Impact Driver Showdown: Flex 24V vs Milwaukee M18 Fuel vs DeWalt

Introduction

When Flex staged its head-to-head event pitting its newest tools against the competition, the impact driver tests drew the most attention, and the most skepticism. Was the testing fair? Were the right batteries used? Why were two DeWalt drivers left out entirely?

To answer those questions, the Flex testing rig was rebuilt from scratch, reverse-engineered from the published setup down to the steel frame, pulley system, and weighted rope. Then five impact drivers went head to head under increasing loads. The results were surprising.

How We Put the Tests Together

The original rig looked like heavy steel construction with a pulley system and rope, so sourcing materials and getting the dimensions right took real effort. Measurements were estimated by scaling a human against the building materials, then components were remade more than once as things broke or failed to work.

Even after completion, there was no guarantee a single usable test would come out of it. In the end, the rig held up, and the numbers it produced settled the argument fairly decisively.

The Battery Question Nobody Talked About

On paper, Flex should be the clear winner based on specification alone. But there is a caveat worth understanding. The published specs for the Milwaukee M18 Fuel impact driver date from its original release, when it ran a 5 amp hour battery built on 18 650 cells.

During the head-to-head event, all three tools ran 6 amp hour batteries built on 21 700 cells. That means Milwaukee may have been at a better advantage than the raw specifications suggest. DeWalt’s DCF887 sat at the largest disadvantage on paper, which raised two fair questions: why weren’t the DCF850 and DCF840 included, and why did DeWalt run its 6 amp hour 21 700 cell battery instead of its own PowerStack pack?

The Five-Pound Test

Every tool started fully charged, and every tool ran a 6 amp hour battery with 21 700 cells, except the Flex, which ran its Stack Lithium pouch cells. At five pounds of load, the Flex finished fastest, so fast it barely registered on camera. Milwaukee and the field followed in a near dead heat for second, while the DeWalt DCF887 turned in the slowest run of the group.

The Seven-and-a-Half-Pound Test

Increasing the weight to seven and a half pounds separated the field further. The Flex kept its lead, and the DCF887 again struggled to keep pace. It was also at this stage that the two previously missing DeWalt drivers got their chance: the DCF850 was run with the PowerStack battery, and the results turned heads.

The 10-Pound Test: Where Bits Break

Plenty of observers found it odd that Flex chose not to push beyond eight pounds in its own event, given how capable these drivers clearly are. So the weight went up to ten pounds. The Flex powered through. Then the bits started snapping.

The DeWalt DCF840, running a 6 amp hour battery, broke a bit mid-run, and the pattern repeated with other drivers as well. One driver did not even lift the load off the ground before its bit gave out. That is likely why the original event stopped at eight pounds: past that threshold, the test becomes as much about the bits as the tools.

Which One Should You Buy

A practical caveat on the whole exercise: running a 6 amp hour battery on any of these tools is not the ideal configuration. All three platforms make very compact drivers, and a large battery undercuts that advantage. Flex deserves credit for offering a smaller 3.5 amp hour Stack Lithium battery that still delivers the full amperage in a compact form factor.

Honest takeaway: Flex outperformed the competition even running its previous generation of cylindrical cell batteries, and the true potential of Stack Lithium probably will not be realized until those batteries pair with much larger tools such as bigger saws, rotary hammers, breakers, and cut-off saws. The potential is there.

For Milwaukee fans, the M18 Fuel remains a strong all-around driver, and the battery caveats around its published specs arguably flatter it more than the spec sheet suggests.

The DeWalt DCF887, the oldest design of the group, showed its age across every load level and is best treated as a proven, budget-friendly option rather than a performance leader.

Wait, one note on the compact DeWalt options: the DCF850 with the PowerStack battery delivered the shock result of the heavy tests, outperforming expectations for its size.

Conclusion

Under a rebuilt, independently constructed testing rig, the Flex 24V impact driver was the fastest at every load level, from five pounds through ten. The Milwaukee M18 Fuel held a solid middle position once battery configurations were equalized. The DeWalt DCF887 lagged at every stage, while the newer DeWalt DCF850 and DCF840, especially with a PowerStack battery, proved far more competitive than the original event implied. If raw driving performance is the priority, the Flex is the standout; if you value compact form factors and battery flexibility, weigh the smaller pack options before choosing.