Ultralight Water Filter Cap

For water purification, I use MicroPUR Tablets. They work great, but depending on where I fill my water bottle up from – I sometimes get "floaty bits" in the water. A traditional water filter would of course remove these.

What I need is a way to filter out these bigger particles when filling the bottle.

Read on for instructions on my home-made solution.


1. Buy yourself some replacement caps for the platypus bottle.

2. Cut a hole in the top of the cap. I did this with a 3/4" spade bit and a drill, which cut a perfect sized hole. You could use an exacto knife and a little care.

3. Using some landscaping fabric, cut out a small disk of fabric the same diameter as the outer rim of the cap.

4. Insert the filter fabric into the inner cap

5. Screw the cap onto the bottle.

Cut out 2 or 3 more disks of fabric and put them in your first-aid kit – now you have a field-serviable filter in case one should tear. Voila!

Total weight? Irrelevant – it's less than 0.1 oz, as it wont register on my scale.

Update on Aug 8th:

Hhhmm… Having this filter slows the rate of flow for filling up the bottle of course, but in a running water stream – it's no problem at all. In a lake, it simply takes too long 'scoop' water in with a sideways action – I think I'm going back to the drawing board on this one and looking for a fabric or mesh that is less dense….

Update on Aug 17th:

I've tried using 'metal screen mesh' cut from a small loose-tea strainer into a similar disk and it seems to be working well. I'll post pics later.

7 thoughts on “Ultralight Water Filter Cap

  1. Matt Ahonen

    Brett –

    I just made a similar filter using a screen out of a fuel funnel. I have been using platys for
    quite some time, but have not taken them on a trip without a traditional water filter / pump.

    What is your secret to filling them in shallow water?


  2. brett Post author


    If the shallow water is flowing, there’s usually enough flow to push it in there – you just have to be patient. Also – squeezing the air out of the bag helps as it fills. In shallow water with no flow – I have no secret – if it won’t fill – I fall back to filling without the filter cap. The chemicals take care of the bad guys anyway – it just means I have to suffer any floaty bits – usually never that bad. 🙂

    In a still lake / deeper water – you can just use sideways motion of the bag to help generate flow.

    Happy filtering!

  3. Miguel Marcos

    Since the Platypus + cap is so light why not have two, a filtering one you fill it up without the cap, screw the filter cap on when full, then pour the filtered water into the drinking Platypus with a really small funnel? Sure it’s extra equipment but it weighs nothing and folds down to nothing.

  4. brett Post author

    hi miguel ,

    yeah that would work for sure. an extra platy is 1.0 oz. while that sounds like nothing i’ve learned over the years that it’s easy to get tempted to bring an extra something or other. but it’s a slippery slope and things start adding up to half pounds and pounds pretty quickly. philosophically, i prefer to lean on technique over additional items if possible. this is one case where a little extra time spent by the water source can save some weight – so it’s worth the trade off for me.

    great suggestion though! keep them coming.

  5. Alex

    Hi. Great blog – thanks for the good work!
    To get rid of the floaty bits I put a bandana (or shirt) over the opening to my platy and pour water through the bandana with my cookpot. I use the platy with the zip-lock end, so i have a nice wide opening. As long as I can get water into my cookpot, then I’m good to go.

  6. brett Post author

    good tip on the bandana. i recently did a hike with another guy who had the zip-lock platy also. i was skeptical at first – as it seemed that a blow-out could be devastating to your pack contents. but it seem pretty darn secure and it was REALLY convenient for scooping up water etc.

  7. The water filter champion

    Great idea, I love well thought out ideas that cost next to nothing

    If you have low flow how about filling your platy without the filter and then drinking from it with the filter fixed on. Or is it not strong enough to take the weight of water coming back?

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