Katadyn BeFree AC 0.5 L Filter Bottle (slate blue)

What it removes from water, and how well that is proved. 8: Clearly better than most.

Removes bacteria and protozoan cysts with a 0.1 µm hollow-fibre membrane. Katadyn's manual says it meets the US EPA's microbiological water purifier standards for bacteria (99.9999%, Klebsiella terrigena) and protozoan cysts (99.9%, Giardia and Cryptosporidium): a named standard for bacteria and protozoa, so 4. It does not remove viruses (the manual says so), which is why it is not a 5.

How fast clean water comes through when it is new. Real flow drops as it clogs: see Ease of use. 7: Better than most.

Katadyn gives 2 L/min when new, in the 1.8–2.49 band. Testers say the fast flow is real but the carbon insert slows it: Adventure Alan found it cuts new flow roughly in half, and flow keeps falling as the membrane loads (the BeFree cannot be backflushed). Banded on Katadyn's figure.

How much it adds to your pack. 9: Among the best you can buy for this.

72 g for the filter, cap and 0.5 L soft flask (Katadyn, and Ultralight Outdoor Gear), under the 90 g line.

Filling, squeezing or pumping effort, how fast it clogs in peaty or silty water, and how easy it is to clean. 9: Among the best you can buy for this.

Outdoors Magic calls it the best flow rate of any portable filter he has used and cleaning easy; Adventure Alan picks this size as the best BeFree because it fits a shoulder-strap or vest pocket, so you can drink without taking the pack off. Hiking Daily and OutdoorGearLab are a notch lower: no backflush, it clogs in silty water and the membrane has to be wet first. OutdoorGearLab reviews the BeFree AC range (0.5 L to 1 L) as one filter, and ease is the same cap, filter and neck on every size. Two sources score 5 and two score 4: average 4.5 out of 5, so 9 out of 10. Better Trail's 1 L review and the Whiskey Wanderer video are not counted here: they used the 1 L.