DIY Stir Plate for Homebrew Yeast Starters

Up until now I have only used dry yeast for my homebrew creations. This limits me in what types of brews I can create as the various strains of liquid yeast far outnumber the dry strains. As an example, I am planning to brew a Kolsch soon, and there is no available dry yeast strain for that.

The purpose of a yeast starter is to build up the number of viable yeast cells from the amount that comes in a vial of yeast from the store. A vial of California Ale Yeast (WLP001) will have roughly 96 billion yeast cells. For a 5.5 gallon beer with a 1.050 original gravity, you will want to use about 192 billion. A starter is a mini fermentation that builds up the yeast as they feed on the sugars in the mash you’ve made.

A magnetic stir plate escalates the growth in the yeast starter by spinning a magnetic stir bar at the bottom of the flask containing the starter. I can grow that 96 billion to 192 billion with a .75 liter starter on the stir plate, opposed to 131 billion without. These numbers we calculated from the beersmith app.

Here is a video of a stir plate in action from Beer Geek Nation:

There are several, resources, for building, your own, stir plate online, so add this post to the list.

Parts List


The beauty of this fan is that it contains most of the parts that DIY’ers have to locate to build  a magnetic stir plate: computer fan, power supply, speed control (potentiometer). All for about $20 shipped on Amazon. Granted you can get these parts free from defunct electronics you may have laying around.

It has to be disassembled, but it’s real easy. Unscrew the bolts at the four corners, take off the stand and grilles. Disconnect the power and the potentiometer. Done.

It’s a USB connection, but a standard iPhone wall wart will allow you to plug the USB end into it, then plugged into the wall. There is no on/off, if it’s plugged in it’s on.

I started with a cigar box that was once a wall clock (note the little hole under the ‘n’ in Sinatra, that’s where the clock mechanism came through). This was given to me by my good friend Jeremy back in the Twitter Poker Tour days. The clock mechanism failed so I decided to give it a new life as a stir plate!

I measured the center and placed the fan making sure to give enough room for the speed dial to fit in the front right. I then drilled through the holes at the corners of the fan through the bottom of the cigar box.

I inserted the #8-32 bolts up from the bottom through a washer then through the holes I just made. From the inside, I used another washer and a few nuts to secure the bolt and to act as a spacer to get the fan closer to the top of the cigar box.

I fitted the fan over the four bolts and used another washer/nut combo to secure the fan in place.

Once I had the fan in place, I used a step-bit to drill a hole large enough to fit the knob from the speed controller (potentiometer) while mounted from the inside. While the drill was out, I drilled a similarly sized hole in the rear-right to lead the power cord into the cigar box.

Here is a view of the speed dial just prior to mounting it inside.

I mounted a fender washer to the top of the fan center with velcro, but will use something stronger at some point. The hard drive magnets are stuck to the washer. It may take a little work to center the magnets so that it spins true. If it’s off-center it can throw the stir bar.

Fill the flask with some water and place it on the stir plate. Drop the stir bar in and it will be drawn to magnet at the center. Turn on the fan and as the fan spins, so will the stir bar.

7 thoughts on “DIY Stir Plate for Homebrew Yeast Starters

  1. Nice work Geoff, I really like your wooden enclosure.

    Just a week or two ago I’ve started selling kits with all the hard-to-source parts for brewers like yourself wanting to build DIY stir plates. I know it’s a bit late to help you now, but if you ever want to conduct a review on your blog, let me know and I’d love to send you one of our kits.

    Cheers mate,
    Michael from Digital Homebrew.

  2. Hey I’m just wondering how big of a vessel this thing can max out on mixxing?
    for example, could it keep a 30 gallon container well contained?

  3. Victor: Not sure if it would be able to get 30 gallons mixed. Would probably need a bigger stir bar and thus a more powerful motor to move it. That’s just a guess though!

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