How to Know What Dropper Post Fits your Bike

A longer dropper post is one of the best upgrades you can make to your bike. A longer dropper will give you more confidence and control on every ride. And the award-winning OneUp Dropper Post will give you more drop than any other post on the market.

Today we're going to help you find the longest travel dropper post that will fit you and your bike. We’ll show how to take the two key measurements you need to use our Dropper Post Length Calculator or our 27.2 Dropper Post Length Calculator


Step 1: Identify your Seatpost Diameter

There are four common sizes for seatpost diameter: 27.2, 30.9, 31.6 and 34.9mm. 27.2mm is for XC and gravel bikes mainly. For mountain bikes, the common sizes are 30.9, 31.6, and 34.9. 

The diameter should be listed on your current seat post or on your bike manufacturer's website.

Step 2: Measure Ride Height (X)

Ride Height is measurement X on our Dropper Post Length calculator tool. This measurement is the amount of post sticking out of your frame when your dropper post is at full extension.

To measure: 

  • Put your dropper post at full extension
  • Measure from the top of the seat post collar to the middle of the saddle rails. 
  • This will give us measurement X for the Dropper calculator.

The Ride Height measurement in this image is 304mm.

Step 3: Remove your Seatpost

The easiest way to remove your post is to disconnect the remote from your handlebar, leaving the cable connected to the remote.  

Next, loosen off your seat post collar. That will allow your current seatpost to come out.

Now, with the cable clamped in the remote at one end and connected to the post at the other you’ll be able to move everything together. The technique is to feed the cable in with one hand at the front of the bike and gently remove the post with the other hand at the same speed.

Step 4: Measure Insertion Depth

Insertion Depth tells us how much dropper post can fit inside your frame. The measurement is taken from the top of your seat tube down to anything in your frame that might obstruct a dropper post upon insertion. Bends, internal welds and frame pivots in the seat tube could obstruct your post.

To measure:

  • Carefully insert a tape measure until the tape makes contact with an obstruction.
  • Spin the tape measure inside the frame to see if it gets hung up on anything. 
  • Measure from the depth where the tape measure first makes contact with an obstruction to the seat post collar. 

The Insertion Depth in the example is 310mm.

Step 5: Using the calculator

Put your two measurements into our Dropper Post Length Calculator below, or proceed to our 27.2 Dropper Post Calculator and enter your measurements

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Now you know the longest size dropper post you can fit in your bike.

Any questions, concerns, or trouble, please email

We're here to help.