18V battery has four, which contradicts the arithmetic every credible source gives and is simply wrong.