Is 20V MAX the Same as 18V?

Two honest numbers describe the same battery. One is the voltage a full pack shows at rest, before any tool draws current. The other is the nominal voltage, the figure the pack is rated at and the one the regulations use. DeWalt's US pages print the first on the pack and the second in a footnote, and its UK range is named by the second. Only the nominal figure belongs in an energy calculation, and mixing the two across brands is how an 18 V pack ends up compared with an 18 V pack as if they differed.

Where the two numbers come from

A tool pack is a string of lithium-ion cells in series, and its voltage is the cell voltage times the number of cells. Molicel's P42A, a 21700 cell, is a convenient reference. Molicel lists it at 3.6 V nominal, and its data sheet figures, as reproduced by a cell retailer, give a charge voltage of 4.20 V and a discharge cut-off of 2.50 V.

So one cell has three voltages that matter: about 4.2 V full, 3.6 V nominal and 2.5 V empty. Multiply by the cell count and the pack figures follow. A five-cell pack runs from 21.0 V full to 12.5 V at cut-off, and 18.0 V is the rating in between. A voltmeter reading anywhere in that range neither proves nor disproves a label.

Notice that the max-style labels sit below the full charge column. A five-cell pack of 4.20 V cells reads 21.0 V at the top of charge, not 20. DeWalt's footnote defines its 20 as the "maximum initial battery voltage (measured without a workload)," and says nothing more about how it is measured. What can be said is that every max-style label in this article is exactly 10/9 of the maker's nominal figure, 4.0 V per 3.6 V cell. That is a naming rule shared across brands, not a measurement of any one pack.

What each maker actually rates its packs at

These are the nominal figures from each maker's own footnote, product page or safety data sheet.

Two conventions, and the split is not by brand. Makita names its LXT line by the nominal figure and its XGT line "40V max," while its own safety data sheet gives the BL4050F an average operating voltage of 36.0 V. DeWalt uses 20V MAX in the US and 18V XR in the UK. So "20V MAX vs 18V" and "40V max vs 36V" are the same question with the same answer. Between Makita's two lines the real gap is 18 V against 36 V nominal, double, not the 22 V the names suggest.

Watt hours are the only figure that compares packs

The federal definition settles what goes into an energy rating. 49 CFR 171.8 says the watt hour rating of a lithium-ion battery "is determined by multiplying the rated capacity of a cell or battery in ampere-hours, by its nominal voltage." Nominal, not maximum. That is the rating the airline rules use, as worked through in the guide to flying with power tool batteries, and it is the right basis for a buying comparison too.

Amp hours alone compare packs only within one platform. Across platforms, a 5.0 Ah pack at 36 V holds twice the energy of a 5.0 Ah pack at 18 V. Normalized to watt hours, packs with very different labels line up:

Three labels, one energy. A DeWalt 20V MAX 5.0 Ah, a Milwaukee M18 5.0 Ah and a Makita 40V max 2.5 Ah all rate 90 Wh. The XGT pack carries half the amp hours at twice the nominal voltage.

The MAX arithmetic overstates by 11 percent. DeWalt's own FlexVolt page promises "180 watt hours of energy in each battery pack" for the 9.0 Ah pack. That is 20 V times 9.0 Ah. The rated figure on the nominal basis is 18 V times 9.0 Ah, 162 Wh, the same as Milwaukee publishes for its 9.0 Ah M18 pack. Makita's 180 Wh for the BL4050F is on the nominal basis (36 V x 5.0 Ah), so the two "180 Wh" figures on two makers' pages are not the same measurement.

FlexVolt does not change its energy by switching modes. 18 V x 9.0 Ah and 54 V x 3.0 Ah are both 162 Wh.

The 12 V exception: same label, different rating

The one place where the label actively misleads is the 12 V class, and it is not a MAX problem. DeWalt rates 12V MAX at 10.8 V nominal. Milwaukee's safety data sheet rates M12 packs at 12 V nominal and computes their energy from it: 18 Wh for the 1.5 Ah pack, 48 Wh for the 4.0 Ah, 60 Wh for the HB5 at 5.0 Ah and 72 Wh for the 6.0 Ah.

So two packs that both say 12 and 5.0 Ah are rated 60 Wh and 54 Wh, 11 percent apart. The documents do not show whether that is a real difference in stored energy or only in rating convention. 12 V is not a whole multiple of the 3.6 V cell nominal used above, and the TTI sheet does not say how it was set. What is certain is that the rated Wh on two "12V 5.0Ah" labels will not match.

Same voltage, different plug

Equal nominal voltage does not make packs interchangeable. Each platform has its own slide rail, latch and terminal layout, and a pack fits only the tools built for that interface. The same split runs through tool storage, where a stacking box only couples to its own family, as the cross-brand modular compatibility guide shows, and the decision has the same shape as choosing a modular box ecosystem: you are buying into a platform for years. Third-party cross-brand adapters are outside what maker documents cover, so this page does not rate them.

Within DeWalt, the 18V and 20V MAX lines are a documented case. The DCA1820 adapter "allows for the use of 20V MAX batteries in most 18V DEWALT tools." DeWalt attaches four conditions to it:

1. It requires a 20V MAX charger for the packs.
2. It works only with 1.5, 2.0, 3.0, 4.0 and 5.0 Ah XR packs.
3. It will not work with 20V/60V FlexVolt packs.
4. It should not be used with premium DeWalt batteries in the DC970 or DC759 tools.

The footnote on the same page reads 20 V maximum, 18 V nominal. The old tools and the new packs share a nominal voltage. The adapter exists because the plug changed, not the voltage.

A worked comparison across three platforms

A crew runs three platforms and wants to know which spare pack carries the most energy. Weight is the other half of that choice when the spare rides in a bag, where two 5.0 Ah packs alone can pass 3 lb, as the electrician's bag weight budget shows. The candidates are a 24V 2.0 Ah rated at Sun Joe's 21.6 V, a 20V MAX 5.0 Ah and a 40V max 2.5 Ah.

The printed numbers rank these the same way, because every label here carries the same 10/9 inflation. The error bites when one label is nominal and the other a MAX figure. Compare an M18 5.0 Ah (printed 18) with a 20V MAX 5.0 Ah (printed 20) on the printed number and the DeWalt looks 11 percent larger. On rated Wh they are both 90.

The rule for any mixed fleet: get the nominal figure from the footnote, data sheet or safety data sheet, multiply by the amp hours, and compare watt hours. If the rating label on the underside prints a Wh figure, that number beats any arithmetic. Logging that figure alongside make, model and serial is also what makes a battery claim go smoothly, as covered in tool insurance and inventory.

Where this answer stops applying

• The rating is not usable energy. A Wh rating is nominal volts times rated capacity. The energy a tool actually gets depends on load, temperature and the pack's cut-off. A cold pack delivers less, as the battery storage temperature guide explains.
• Rated capacity has its own basis. Molicel lists the P42A at 4200 mAh typical and 4000 mAh minimum, about 5 percent apart. Pack documents checked here do not say which basis their Ah figure uses, so two packs with the same rated Wh can still differ slightly in use.
• Higher voltage is not more power by itself. Power depends on the current a pack and tool can sustain as well as voltage. Nothing on this page ranks tools by output. It ranks packs by rated energy.
• Other 24V and 40V brands need their own check. The 24V figure here is Sun Joe's footnote and the 40V figure is Makita's. Another brand using the same label may rate its pack differently, as the 12 V class shows. Read the footnote or the safety data sheet for the pack in your hand.

The thirty-second check

1. Find the nominal voltage. If the pack says MAX, the footnote gives it: DeWalt 12V MAX is 10.8, 20V MAX is 18, Sun Joe 24V is 21.6, Makita 40V max is 36, DeWalt 60V MAX is 54.
2. Multiply by the amp hours to get rated watt hours.
3. Compare packs on watt hours, not on volts or amp hours alone.
4. If the rating label prints a Wh figure, use that instead of your arithmetic.
5. A 20V MAX pack and an 18V pack are the same voltage. Whether one fits the other's tools is a question about the plug, not the volts.